TOPIC 5: ENVIRONMENTAL PHYSICS | FORM 6
(i) Agriculture physics
– Influence of solar radiation on plant growth.
– Influence of wind, humidity, rainfall and air temperature on plant growth.
– Soil environmental component which influence plant growth.
(ii) Energy from the environment
(iii) Geophysics (Earth quakes)
Elastic rebound theory
Types of seismic waves
Propagation of seismic waves
(iv) Environmental pollution
Types of pollutant in the atmosphere
Transport mechanisms of atmospheric pollutant
Nuclear waste and their disposal
Effects of pollution on visibility and optical properties of materials.
Environmental physics is an interdisciplinary subject that integrates the physics processes in the following disciplines: the atmosphere, the biosphere, the hydrosphere, and the geosphere.
Environmental physics can be defined as the response of living organisms to their environment within the framework of the physics of environmental processes and issues.
It is structures within the relationship between the atmosphere, the oceans (hydrosphere), land (lithosphere), soils and vegetation (biosphere).
It embraces the following themes:
(i) Human environment and survival physics,
(ii) Built environment
(iii) Renewable energy
(iv) Remote sensing
(v) Weather, climate and climate change, and
(vi) Environmental health.
The environment may be defined as the medium in which any entity finds itself, For example, for a cloud its environment may be the region of the atmosphere in which it is formed.
Agriculture physics is concerned with physics environment in relation to plant growth.
(a) Influence of Radiation Environment on Plant Growth
Radiation environments. Refer to radiations present in the atmosphere, commonly coming from the sun.
Components of solar radiation
The main components of solar radiation are:
(i) Visible light
(ii) Infrared radiation, and
(iii) Ultraviolet radiation.
HEATING EFFECT OF SOLAR RADIATION ON PLANTS
An optimum amount of heat on plant favours the process of photosynthesis. This enables a plant to make its own food and hence provide its growth.
(i) Excessive solar radiation (ultraviolet light) on plants leads to bleaching of green pigment (chlorophyll). This lowers the amount of food produced by photosynthesis to plant and hence a plant may die.
(ii) Excessive solar radiation on plants leads to excessive water loss in the form of water vapour commonly on plant leaves (transpiration). Hence wilting (drying) of plants may occur.
(b) Influence of Aerial Environment on Plant Growth
Aerial environments refer to the atmospheric condition resulting from a series of processes occurring in the atmosphere. These include air temperature, wind, humidity and rainfall.
WIND EFFECT ON PLANT GROWTH
(a) Wind acts as pollinating agent for some plants and hence favours plant productivity.
(b) Wind also favours evaporation of water from plant leaves and thus maintains water balance for proper plant growth.
(a) Excessive wind on environments leads to plant breaking or cutting of tree branches. This may lead to the death of plant.
(b) As the wind speed increases further, cell and Cuticular damage occurs, followed by death of plant tissue, and a gnarled appearance becomes more apparent.
(c) At low wind speeds, the effect seems to be an increase in transpiration, which results in water stress. This stress causes the plant to adapt by decreasing leaf area and internodes length, while increasing root growth and stem diameter.
(d) Strong wind may also cause shade off flowers; this lowers plant productivity.
Effect of Rainfall on Plant Growth
An optimum amount of rainfall on plants favours its growth. Water is a raw material for the process of photosynthesis from which plants obtain their food and hence their growth.
Excessive rainfall leads to water logging in soil which in turn leads to root spoil and hence the death of plant.
Effect of Humidity on Plant Growth
Favourable humidity on plants help plants to conserve water for various activities and in seeds helps the development of new leaves.
Low humidity results into a greater rate of transpiration and hence may result into plant drying.
Effect of Air Temperature of Plant Growth
An Optimum temperature on plants enhances enzymic activities which in turn gives favourable conditions for plant growth.
(a) High temperature denature enzymes commonly for photosynthesis and hence the death of plant.
(b) Low temperature inactivates the plant growth enzymes, hence low growth rate.
Wind belts are seasonal strong wind moving in a specified direction in a certain region of the earth.
The global wind belts are formed by two main factors:
(i) The unequal heating of the earth by sunlight and
(ii) The earth’s spin.
Here is a simple explanation of the process
The unequal heating makes the tropical regions warmer than the Polar Regions. As a result, there is generally higher pressure at the poles and lower at the equator. So the atmosphere tries to send the cold air toward the equator at the surface and send warm air northward toward the pole at higher levels.
Unfortunately, the spin of the earth prevents this from being a direct route, and the flow in the atmosphere breaks into three zones between the equator and each pole.
These form the six global wind belts: 3 in the Northern Hemisphere (NH) and 3 in the Southern (SH). They are generally known as:
(1) The Trade winds, which blow from the northeast (NH) and southeast (SH), are, found in the sub tropic regions from about 30 degrees latitude to the equator.
(2) The Prevailing Westerlies (SW in NH in SH) which blow in the middle latitudes.
(3) The Polar Easterlies which blow from the east in the Polar Regions.
Effects of wind belts to plant
1. Wind belts because the loss of plant leaves and flowers hence lower plant productivity and growth. Loss of leaves lowers the rate of photosynthesis.
2. Wind belts sometimes cause plants to lean in direction of moving wing. This changes their direction of growth
3. Trees are broken by the strong wind.
(c) Soil Environment Components Which Influence Plant Growth
Soil is composed of both rock particles and organic matter (humus) – the remains of plants and animals in various stages of decomposition. The humus serves as food for many living organisms. Within the soil is a large population of animals, plants. These break down the humus into soluble substances that can be absorbed by the roots of large plants.
Components of a soil
Soil is composed of:
( (a) Air, 25% by volume which supports life of soil organisms,
(b) Water, 25% which dissolves minerals so that are easily absorbed by plants,
(c) Organic matter (humus), 5% by volume,
(d) Inorganic matter (minerals), 45% by volume,
(e) Biotic organisms, micro – organisms like earth worm, centipedes, millipede, bacteria which decompose organic matter.
