1.7 Energy, Work and Power
Chapter 1: General Physics
1.7.1 Energy
Energy is the capacity to do work.
• It is a property a body has that helps it move against a force.
• SI Unit: Joule (J).
• It is a scalar quantity.
Types of Energy:
Chemical Potential Energy – Fuels such as coals or petrol stores chemical energy. When they burn,
the stored energy is released as heat and light. Also stored in food which in turn transforms into kinetic
energy of muscles and heat in the body. Chemical energy in batteries transform to heat and electric
energy.
Kinetic Energy – A moving object has kinetic energy. The faster an object is the more its kinetic
energy.
Gravitational Potential Energy – Energy of an object due to its position. Object at height has gpe.
Light Energy – Very hot objects gives of a glow called light energy.
Thermal (Heat) Energy – Energy transfers from hot object to cold in the form of thermal energy.
Sound Energy – Vibrating objects produce sound energy transferred by sound waves.
Electrical Energy – An electric current is a good way of transferring energy from one place to another.
It carries electrical energy.
Nuclear Energy – Energy released when the nucleus of an atom splits/disintegrates or fuses.
Wind Energy – Wind energy is caused due to the thermal energy of the sun. Huge convectional current
set up in the atmosphere and wind occurs.
Strain Energy – If you stretch a rubber band, it becomes a store for strain energy. It is also called
elastic potential energy.
Energy Transfer:
Heating - Some objects are hotter than others. Energy is transferred from the hotter object to the cooler
one, and the difference in temperature between them decreases.
Mechanically - Energy can be transferred mechanically through the movement of the parts in
machines, and when the motion or position of an object changes. Sound waves and seismic waves
(formed during earthquakes) are mechanical waves that transfer energy through materials and from
place to place.
Electrically - Energy is transferred when an electrical circuit is complete. A simple circuit may consist
of a battery, lamp and wires. Internal energy stored in the battery is transferred to moving charged
particles in the wire.
Radiation - Visible light, infrared light, microwaves and radio waves are forms of radiation. They are
carried by waves (although unlike sound, these are not mechanical waves and can travel through empty
space). Electric lamps and burning fuels transfer visible and infrared light to the surroundings.
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Energy Conservation:
Energy can change from one form to another. The principle of energy conservasion states:
Energy cannot be created or destroyed but can change from one form to another.
The total amount of energy always stays the same.
Kinetic Energy:
Kinetic energy is the energy a body possesses due to its motion.
Any object which is moving has kinetic energy. The kinetic energy Ek of an object of mass m moving
with a speed v is defined as:
2
1
mass x velocity
2
1
K.E=
mv2
2
Kinetic Energy =
Potential Energy:
Gravitational potential energy is the energy which body possesses because of its position (height)
relative to the ground.
When an object with mass m near the Earth’s surface is raised through a height h, the change in
potential energy is given by:
Potential Energy = mass x gravitational field strength x height
PE = m g h
When an object with GPE starts to fall, its GPE is
transferred into KE.
The further the object falls, the less GPE it has and
the more KE it has.
When the object hits the ground, all of its GPE has
been transferred into KE.
Lost Potential Energy = Gained Kinetic
Energy
P.E = K.E
1
mgh=
mv 2
2
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1.7.2 Energy Resources
Major Sources of Energy
Renewable Sources
Non-Renewable Sources
Energy sources that can be replenished over short Non - renewable energy sources are the energy
period of time.
sources that cannot be replenished over short
period of time.
Hydroelectric, Tidal, Wave, Wind, Solar,
Geothermal, Biomass
Fossil Fuels – Coal, Oil & Gas
Nuclear Power
Power Stations – Power stations convert a primary energy resource into electrical energy.
Electrical energy is called a secondary energy source.
Solar Energy:
Advantages
Energy from the sun. Sun has nuclear fusion at its
core resulting in heat and light energy.
Solar Panel – Absorb energy from the sun and
convert it into electricity. It has photovoltaic cells
made of silicon – as sun’s light hits it, it releases
free electrons.
Disadvantages
Low running cost
Low efficiency
No major pollution
Takes large area
Renewable
Availibility varies
Amount of electricity
produced too small to
power large devices.
Wind Energy:
Form of solar energy. Winds are caused by uneven
Advantages
heating of the atmosphere by Sun, the irregularties
of the Earth’s surface and the rotation of the earth Low running cost
causing convection currents and wind to flow.
No major pollution
Wind is then used by humans to form electrical
energy.
