Some basic concepts of energy

1. Some basic concepts of energy (A) Conservation of energy Energy cannot be created nor destroyed. It can be transformed from one form to another, but the total amount of energy remains unchanged. Examples • A light bulb changes electrical energy into light energy and heat. • A motor changes electrical energy into kinetic energy. • An electric generator changes mechanical energy into electrical energy.

2. (B) Units • In physics, all forms of energy are measured in joules (J). • However, kilowatt-hour (kWh) and British thermal unit (Btu) are also units of energy.

3. (C) Degradation • Some forms of energy are more useful than others because they are more suitable for doing work and being transformed into other forms of energy. • Electrical and chemical energy are in this category and are called high-grade energy. Internal energy is low-grade energy that is not easily transformed into anything else. • By the law of conservation of energy, energy can be transformed from one form to another. Eventually, all energy will be transformed to the low-grade internal energy, such as work done against friction. This process is called degradation of energy.

4. (D) Energy density • In an energy source such as Uranium fuel and coal, which has a high energy density, the energy is concentrated and the converter need to release it (i.e. nuclear reactor and a furnace) is relatively small. • On the contrary, the energy density of solar energy is low and as a result a solar converter must be larger for the same power output.

5. Non-renewable sources and Renewable sources • Non-renewable energy refers to energy that cannot be renewed or can only be recreated in nature in a period of time much longer than that human beings consume the energy. • Examples • Coal, petroleum oil, natural gas and other fossil fuels.

6. Nonrenewable energy • Coal is formed by dead plants covered by sand and mud. It contains mainly carbon.

7. Oil is formed by dead marine plants and animals covered by sand and mud. It contains mainly hydrocarbons and is the largest energy source in the world.

8. Renewable energy • Renewable energy can be regenerated or recycled through natural processes. While this energy must be finite, the supply is so large that there is no foreseeable end in our or our children’s lifetimes. • Examples • Solar energy, wind energy, hydroelectric energy, tidal energy, geothermal energy, biomass energy etc.

9. Solar energy • The sun’s energy is produced by nuclear fusion. • It consists of electromagnetic radiation ranging from short-wavelength X-rays to long-wavelength radio waves • 99% of the energy is in the form of light, infrared and ultraviolet radiation.

10. direct solar heating • There is direct solar heating in which the light energy is allowed to heat a house through transparent windows or heat a mass of water which can then be circulated to heat the home.

11. There are photovoltaics or solar cells where the light energy is transferred into electrical energy by the use of solar panels. For example, these panels can be attached to the roof of a house. Also many satellites use solar panels for their power needs.

12. Finally there is solar power station where the sun light is reflected and focused using a huge parabolic mirror. This reflected light energy can then be used to heat water to create steam which can be used to drive a turbine to produce electricity just like in a conventional power plant.

13. Wind energy • Wind energy is the kinetic energy contained in the movement of a mass of air molecules. The wind can be used to turn a turbine, which can then produce electricity using a generator.

14. Advantages: • Wind energy is unlimited and does not cause any pollution problems. • Building and running windmills are relatively cheap. Disadvantages: • A large number of windmills are required to produce appreciable amounts of energy. • The supply of wind energy is not steady since wind may be weak in some days.

15. Hydroelectric power • Potential energy of water in a high-level reservoir becomes kinetic energy when it flows from a dam. The flowing water turns a turbine at the bottom of a dam to generate electricity.

16. Hydroelectric power • Advantages: Hydroelectric energy is unlimited. • Disadvantages: The hydroelectric power stations are only suitable to be built in mountainous regions having high rainfall.

17. Tides rise and fall each day. When tides come into the shore, they can be trapped in reservoirs behind dams. When the tide drops, the water behind the dam can be let out just like in a regular hydroelectric power plant.

18. In order for tidal energy to work well, at least a 16-feet difference between low tide and high tide is needed. There are only a few places where this tide change occurs around the earth. • One plant in France makes enough energy from tides (240 megawatts) to power 240,000 homes. • However, the aquatic life may be affected by interfering the rise and fall of tides.

19. Geothermal energy • Geothermal energy is heat energy that comes from the molten interior of the earth. There are places where this molten material is close enough to the surface.

20. Geothermal energy • A long tunnel is drilled into the ground to reach the hot rock. Cold water is pumped into the tunnel and is heated up under the ground. The heated water or steam will emerge from the tunnel to generate electricity by turning a turbine.

21. Biofuels • To many people, the most familiar forms of renewable energy are the wind and the sun. • But biomass (plant materialand animal waste) supplies almost 15 times as much energy in the United States as wind and solar power combined and has the potential to supply much more

22. Biofuels • Biomass is a renewable energy source because the energy it contains comes from the sun. Through the process of photosynthesis, chlorophyll in plants captures the sun’s energy by converting carbon dioxide from the air and water from the ground into carbohydrates, complex compounds composed of carbon, hydrogen, and oxygen. When these carbohydrates are burned, they turn back into carbon dioxide and water and release the sun’s energy they contain.

23. Biomass energy can also generated by the decay of large masses of plant or animal material or waste which forms methane and other combustible gases. These gases contain chemical energy, which when burned can be used to generate electricity.

24. Alternative energy sources (Nuclear energy) • Nuclear energy from controlled nuclear fission can be used to generate electricity. Advantages: • 1. Unlike coal- or oil-fired power stations, nuclear fuel is small in size and so there is no fuel transportation problem. • 2. It is cheaper than coal or oil for generating electricity. • 3. It is clean and causes little environmental pollution while coal- and oil- fired power stations emit large amount of fly-ash.

25. Disadvantages • 1. However, nuclear energy constitutes an unacceptable hazard to the public. • 2. Although the chance of an accident happening is very small, the consequence is extremely serious. • 3. Large sums of money have to spend on maintaining and upgrading the safety standards of the reactor. • 4. Besides, the wide spread use of nuclear energy will lead to the growth of nuclear weapons.

27. Efficiency of energy transform • Consider a light bulb which changes electrical energy into light energy. Electrical energy (100 J) ⇒ Light energy (30 J) + Heat (70 J) • Efficiency = (useful energy output)/(total energy input) x 100% • Efficiency = (useful energy output)/(total energy input) x 100% • Efficiency of the light bulb = 30 / 100 x 100%

28. Note: • 1. The efficiency of an ideal transformer is 100% while that of a practical one is about 90%. • 2. In a real case, the efficiency must be smaller than 100% as some energy is lost, such as work done against friction.

30. Example 1Calculate the efficiency of a 1250 W electrical kettle if it takes 15 minutes to boil 2.2 kg of water from 20 oC to 100 oC. It is given that the specific heat capacity of water is 4200 J kg-1 oC-1.Explain why the efficiency is not equal to 100 %. • Solution: • Energy input (electrical energy) = Pt =1250 x 15 x 60 = 1125000 J • Energy output (heat) = mcDT = 2.2 x 4200 x 80 = 739200 J • Efficiency of the kettle = 739200 / 1125000 x 100% = 65.7 %