Types of soil
(i) Sandy soil
(ii) Silt soil,
(iii) Clay soil, and
(iv) Loamy soil (sand + silt + clay soil mixture)
Water Movement in the soil
Two forces primarily affect water movement through soils, (a) gravity and (b) capillary action.
Capillary action refers to the attraction of water into soil pores – an attraction which makes water move in soil. Capillary action involves two types of attraction – adhesion and cohesion.
Adhesion is the attraction of water to solid surfaces.
Cohesion is the attraction of water to itself.
Speed of water in a particular soil type depends on:
(i) How much water is in the soil, and
(ii) Porosity of the soil.
The movement of water in the solid is mainly due to gravity. The porosity gives a measure of how much water the soil can hold and the rate at which water flows through the soil. Large pore spaces give a faster rate and vice versa.
An experiment to study water movement in soil
An experiment to demonstrate the rate of flow of water in the soil is done using a glass tube and sand type filled in it. Water is poured into the tube and the time taken for water to reach the bottom of the tube in notes.
i. Sand soil have large pore spaces thus allows water to travel downwards through it at a fastest rate.
ii. Clay soil can hold water as has very fine pore spaces.
iii. Loamy soil allows water movement at a medium rate.
Heat transfer in the soil
Within the soil heat is transferred by a conduction process. Since soil is poor conductor of heat most of the heat from the atmosphere appears at the surface of the earth.
An optimum soil temperature favours plants growth but a high temperature can lead to the rotting of plant roots.
(d) Techniques for the Improvement of the Plant Environment
Plant environment can be improved by using wind breaks, shading and mulching.
Shading is the process of obstructing plants from excessive solar radiation.
Positive Impacts of Shading
1. Prevents excessive loss of water by plants through transpiration. This enhances plant productivity.
2. Preserve moisture in the soil and hence water supply to plant.
Mulching is the process of covering the soil by dry leaves, grasses and or papers.
Benefits (Advantages) of Mulching
1. Improve soil moisture. Bare soil is exposed to heat, wind and compaction loses water through evaporation and is less able to absorb irrigation or rainfall. Using mulches, the soil has greater water retention, reduced evaporation, and reduced weeds. Mulch can also protect trees and shrubs from drought stress and cold injury
2. Reduce soil erosion and compaction. Mulches protect soils from wind water, traffic induced erosion and compaction that directly contribute to root stress and poor plant health.
3. Maintenance of optimal soil temperatures. Mulches have shown to lower soil temperatures in summer months. Extreme temperatures can kill fine plant roots which can cause stress and root rot. Mulches protect soils from extreme temperatures, either cold or hot.
4. Increase soil nutrition. Mulches with relatively high nitrogen content often result in higher yields, but low nitrogen mulches, such as straw, sawdust and bark, can also increase soil fertility and plant nutrition.
5. Reduction of salt and pesticide contamination. In arid landscapes, evaporating water leaves behind salt crusts. Because mulches reduce evaporation, water is left in the soil and salts are diluted. Organic mulches can actively accelerate soil desalinization and help degrade pesticides and other contaminants.
6. Improve plant establishment and growth. Mulches are used to enhance the establishment of many woody and herbaceous species. Mulches improve seed germination and seed survival, enhance root establishment, transplant survival, and increase plant performance.
7. Reduction of disease. Mulches will reduce the splashing of rain or irrigation water, which can carry spores of disease organisms to stems and leaves of plants. Populations of beneficial microbes that reduce soil pathogens can be increased with mulches. Mulches can combat disease organisms directly as well.
8. Reduction of Weeds. Using mulches for weed control is highly effective. Mulches can reduce seed germination of many weed species and reduce light, which stresses existing weeds.
9. Reduce pesticide use. Mulches reduce weeds, plant stress, and susceptibility to pests and pathogens which translates to reduced use of herbicides, insecticides, and fungicides.
Mulch Problems (disadvantages of mulching)
1. i. Acidification. Some types of mulches can increase soil acidity.
2. ii. Disease. Many mulches made from diseased plant materials can be composted or treated at temperatures that kill pathogens that can be transmitted to healthy plants.
3. iii.Pests. Many organic mulches, especially wood – based mulches, have the reputation as being “pest magnets”.
4. iv. Weed contamination. Improperly treated crop residues and composts as well as bark mulches are often carriers of weed seed. Mulch must be deep enough to suppress weeds and promote healthy soils and plants. Weed control and enhanced plant performance are directly linked to mulch depth.
v. Wind Breaks
Wind breaks are long rooted strong plants (trees) that are used to obstruct the path of wind or to slow down the wind.
Windbreaks provide many benefits to soil, water, plants, animals and man. They are an important part of the modern day agricultural landscape. Windbreaks come in many different sizes and shapes to serve many different conservation purposes.
In agriculture, wind breaks protect small growing plants from strong blowing wind
Advantages of Windbreaks to Plant Environment
1. i. Control soil erosion. Windbreaks prevent wind erosion from causing loss of soil productivity. This eliminates plant roots stresses and thus favours plant growth condition.
2. ii. Increase plant yield. Windbreak research substantiates that field windbreaks improve crop yields which offsets the loss of production from the land taken out of cultivation.
3. Pesticide sprays. Windbreaks control pesticide spray drift and provide buffers to delineate property lines and protect neighbors.
EXAMPLES: SET A
(a) What is agriculture physics? (02 marks)
(b) What are the components of a soil? How do they support the life of a plant? (06 marks)
(c) Explain briefly how soil temperature affects plant growth. (02 marks)
(a) What do you understand by the word environmental physics? (01 marks)
(b) Explain how the following climatic factors influence plant growth: air temperature, humidity, rainfall and wind. (06 marks)
(c) What are wind belts? Explain the effect of wind belts on plant productivity. (03 marks)
(a) What is mulching? (02 marks)
(b) Give two advantages and two disadvantages of mulching. (04 marks)
(c) Discuss the heating effect of solar radiation to plant growth. (04 marks)
(a) Explain two factors that primarily affect water movement in the soil (03 marks)
(b) Explain the soil environment that favours high crop yield (04 marks)
(c) What is shading and what is its purpose? (03 marks)
(a) (i) Mention the components of solar radiation.