Renewable
Wind Turbine – convert kinetic energy of the
wind into mechanical power. The generation then
convert this mechanical power into electricity.
Disadvantages
Wind is unpredictable
Takes large area and
causes sight pollution
Noisy
Power generated is
small
Geothermal Energy:
The heat from the earth's own molten core can be
converted into electricity. This core consists
primarily of extremely high temperature liquid
rock known as magma.
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Advantages
Disadvantages
High Power generated
Costly to set up
No major pollution
Works only in areas
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Cold water is pumped down a bore hole several
kilometers deep into the hot granite rocks
underground. Waters comes up through a second
hole as steam at over 200°C. This high pressure
steam can be used to drive a turbine and generate
electricity
where there is volcanic
activity
Renewable
Becomes useless once
the volcanic activity
dies down.
High power can be
generated
Hydroelectric Power:
Advantages
Heat from solar energy is used to evaporate water Low running cost
from seas and ocean which collects as clouds and
No major pollution
gives us rain. Rivers and rain water fill up a lake
behind dam. Water rushes down from the lake and
spin the turbine. The turbine is connected to the
Renewable
generator, when the turbine rotates generator also
rotates and produce electricity.
Disadvantages
Costly to set up
Can cause flooding in
surrounding areas
Have major ecological
impact – affects the
surrounding animal and
plant life.
Tidal Energy:
Advantages
Tidal power converts the natural rise and fall of
water tides into electricity. Tides are caused by the
combined effects of gravitational forces exerted
by the Moon, the Sun, and the rotation of the
Earth.
Disadvantages
Potential to generate
lots of energy
Costly to set up
No major pollution
Affects wildlife
Renewable
Reduce tidal flow
Predictable - Tide in
and out twice a day
Thermal Power Stations:
Advantages
Thermal Power Station have a boilers, turbines
and generator.
Takes less space
It uses non-renewable energy sources such as coal,
Generate lots of power
oil & gas to produce thermal energy for the
generator. As the heat is transferred to boiler, it
boils the water there to generate steam. Steam
goes to turbine, and turbine runs the generator to
generate electricity.
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Disadvantages
Costly to set up
Non - renewable
Pollution, Global
warming, acid rain –
Huge environmental
impact.
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Nuclear Energy:
Advantages
Less Pollution
In nuclear power stations, the energy is released
by fission, a process in which the nuclei of
uranium atoms are split. After the energy is
released the rest of the process works similar to
that of a thermal power station.
Disadvantages
Expensive to run
Little amount of nuclear Non – renewable
material gives immense
energy
Raw material is cheap
Leakage is dangerous to
all living beings.
Nuclear waste is highly
toxic, expensive and
hard to store.
Nuclear Fission
Nuclear fission is the splitting of a heavy
nucleus into two lighter nuclei, when the
nucleus of an atom is bombarded with a
neutron.
The energy of the neutron causes the target
nucleus to split into two (or more) nuclei that
are lighter than the parent nucleus, releasing a
large amount of energy during the process.
Nuclear Fusion
Nuclear fusion is the combining of two lighter nuclei
to form a heavier nucleus, releasing a vast amount of
energy during the process.
Nuclear fusion is believed to be the process by which
energy is released by the Sun. When two hydrogen-2
nuclei moving at high speeds collide, they can join
together to produce heavier nucleus (Helium). A large
amount of energy is released.
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1.7.3 Work
Work is done whenever force is applied on an object and object move direction of force applied.
• Work can only be done if the object or system has energy. When work is done energy is
transferred and this energy transfer is equals to work done.
Work done = Force x distance moved in the direction of force
W=Fxd
•
•
Unit of Work – Newton meter (N m) or Joules (J)
It is a scalar quantity
1.7.4 Power
Power is defined as the rate of doing work or rate of transferring energy.
The more powerful a machine is, the quicker it does a fixed amount of work or transfer fixed amount of
energy.
Power=
Work done
time taken
P=
W
t
Unit: Joule/second (J/s) or Watt (W)
Efficiency:
In any energy transformations, there are always some non-useful forms produced.
In converting one form of energy to another, we sometime
consider the efficiency of energy conversion or the efficiency
of a machine.
Efficiency=
useful energy
×100
total energy input
Efficiency=
power output
×100
total power input
The efficiency of a machine can never exceed 100% because
energy can neither be created or destroyed i.e. energy output
can never be greater than energy input, or work output can
never be greater than work input. If a system is very efficient,
its efficiency will be closed to 100%. This implies that work
output is always less than work put in.
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