Some basic concepts of energySome basic concepts of energy(A)(A) Conservation of energyConservation of energy

Energy cannot be created nor destroyed. It can be tranEnergy cannot be created nor destroyed. It can be transformed from one form to another, but the total amousformed from one form to another, but the total amount of energy remains unchanged.nt of energy remains unchanged.ExamplesExamples

A light bulb changes electrical energy into light energA light bulb changes electrical energy into light energy and heat.y and heat.

A motor changes electrical energy into kinetic energy.A motor changes electrical energy into kinetic energy. An electric generator changes mechanical energy into An electric generator changes mechanical energy into

electrical energy. electrical energy.

(B)(B) UnitsUnits In physics, all forms of energy are measured in jouIn physics, all forms of energy are measured in jou

les (J). les (J). However, kilowatt-hour (kWh) and British thermaHowever, kilowatt-hour (kWh) and British therma

l unit (Btu) are also units of energy.l unit (Btu) are also units of energy.

1 kWh = 3.6 x 106 J or 3.6 M J

1 Btu = 1.1 x 103 J or 1.1 k J

(C)(C) DegradationDegradation Some forms of energy are more useful than others beSome forms of energy are more useful than others be

cause they are more suitable for doing work and beincause they are more suitable for doing work and being transformed into other forms of energy. g transformed into other forms of energy.

Electrical and chemical energy are in this category anElectrical and chemical energy are in this category and are called high-grade energy. Internal energy is lod are called high-grade energy. Internal energy is low-grade energy that is not easily transformed into anyw-grade energy that is not easily transformed into anything else.thing else.

By the law of conservation of energy, energy can be tBy the law of conservation of energy, energy can be transformed from one form to another. Eventually, all ransformed from one form to another. Eventually, all energy will be transformed to the low-grade internal eenergy will be transformed to the low-grade internal energy, such as work done against friction. This procesnergy, such as work done against friction. This process is called degradation of energy.s is called degradation of energy.

(D)(D) Energy densityEnergy density In an energy source such as Uranium fuel and In an energy source such as Uranium fuel and

coal, which has a high energy density, the enercoal, which has a high energy density, the energy is concentrated and the converter need to regy is concentrated and the converter need to release it (i.e. nuclear reactor and a furnace) is release it (i.e. nuclear reactor and a furnace) is relatively small. latively small.

On the contrary, the energy density of solar enOn the contrary, the energy density of solar energy is low and as a result a solar converter muergy is low and as a result a solar converter must be larger for the same power output.st be larger for the same power output.

Non-renewable sources and RenewaNon-renewable sources and Renewable sources ble sources

Non-renewable energy refers to energy that Non-renewable energy refers to energy that cannot be renewed or can only be recreated in cannot be renewed or can only be recreated in nature in a period of time much longer than nature in a period of time much longer than that human beings consume the energy.that human beings consume the energy.

ExamplesExamples Coal, petroleum oil, natural gas and other Coal, petroleum oil, natural gas and other

fossil fuels.fossil fuels.

Nonrenewable energyNonrenewable energy CoalCoal is formed by dead plants covered by sand is formed by dead plants covered by sand

and mud. It contains mainly carbon.and mud. It contains mainly carbon.

OilOil is formed by dead marine plants and anima is formed by dead marine plants and animals covered by sand and mud. It contains mainly ls covered by sand and mud. It contains mainly hydrocarbons and is the largest energy source ihydrocarbons and is the largest energy source in the world. n the world.

Renewable energyRenewable energy Renewable energy can be regenerated or Renewable energy can be regenerated or

recycled through natural processes. While this recycled through natural processes. While this energy must be finite, the supply is so large energy must be finite, the supply is so large that there is no foreseeable end in our or our that there is no foreseeable end in our or our children’s lifetimes. children’s lifetimes.

ExamplesExamples Solar energy, wind energy, hydroelectric Solar energy, wind energy, hydroelectric

energy, tidal energy, geothermal energy, energy, tidal energy, geothermal energy, biomass energy etc.biomass energy etc.

Solar energySolar energy The sun’s energy is produced by nuclear fusioThe sun’s energy is produced by nuclear fusio

n. n. It consists of electromagnetic radiation ranging It consists of electromagnetic radiation ranging

from short-wavelength X-rays to long-wavelenfrom short-wavelength X-rays to long-wavelength radio wavesgth radio waves

99% of the energy is in the form of light, infrar99% of the energy is in the form of light, infrared and ultraviolet radiation.ed and ultraviolet radiation.

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

direct solar heating

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

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

Wind energyWind energy

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

Advantages:Advantages: Wind energy is unlimited and does not cause any poWind energy is unlimited and does not cause any po

llution problems. llution problems. Building and running windmills are relatively cheap. Building and running windmills are relatively cheap. Disadvantages:Disadvantages: A large number of windmills are required to producA large number of windmills are required to produc

e appreciable amounts of energy. e appreciable amounts of energy. The supply of wind energy is not steady since wind The supply of wind energy is not steady since wind

may be weak in some days.may be weak in some days.

Hydroelectric power Hydroelectric power

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

Hydroelectric power Hydroelectric power

Advantages: Advantages: Hydroelectric energy is unlimited. Hydroelectric energy is unlimited.

Disadvantages:Disadvantages:The hydroelectric power stations are only suitable to bThe hydroelectric power stations are only suitable to be built in mountainous regions having high rainfall. e built in mountainous regions having high rainfall.

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

In order for tidal energy to work well, at least a 16-feIn order for tidal energy to work well, at least a 16-feet difference between low tide and high tide is needed.et difference between low tide and high tide is needed. There are only a few places where this tide change oc There are only a few places where this tide change occurs around the earth. curs around the earth.