(ii) How do those components affect plant growth? (04½ marks)
(b) What are wind breaks? (02 marks)
(c) What are the advantages of wind breaks to plant environment? (03½ marks)
ENERGY FROM THE ENVIRONMENT
Energy is defined as the capacity to do work Or is defined as ability to do work.
Energy is measured in Joules (symbol J)
Types of energy according to their usefulness
(i) High grade energy
(ii) Low grade energy
i. High grade energy is the energy that is easily transformed into other forms of energy and is more suitable for doing works.
Examples are chemical and electrical energy.
ii. Low grade energy is the one that is not easily transformed into anything else.
Examples are the kinetic energy of molecules due to their randomness and the potential energy due to the forces between molecules.
There are two types of energy sources, namely:
(i) Primary energy sources,
(ii) Secondary energy sources.
i. Primary energy sources
Primary energy sources are sources of energy that are used in the form in which they occur naturally.
Primary energy sources fall into two groups:
(a) Finite energy sources,
(b) Renewable sources.
a. Finite energy sources are those energy sources that last after a number of years when exploited.
Examples are coal, oil, natural gas, and nuclear fuels.
b. Renewable energy sources: these cannot be exhausted. Examples are solar energy, biofuels, hydroelectric power, wind power, wave power, tidal and geothermal power, wind power, wave power, tidal and geothermal power.
ii. Secondary energy sources
Secondary energy sources are used in the non – natural form.
Nature of solar energy
The sun’s energy is produced by thermonuclear fusion.
Not all of the solar radiation arriving at the edge of the Earth’s atmosphere reaches the Earth’s surface.
About 30% is reflected back into space by atmospheric dusts and by the polar ice caps.
About 47% is absorbed during the day by the land and sea and becomes internal energy (i.e. heats the Earth). At night this is radiated back into space as infrared.
23% causes evaporation from the oceans and sea to form water vapour. This results into rain and hence hydroelectric power.
-0.2% causes convection currents in the air, creating wind power which in turn causes wave power.
-0.02% is absorbed by plants during photosynthesis and is stored in them as chemical energy. Plants are sources of biofuels
Solar constant is defined as the solar energy falling per second on a square meter placed normal to the sun’s rays at the edge of the Earth’s atmosphere, when the Earth is at mean distance from the sun.
Its value is about 1.35 kWm2
The amount of solar radiation received at any point on the earth’s surface depends on:
(i) The geographical location,
(ii) The season, (summer or winter)
(iii) The time of the day, the lower the sun is in the sky the greater is the atmospheric absorption.
(iv) The altitude; the greater the height above sea level the less is the absorption by the atmosphere, clouds and pollution
PHOTOVOLTAIC DEVICES (SOLAR CELLS)
A solar cell (PV, cells) is a PN junction device which converts solar energy directly into electrical energy.
How it Works
PV cells are made of at least two layers of semiconductor material. One layer has a positive charge (p – type material), the other negative (n-type material). When light enters the cell, some of the photons from the light are absorbed by the semiconductor atoms, freeing electrons from the cell’s negative layer to flow through an external circuit and back into the positive layer. This flow of electrons produces electric current.
Uses of the solar cell
1. (i)Are used to power electronics in satellite and space vehicles.
2. (ii)Are used as power supply to some calculators.
3. (iii)Are used to generate electricity for home, office and industrial uses.
Series arrangement of solar cells
Solar panel (module) is a sealed, weatherproof package containing a number of interconnected solar cells so as to increase utility of a solar cell.
When two modules are wired together in series, their voltage is doubled while the current stays constant.
When two modules are wired in parallel, their current is doubled while the voltage stays constant.
To achieve the desired voltage and current, modules are wired in series and parallel into what is called a PV array.
The flexibility of the modular PV system allows designers to create solar power systems that can meet a wide variety of electrical needs, no matter how large or small.
Efficiency of a photovoltaic system
The output power of a solar cell depends on:
(i) The amount of light energy from the sun falling on a solar panel (the intensity of light).
(ii) The orientation of the solar panel. More electricity is produced if light falls perpendicular to panels.
(iii) The surface area of the panel. Large area collects more solar energy and hence greater electricity.
The best designed solar cell can generate 240 Wm-2 in bright sun light at an efficiency of about 24%.
Advantages of photovoltaic systems
1. Solar cells can produce electricity without noise or air pollution.
2. A photovoltaic system requires no fuels to purchase.
3. Panels of photovoltaic cells are used for small – scale electricity generation in remote areas where there is sufficient sun.
4. Net metering: This has the potential to help shave peak loads, which generally coincide with maximum PV power production.
5. The electricity from a PV system is controllable.
Disadvantages of photovoltaic systems
1. They require an inverter to convert the d.c output into a. c for transmission.
2. They produce electricity only when there is sunlight. Hence they need backup batteries to provide energy storage.
3. Suitable in areas which receives enough sunlight.
4. Photovoltaic large scale power generation is cost effective. This is due to large surface area of cells required for generating high power outputs and the need to convert d.c to a.c for transmission.
5. Compared to other energy sources, PV systems are an expensive way to generate electricity.
6. The available solar resource depends on two variables: The latitude at which the array is located and the average cloud cover.
Winds are due to conventional currents in the air caused by uneven heating in the earth’s surface by the sun.
Wind energy is extracted by a device called wind turbine.
Wind speed increases with the height; it is greatest in hilly areas. It is also greater over the sea and coastal areas where there is less surface drag.
Wind turbines are also called aerogenerator or wind mills (old name)
Types of wind turbines
There are two types of wind turbines;
(i) Horizontal axis wind turbines (HAWT)
(ii) Vertical axis wind turbines (VAWT)
Horizontal axis wind turbine (HAWT)
HAWT has two or more long vertical blades rotating about a horizontal axis. Modern HAWTs usually feature rotors that resemble aircraft propellers, which operate on similar aerodynamic principles, i.e. the air flow over the airfoil shaped blades creates a lifting force that turns the rotor. The nacelle of a HAWT houses a gearbox and generator (alternator).
Advantage of HAWT
1. HAWTS can be placed on towers to take advantage of higher winds farther from the ground.
Disadvantages of HAWT
1. The alternator (generator) is paced at the top of the supporting tower.
2. Can produce power in a particular wind direction.
Vertical axis wind turbine (VAWT)
In vertical axis, the blades are long and vertical and can accept wind in any direction. The blades are propelled by the drag force on the blades as the wind flows.
Advantages of VAWT
1. It can harness wind from any direction
2. Typically operate closer to the ground, which has the advantage of allowing placement of heavy equipment, like the generator and gearbox, near ground level rather than in the nacelle.
Disadvantages of VAWT
1. Winds are lower near ground level, so for the same wind and capture area, less power will be produced compared to HAWT.
2. Time varying power output due to variation of power during a single rotation of the blade.
3. The need for guy wires to support the tower.
4. Darrieus VAWTS are not self starting like HAWTS. (More colorful picture and videos during lecture)
Power of a Wind Turbine
Consider a wind turbine with blades of length, r (area A), the wind speed is v and the air density is ρ. Assuming that the air speed is reduced to zero by the blades.
Kinetic energy of the wind, K.E =
Kinetic energy per unit volume
K.E per volume = ÷ volume =
The blades sweeps out an area A in one turn, so the volume of air passing in one second is Av.
Kinetic energy per second
= K.E per unit volume x volume per second
K.E per second = =
The available wind power is P =
The power extracted by the rotating blades is much less than the available wind power. This is because:
(i) The velocity of the wind is not reduced to zero at the blades
(ii) Losses due to friction at the turbine and alternator
(iii) Due to losses in both the gear train and generator.
The power actually captured by the wind turbine rotor, PR, is some fraction of the available power, defined by the coefficient of performance, Cp, which is essentially a type of power conversion efficiency:
The extractable power (electrical power output) is given by
Where ns and nb are efficiencies (power output over power input) for the generator and the gearbox.
Variations of power with wind speed
The power curve for a wind turbine shows this net power output as a function of wind speed.
i. Cut in wind speed: This is the lowest speed at which the wind turbine will start generating power.
Typical cut – in wind speeds are 3 to 5 m/s.
ii. Nominal wind speed: This is the lowest speed at which the wind turbine reaches its nominal power output.
Above this speed, higher power outputs are possible, but the rotor is controlled to maintain a constant power to limit loads and stresses on the blades.
iii. Cut – out wind speed: This is the highest wind speed which the turbine will operate at.
Above this speed, the turbine is stopped to prevent damage to the blades.
Advantages of Wind Energy
1. Wind Energy is an inexhaustible source of energy and is virtually a limitless resource.
2. Energy is generated without polluting environment
3. This source of energy has tremendous potential to generate energy on large scale.
4. Like solar energy and hydropower, wind power taps a natural physical resource,
5. Windmill generators don’t emit any emissions that can lead to acid rain or greenhouse effect.
6. Wind Energy can be used directly as mechanical energy
7. In remote areas, wind turbines can be used as great resource to generate energy
8. In combination with Solar Energy they can be used to provide reliable as well as steady supply of electricity.
9. Land around wind turbines can be used for other uses, e.g. Farming.
Disadvantages of Wind Energy
1. Wind energy requires expensive storage during peak production time.
2. It is unreliable energy source as winds are uncertain and unpredictable.
3. There is visual and aesthetic impact on region
4. Requires large open areas for setting up wind farms.
5. Noise pollution problem is usually associated with wind mills.
6. Wind energy can be harnessed only in those areas where wind is strong enough and weather is windy for most parts of the year.
7. Usually places, where wind power set-up is situated, are away from the places where demand of electricity is there. Transmission from such places increases cost of electricity.
8. The average efficiency of wind turbine is very less as compared to fossil fuel power plants. We might require many wind turbines to produce similar impact.
9. It can be a threat to wildlife. Birds do get killed or injured when they fly into turbines.
10. Maintenance cost of wind turbines is high as they have mechanical parts which undergo wear and tear over the time.
NB: Even though there are advantages of wind energy, the limitations make it extremely difficult for it to be harnessed and prove to be a setback
Geothermal energy is the energy from nuclear energy changes deep in the earth, which produces hot dry rock.
Geothermal energy originates from the heat retained within the Earth since the original formation of the planet, from radioactive decay of minerals, and from solar energy absorbed at the surface.
Harnessing Geothermal Energy
Most high temperature geothermal heat is harvested in regions close to tectonic plate boundaries where volcanic activity rises close to the surface of the Earth. In these areas, ground and groundwater can be found with temperatures higher than the target temperature of the application.
Geothermal energy is extracted by using two methods:
(i) A heat pump system
(ii) Hot dry rock conversion
The heat pump system
Hot aquifers are layers of permeable (porous) rock such as sandstone or limestone at a depth of 2 – 3 km which contains hot water at temperatures of 60 – 1000C.
A shaft is drilled to aquifer and the hot water pumped up it to the surface where it is used for district space and water heating schemes or to generate electricity. A second shaft may be drilled to return the cool water to the rock.
The hot dry rock conversion
These are impermeable hot dry rocks found at depth of 5 – 6 km, have temperature of 2000C or more.
Two shafts are drilled and terminate at different levels in the hot rock about 300 m apart. The rocks near the end are fractured by explosion or by methods to reduce the resistance
to the flow of cold water which is pumped under very high pressure (300 atm) down the injection shaft and emerges as steam from the top of the shallower shaft.