One plant in France makes enough energy from tides One plant in France makes enough energy from tides (240 megawatts) to power 240,000 homes. (240 megawatts) to power 240,000 homes.

However, the aquatic life may be affected by interferiHowever, the aquatic life may be affected by interfering the rise and fall of tides.ng the rise and fall of tides.

Geothermal energyGeothermal energy

Geothermal energy is heat enGeothermal energy is heat energy that comes from the molergy that comes from the molten interior of the earth. Therten interior of the earth. There are places where this moltee are places where this molten material is close enough to n material is close enough to the surface. the surface.

Geothermal energyGeothermal energy A long tunnel is drilled intA long tunnel is drilled int

o the ground to reach the o the ground to reach the hot rock. Cold water is puhot rock. Cold water is pumped into the tunnel and imped into the tunnel and is heated up under the grous heated up under the ground. The heated water or stnd. The heated water or steam will emerge from the eam will emerge from the tunnel to generate electricitunnel to generate electricity by turning a turbine. ty by turning a turbine.

To many people, the most familiar forms of reTo many people, the most familiar forms of renewable energy are the wind and the sun. newable energy are the wind and the sun.

But biomass (plant material and animal waste) But biomass (plant material and animal waste) supplies almost 15 times as much energy in the supplies almost 15 times as much energy in the United States as wind and solar power combinUnited States as wind and solar power combined and has the potential to supply much moreed and has the potential to supply much more

BiofuelsBiofuels

BiofuelsBiofuels

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

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

Alternative energy sources (Nuclear Alternative energy sources (Nuclear energy) energy)

Nuclear energy from Nuclear energy from controlled nuclear fissioncontrolled nuclear fission can be can be used to generate electricity. used to generate electricity.

Advantages:Advantages: 1.1. Unlike coal- or oil-fired power stations, nuclear Unlike coal- or oil-fired power stations, nuclear

fuel is small in size and so there is no fuel fuel is small in size and so there is no fuel transportation problem. transportation problem.

2.2. It is cheaper than coal or oil for generating It is cheaper than coal or oil for generating electricity. electricity.

3.3. It is clean and causes little environmental It is clean and causes little environmental pollution while coal- and oil- fired power stations pollution while coal- and oil- fired power stations emit large amount of fly-ash.emit large amount of fly-ash.

DisadvantagesDisadvantages 1.1. However, nuclear energy constitutes an However, nuclear energy constitutes an

unacceptable hazard to the public. unacceptable hazard to the public. 2.2. Although the chance of an accident happening Although the chance of an accident happening

is very small, the consequence is extremely serious. is very small, the consequence is extremely serious. 3.3. Large sums of money have to spend on Large sums of money have to spend on

maintaining and upgrading the safety standards of the maintaining and upgrading the safety standards of the reactor. reactor.

4.4. Besides, the wide spread use of nuclear energy Besides, the wide spread use of nuclear energy will lead to the growth of nuclear weapons.will lead to the growth of nuclear weapons.

Efficiency of energy transformEfficiency of energy transform Consider a light bulb which changes electrical energy Consider a light bulb which changes electrical energy

into light energy.into light energy.Electrical energy (100 J) ⇒Electrical energy (100 J) ⇒ Light energy (30 J)Light energy (30 J)+ + Heat (70 J)Heat (70 J)

Efficiency Efficiency = (useful energy output)/(total energy input) = (useful energy output)/(total energy input) xx 100% 100%

Efficiency Efficiency = (useful energy output)/(total energy input) = (useful energy output)/(total energy input) xx 100% 100%

Efficiency of the light bulb = 30 / 100 Efficiency of the light bulb = 30 / 100 xx 100% 100%

Note:Note:

1.1. The efficiency of an ideal transformer is The efficiency of an ideal transformer is 100% while that of a practical one is about 100% while that of a practical one is about 90%.90%.

2.2. In a real case, the efficiency must be In a real case, the efficiency must be smaller than 100% as some energy is lost, such smaller than 100% as some energy is lost, such as work as work done against friction.done against friction.

DeviceDevice EfficiencyEfficiency Energy transferEnergy transfer

Large electric motorLarge electric motor 90 %90 % Electrical to mechanicalElectrical to mechanical

Large electric Large electric generatorgenerator 90 %90 % Mechanical to electricalMechanical to electrical

Domestic gas boilerDomestic gas boiler 75 %75 % Chemical to internal Chemical to internal energyenergy

Steam turbineSteam turbine 45 %45 % Internal energy to Internal energy to mechanicalmechanical

Car engineCar engine 25 %25 % Chemical to mechanicalChemical to mechanical

Example 1Example 1 Calculate the efficiency of a 1250 W electrical kettle if it takes 15 Calculate the efficiency of a 1250 W electrical kettle if it takes 15 minutes to boil 2.2 kg of water from 20 minutes to boil 2.2 kg of water from 20 ooC to 100 C to 100 ooC. It is given tC. It is given that the specific heat capacity of water is 4200 J kghat the specific heat capacity of water is 4200 J kg-1 o-1 oCC-1-1..Explain why the efficiency is not equal to 100 %.Explain why the efficiency is not equal to 100 %.

Solution:Solution: Energy input (electrical energy) = PtEnergy input (electrical energy) = Pt

=1250 =1250 xx 15 15 xx 60 = 1125000 J 60 = 1125000 J Energy output (heat) = mcEnergy output (heat) = mcT T

= 2.2 = 2.2 xx 4200 4200 xx 80 = 739200 J 80 = 739200 J Efficiency of the kettleEfficiency of the kettle

= 739200 / 1125000 = 739200 / 1125000 xx 100% = 65.7 % 100% = 65.7 %