Uses of geothermal energy
Geothermal energy can be used for electricity production, for direct use purposes, and for home heating efficiency (through geothermal heat pumps).
Advantages of geothermal energy
1. Geothermal power plants provide steady and predictable base load power.
2. New geothermal power plants currently generate electricity at low cost.
3. Responsibly managed geothermal resources can deliver energy and provide power for decades.
4. Geothermal power plants are reliable, capable of operating about 98 percent of the time.
5. Power plants are small, require no fuel purchase and are compatible with agricultural land uses.
6. Geothermal plants produce a small amount of pollutant emissions compared to traditional fossil fuel power plants.
Disadvantages of geothermal energy
1. Many of the best potential resources are located in remote or rural areas, often of federal or state lands
2. Although costs have decreased in recent years, exploration and drilling for power production remain expensive
3. Using the best geothermal resources for electricity production may require an expansion or upgrade of the transmission system.
4. The productivity of geothermal wells may decline over time. As a result, it is crucial that
developers manage the geothermal resources efficiently.
Wave energy is the energy extracted from the ocean surface wave. Energy that comes from the waves in the ocean sounds like a boundless, harmless supply.
Machinery able to exploit wave power is generally known as a wave energy converter (WEC)
Waves in the sea have kinetic energy and gravitational potential energy as the rise and fall.
Consider a sine wave of wave length λ spread over a width d the amplitude of the wave is a and the time period is T.
The power in a wave come from the change in potential energy of the water as it rotates on the circuit paths beneath the surface. It can be shown that the power carried forward by a wave is given by:
Wave Energy Flux
The mean transport rate of the wave energy through a vertical plane of unit width , parallel to a wave crest, is called wave energy flux.
Harvesting wave energy
There are two type of system:
1. i. Offshore systems in deep water more than 141 feet deep. The Salter duck method.
(a) Pumps that use bobbing motion of waves.
(b) Hoses connected to floats on surface of waves. As float rises and falls , the hose stretches and relaxes, pressurizing the water which then rotates a turbine
2. ii. Onshore systems are built along shorelines and harvest energy from braking waves.
(a)Oscillating water columns are of concrete or steel and have an opening to the sea below the waterline. It uses the water to pressurize an air column that is drawn through the turbine as waves recede.
(b)A Tapchan is a tapered water system in sea cliffs that forces waves through narrow channels and the water that spills over the walls is fed through a turbine.
(c)A Pendulor device is a rectangular box with a hinged flap over one side that is open to the sea .Waves cause the flap to swing back and forth and this powers a hydraulic pump and generator.
Advantages of Wave energy
1. Renewable: It will never run out.
2. Environment friendly: Creating power from waves creates no harmful byproducts such as gas, waste, and pollution.
3. Abundant and widely available: Another benefit to using this energy is its nearest to places that can use it.
4. Variety of ways to harness: Current gathering method range from installed power plant with hydro turbine to seafaring vessels equipped with massive structures that are laid into the sea to gather the wave energy.
5. Easily predictable: The biggest advantage of wave power as against most of the other alternative energy source is that it is easily predictable and can be used to calculate the amount that it can produce.
6. Less dependency on foreign oil cost.
7. Non damage to land.
Disadvantages of wave energy
1. Suitable to certain locations: The biggest disadvantage to getting your energy from the wave is location. Only power plants and town near the ocean will benefit direct from it.
2. Effect on marine ecosystem: Large machine have to be put near and in the water gather energy from waves .These machines disturb the seafloor, changes the habitat of near-shore creatures (like crabs and starfish) and create noise that disturb the sea life around them.
3. Wavelength: Wave power is highly dependent on wavelength i.e. wave speed, wave length, and wavelength and water density.
4. Weak performance in Rough Weather: The performance of wave power drops significantly during rough weather.
5. Noise and Visual pollution: Wave energy generators may be unpleasant for some who live close to coastal regions. They look like large machines working in the middle of the ocean and destroy the beauty of the ocean. They also generate noise pollution but the noise is often covered by the noise of waves which is much more than that of wave generators.
6. Difficult to convert wave motion into electricity efficiently.
7. Difficult to design equipment that can withstand storm damage and saltwater corrosion.
8. Total cost of electricity is not competitive with other energy sources.
9. Pollution from hydraulic fluids and oils from electrical components.
Tidal Power is the power of electricity generation achieved by capturing the energy contained in moving water mass due to tides.
Two types of tidal energy can be extracted: Kinetic energy of currents between ebbing and surging tides and potential energy from the difference in height between high and low tides.
Causes of Tides
Tides are caused by the gravitational pull of the moon, and to a lesser extent the sun, on the oceans. There is a high tide places near the moon and also opposite on the far side.
i. High (spring) tide: Occurs when there is full moon. The moon, sun and earth are in line the moon being between earth and sun. The pulls of the moon and sun reinforce to have extra high tides.
ii. Lowest (neap) tide: Occurs when there is half moon and the sun and moon pulls are at right angles to each other.
iii. Harnessing Tidal Energy
Tidal energy can be harnessed by building a barrage (barrier), containing water turbines and sluice gates, across the mouth of river. Large gates are opened during the incoming (flood) tide, allowing the water to pass until high tides, when they are closed.
On the outgoing tide, when a sufficient head of water has built up, small gates are opened, letting the potential energy of the trapped water drive the turbines and generate electricity.
Advantages of Tidal Energy
1. Decrease reliance on coal driven electricity so less CO2 emissions.
2. Changing technology allowing quicker construction of turbines, which in turn increases likelihood of investment with a shorter return.
3. Once constructed very little cost to run and maintain.
4. Tidal energy is renewable and sustainable.
Disadvantages of Tidal Energy
1. Intermittent energy production based around tides creates unreliable energy source.
2. High construction costs
3. Barrages can disrupt natural migratory routes for marine animals.
4. Barrages can disrupt normal boating pathways.
5. Turbines can kill up to 15% of fish in area, although technology has advanced to the point that the turbines are moving slow enough not to kill as many.
If the tidal height (level) is h and the estuary area is A, then the mass of water trapped being the barrier is and the centre of gravity is h/2 above the low tide level.
The maximum energy per tide is therefore
Potential Energy of tide =
Averaged over a tidal period of T (approx. 12 hours a day), this gives a mean power available of.
Average tidal power =
Note that the efficiency of the turbines (generator) will determine how much of this tidal power will be harnessed.
EXAMPLES: SET B
The power output p of a windmill can be expressed as where A is the area swept out by the windmill blades (sails), is the density of air, v is the wind speed and k is a dimensionless constant
(a) Show that the units on both sides of this expression are the same
(b) Sketch a graph to show how the power increases with wind speed as v rises from zero to 15ms-1
(a) Units on L.H.S = Nms-1
Unit on R.H.S. = m2 (kgm-3) x (ms-1)3
= (kgms-2) ms-1=Nms-1
(b) Variation of power with speed
The radiation received from the sun at the earth’s surface in a certain country is about 600 Wm-2 averaged over 8 hours in the absence of cloud.
(a) What area of solar panel would be needed to replace a power station of 2.0 GW output, if the solar panels used could convert solar radiation to electrical energy at an efficiency of 20%
(b) What percentage is this area of the total of the country (which is about 3 x 1011m2)?
(c) If the total power station capacity is about 140 GW, what percentage of the surface of the country would be covered by solar panels if all the power stations were replaced?
(a) Output of a solar panel
(b) Percentage area to the country
(c) Area of solar panels required
Percentage area to the country
(a) What are aerogenerators?
(b) Estimate the maximum power available from 10m2 of solar panels and calculate the volume of water per second which must pass through if the inlet and outlet temperatures are 200C and 700C. Assume the water carries away energy at the same rate as the maximum power available. The specific heat capacity of water is 4200 Jkg-1 and solar constant is 1.4 kWm-2.
(a) Aerogenerators are devices that convert the kinetic energy of wind into electrical energy. E.g. windmill.
(b) Maximum power available from solar panel
Volume of water per second used is given by
A coal – fired power station has an output of 100mW. Given that its efficiency is 45%, how much coal must be supplied each day? Assume 1 tonne of coal gives 3 x 1010 of energy.
Input power of the station is given by
Total input energy in a day is
The amount of coal required is
Calculate the energy required transport1000 tones of oil along a 100km pipeline; given that 0.05 kW hours of energy is used to shift each tone of oil along each km of pipeline. Given that 1 tonne of oil releases 4.2 x 1010 J if burned, what percentage of the total energy available from 1000 tonnes of oil is used to shift the oil along the pipeline? (Ans: 18GJ, 0.043%)
A hydroelectric power station has efficiency of 25%. The water driving the turbines falls through a height of 300m before reaching the turbines. Calculate the volume of water that must pass through the turbines each second to give a power output of 2mW. Assume the density of water is 1000kg-3.
Power of the falling water
The solar energy flux near the Earth is 1.4W m-2. A solar power station consists of concave mirrors that focus sunlight onto a steam boiler. What must be the minimum mirror area to given an output 1 mW, assuming 100% efficiency? Why in practice, should the mirror area be greater?
Minimum mirror area is given by
The mirror area should be greater to achieve such a power output because part of the incident energy is absorbed by the mirror.
A solar panel attached to the roof of a house is used to heat water from 50C to 400C. If the water flows through the panel at a rate of 0.012kgs-1 Calculate the heat gained per second by the water. Assume the specific heat capacity of water is 4200Jkg-1K-1. (Ans. 1764 Q)
An aerogenerator has a power output that is proportional to (wind speed) 2 and its efficiency varies with wind speed. On a day when there is a steady wind of speed 9 ms-1, the power output is 40kW operating at an efficiency of 20%. If the wind speed on next day is 13.5 ms-1 and the efficiency increases to 25% what is the new power output?
Power output α efficiency × (wind speed)2
Estimate the energy released from a tidal power station if 100 km3 of water raised to height of 1.5m by the tide behind a tidal barrier. What would be the mean power output of such a station if its efficiency is 25% and there are two tides per day?
The tidal power is given by
Note that the centre of gravity of water mass is at the half height up.
Mean power output is
An open boat of width 1.0 m has a total weight of 3000N.Used near a beach, it bobs up and down through 0.5 m once every 5s. Calculate the losses of P.E. every time it drops from a crest to a through. Hence estimate the mean power available per meter of beach waves.
Loss in P.E. is given by
The mean power available per meter is 300 W
(a) If energy is conserved, why is there energy crisis?
(b) Explain the terms high grade and low grade energy and give examples.
(c) Draw an energy flow diagram for a hydroelectric power station. Why does such a station have a much greater efficiency than a thermal power station?
Refer Advanced Physics by Tom Duncan fifth edition for more problems on energy.
Geophysics is the branch of physics which deals with the study of seismic waves and the Earth’s magnetic and gravity fields and heat flow.
Because we cannot directly observe the Earth’s interior, geophysical methods allow us to investigate the interior of the Earth by making measurements at the surface. Without studying these things, we would know nothing of the Earth’s internal structure.
STRUCTURE OF THE EARTH
Major zones of the earth
The earth is divided into two major zones, namely;
(a) Outer zone, and
(b) Inner zone.
a) Outer zone: the earth’s outer zone consists of;
(i) The hydrosphere – water bodies,
(ii) The atmosphere – gaseous envelope
(iii) The biosphere – living organisms, plant and animals
b) Inner zone: the earth’s inner zone consists of;
(i) The crust – lithosphere
(ii) The mantle – mesosphere,
(iii) The core – barysphere
Atmosphere is the envelope of gases that surround the Earth (oxygen, nitrogen, carbon dioxide, etc)
Hydrosphere is the water bodies filling the depressions in the Earth. Examples are rivers, oceans, seas, oasis,
Lithosphere is the solid outer most part of the earth.
Layers defined by composition
Layers are defined by composition because of density sorting during an early period of partial melting, Earth’s interiors not homogeneous.
Crust – the comparatively thin outer skin that ranges from 3 kilometers at the oceanic ridges to 70 kilometers in some mountain belts. It makes up 1% of the Earth’s volume.
Continental crust (SIAL, Silicon and aluminium)
Average rock density about 2.7 g/cm3
Its density varies between 2.0 to 2.8 g/cm3
Composed of silicon and aluminium
Floats higher on the mantle forming the land masses and mountains. It is 30 to 70 km thick.
Oceanic crust (SIMA), silicon and magnesium)
Oceanic crust ranges from 3 to 15 km thick
Density vary between 3.0 to 3.1 g/cm3
Floats lower on the mantle forming the oceanic basins. It is about 8 km thick.
Mantle – a solid rocky (silica-rich) shell that extends to a depth of about 2900 kilometers. It makes up 83% of the Earth’s volume
The mantle can further be dived into:
(i) Upper layer of mantle (Asthenosphere)
(ii) Transition layer and,
(iii) Lower layer of mantle (Mesosphere)
Upper mantle is a rigid layer of rock with average density 3.3kgm-3
Transition layer is the layer that separates upper and lower mantle.
Lower mantle plays an important role in tectonic plate movement which creates earthquakes and volcanoes.
Its density is about 5.7 kgm-3
The mantle rocks are said to be in a plastic state.
The upper part of a mantle has a temperature of about 8700C. The temperature increases downwards through the mantle to about 22000C near the core.
Core – an iron – rich sphere having a radius of 3486 kilometers making up 16% of the Earth’s volume
The core is divided into two parts:
(i) Outer core
(ii) Inner core
i. Outer core is a liquid of molten iron and nickel alloys. The Earth’s magnetic field is generated within the outer core due to convective. It is 2270 kilometers thick.
ii. Inner core is a solid iron and nickel alloys. The temperature within the inner core is higher than the outer core but the inner core is solid, this is because higher pressure in this region causes the melting point to rise. It is a sphere of radius of 1216 kilometers.
Average density is nearly 11 gcm-3and at Earth’s center.
Layers defined by physical properties
Lithosphere (sphere of rock)
Earth’s outermost layer
Consists of the crust and uppermost mantle
Relatively cook, rigid shell
Averages about 100 kilometers in thickness, but may be 250 kilometers or more thick beneath the older portions of the continents
Asthenosphere (weak sphere partially molten)
Beneath the lithosphere, in the upper mantle to a depth of about 660 kilometers
Small amount of melting in the upper portion mechanically detaches the lithosphere from the layer below allowing the lithosphere to move independently of the asthenosphere i.e. allows tectonic plate movement.
Mesosphere or lower mantle
Rigid layer between the depths of 660 kilometers and 2900 kilometers
Earth’s major boundaries
Discontinuity is the name given to any surface that separates one layer from another layer of the Earth.
The Moho (Mohorovicic discontinuity)
Discovered in 1909 by Andriaja Mohorovicic
Separates crustal materials (crust) from underlying mantle.
Discovered in 1914 by Beno Gutenberg
Is the boundary between the outer and inner core.
The Earth’s Structure
TEMPERATURE INSIDE THE EARTH
Earth’s temperature gradually increases with an increase in depth at a rate known as the geothermal gradient.
Temperature varies considerably from place to place
Averages between about 200C and 300C per kilometer in the crust (rate of increase is much less in the mantle and core)
The rate of heat flow within the Earth depends on:
(i) The thermal conductivity of the rock,
(ii) Temperature gradient of the rock
Sources of heat Energy within the Interior of the Earth
Major processes that have contributed to Earth’s internal heat include:
1. Heat emitted by radioactive decay of isotopes of uranium (U), thorium (Th), and potassium (K).
2. Heat released as iron crystallized to form the solid inner core.
3. Heat released by colliding particles during the formation of Earth.
4. Gravitational work done by the Earth due to its rotation through its own axis.
5. Electron motion in the core behaves like an electric current.
Heat Lost by the Earth
Heat in the earth is transferred by the process of;
(i) Convection and
In the solid inner core and in the Earth’s crust heat is transmitted by conduction process. Rates of heat flow in the crust vary.
In the Mantle heat is transmitted by conduction process. Rates of heat flow in the crust vary.
In the Mantle heat is transmitted by convection process. There is not a large change in temperature with depth in the mantle.
Mantle must have an effective method of transmitting heat from the core outward.
Transfer of heat in the Earth by mantle convection
Uses of the Mantle
1. The mantle transfers heat by convection from the earth’s crust to the out regions of the earth and thus help it to regulate its temperature
2. The upper part of the mantle is molten, this allows tectonic plates movements.
An earthquake is a sudden motion or shaking of the earth caused by a sudden release of energy that has accumulated within or along edges of the earth’s tectonic plates.
Earthquakes occur within the Earth’s crust along faults that suddenly release large amounts of energy that have built up over long periods of time.
The shaking during an earthquake is caused by seismic waves.
Seismic waves are propagating vibrations that carry energy from the source of the shaking (earthquake) outward in all directions.
Seismic waves are generated when rock within the crust breaks, producing a tremendous amount of energy. The energy released moves out in all directions as waves, much like ripples radiating outward when you drop a pebble in a pond.
CAUSES OF EARTHQUAKES (SEISMIC WAVES)
The main causes of the Earthquakes and so seismic waves are:
1. Movement of tectonic plate.
2. Volcanic activity.
3. Landslide and avalanches.
4. Rebound of the crust.
5. Human activities.
Movement of tectonic plate
The Earth’s crust is made up of segment (layers) called tectonic plates which are slowly drifting in various directions. Tectonic plates may create a fault.
A boundary is a line where two tectonic plates meet.
A geologic fault is a fracture in the earth’s crust causing loss of cohesion and accompanied by displacement along the fracture.
How an earthquake is formed
Tectonic plates grind past each other, rather than slide past each other smoothly. As the plates move past each other they can become locked together due to friction. For some time, they don’t move and strain energy builds up. Stresses builds between them until fractional force holding the plates together give away. The plates move suddenly, releasing the energy and then held again. This sudden jerk is what is felt as an earthquake.
(a) The Earth’s crusts near tectonic plate edges are forced to bend, compress, and stretch due to the internal forces within the earth, causing earthquakes.
(b) Nearly all earthquakes occur at plate boundaries.
Molten rock “magma” from the mantle is forced through a weak point in the Earth’s crust creating a volcanic eruption. When magma reaches the Earth’s surface it is known as “Lava”. Successive eruptions leads to the buildup of lava on the sides of the vent creating the familiar “cone – shape” of a volcanoes
Earthquakes may be created by the violent explosions which occur if there are sudden movements of the magma.
Landslides and avalanches
A landslide occurs when a large mass of land slips down a slope. An Avalanche occurs when a large mass of snow pours down a mountain side. Both of these effects can start an earthquake
Rebound of the crust
Elastic rebound theory state that “as tectonic plates move relative to each other, elastic strain energy builds up along their edges in the rocks along fault planes”. Since fault planes are not usually very smooth, great amount of energy can be stored (if the rock is strong enough) as movement is restricted due to interlock along the fault. When the shearing stresses induced in the rocks on the fault planes exceed the shear strength of the rock, rupture occurs.
It follows from this that if rocks along the fault are of a certain strength, the fault is a certain length, and the plates are slipping past each other at a defined rate, it is possible to calculate the amount of time it will take to build up enough elastic strain energy to cause an earthquake and its probable magnitude.
When a fault breaks it release elastic strain energy it stored, and hence earthquake.
Human activities such as those caused by nuclear bombs can create earthquake, together with mine actives.
Energy released by an earthquake moves outwards from the origin in the form of concentric waves.
Focus (Hypocenter) is the point in the Earth where seismic waves originate.
Epicenter is the point on the earth’s surface vertically above the focus.
Hypocentral distance is the distance between the focus and the seismic detection station.
Epicentral distance is the distance between the epicentral and the seismic station.
S = Seismic station
E = Epicenter
ES = Epicentral distance
TYPE OF SEISMIC WAVES
i. Seismic waves are elastic waves that propagate within the earth.
There are two type of seismic waves:
1. ii. Body waves, spread outward from the focus in all directions.
2. iii. Surface waves (Long, L – waves) spread outward from the epicenter to the Earth’s surface along the crust, similar to ripples on a pond. These waves can move rock particles in a rolling motion that very few structures can withstand. These waves move slower than body waves.
There are two types of Body Waves
(1) Primary P – wave and
(2) Secondary, S – waves
1. 1. Primary Wave (P – wave): Are longitudinal (compression) wave (travels in the same direction the waves move)
Characteristics of P – waves
1. Are the fastest seismic waves (7 – 14 km/second). Arrives at recording station first, hence the name primary means first.
2. Can pass through solid, gas and liquid, hence can pass through crust, mantle and the cores.
3. Are longitudinal compression waves. The rocks that transmit the P – waves are alternately compressed and expanded.
Velocity of P – waves
The velocity of primary waves depends on the density,bulk modulus B and the shear modulus
In solid, =
In liquid =
A fluid cannot support shear stresses hence
2. Secondary Wave (S – wave): Are transverse (shear) wave (travels perpendicular to the wave movement).
Characteristics of S – waves
1. i. Slower moving (3.5 – 7 km/second) hence are detected after primary waves.
2. ii. Caused by a shearing motion
3. iii. Cannot pass through a fluid (gas or liquid) because they are transverse. Hence are unable to pass through the liquid outer core.
Velocity of S – waves
The velocity of shear waves depends on the density and the shear modulus
In solid, =
In liquid =
Note: Since the density and states of the earth layers varies, the speed of the seismic waves also vary from layer to layer, the solid part showing greater speed and the liquid ones lower speed.
Primary wave and secondary wave
Variation of speed of body waves with depth
SURFACE WAVES/LONG WAVES
Surfaces waves are produced when earthquake energy reaches the Earth’s surface.
These are the slowest moving waves, but are the most destructive for structures on earth
There are two types of L – Waves:
(i) Love long waves
(ii) Rayleigh long waves
i. Love Waves
Love waves are Transverse horizontal motion, perpendicular to the direction of propagation and generally parallel to the Earth’s surface.
They are formed by the interaction of S waves with Earth’s surface and shallow structure and are dispersive waves. The speed at which a dispersive wave travels depends on the wave’s period.
Characteristics of Love Waves
1. i. Love waves are transverse and restricted to horizontal movement (horizontally polarized).
2. ii. The amplitude of ground vibration caused by a Love wave decrease with depth. The rate of amplitude decrease with depth also depends on the period/frequency.
3. iii. Loves wave are dispersive, i.e. wave velocity is dependent on frequency; low frequency – higher velocity.
4. iv. Speed of love waves is between 2.0 and 4.4 km/s
5. v. Love waves travels within the earth’s crust only.
Rayleigh waves are vertically polarized long waves. The slowest of all the seismic wave types and in some ways the most complicated.
Characteristics of Rayleigh Waves
1. Rayleigh waves are transverse and restricted to vertical movements (vertically polarized).
2. The amplitude of Rayleigh wave decreases with depth. The rate of amplitude decrease with depth depends on the period/frequency
3. Rayleigh wave are dispersive, i.e. wave velocity dependent on frequency; low frequency – high velocity
4. Speed of love waves is between 1.0 and 4.2 km/s slowest of all waves.
5. Travels within the earth’s crust only.
6. Depth of penetration of the Rayleigh waves depend frequency, with lower frequencies, penetrating greater depth.