In this chapter you understand and learn:

The different types of Energy;
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About Efficiency;
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How to draw Sankey Diagrams;
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About Gravitational Potential Energy (PE);
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About Kinetic Energy (KE);
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The relation between PE and KE in free-fall;
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About “Work”.
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About “Power”
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Renewable and Non-Renewable Energy Sources;
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Advantages and Disadvantages of:

Fossil Fuels
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Nuclear Power
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Solar Power
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Wind Power
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Hydro-electric Power

Biomass
Energy….making things happen!
Consider this example:
Peter has been playing with his PC for hours…too much for his
little brother to bear…
So, he pulls the PC’s plug!
…and this is how they end up!
The PC no longer worked since the source of energy has been cut off! The PC cannot
work without a source of Electrical Energy!
Energy is continuously being exchanged around us. Just think about it…
While you are reading this text you are receiving Light Energy in
your eyes, which is converted into Electrical Energy in your brain.
In order for your body to function it needs food, and food contains
Chemical Energy. Your body uses this energy to move (Kinetic
Energy) and Heat Energy is given off.
You can try asking the teacher about it, and before you know it, you are giving off
Sound Energy.
All these Energy Exchanges in a few seconds! And yet none of these could take place
unless we eat in the first place.
This happens since Energy changes form. It is neither created nor destroyed.
Let’s learn about the different
forms of Energy!
The Law of Conservation of Energy:
“Energy cannot be created nor destroyed
– it changes from one form to another.”
Total Energy = Total Energy
Input
Output
Energy, Work and Power & Energy Sources
Energy is measured in Joules (J)
page 2
Types of Energy
Chemical (or Stored Energy):
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Heat (or Thermal Energy):
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Sound Energy:
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Radiant (includes Light) Energy:
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Energy, Work and Power & Energy Sources
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Electrical Energy:
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Nuclear Energy:
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Kinetic Energy (KE):
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Potential Energy (PE):
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Energy, Work and Power & Energy Sources
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Fill in the boxes with the name of the Energy involved!
Energy, Work and Power & Energy Sources
page 5
EFFICIENCY
Every machine tends to waste part of the Energy input they are given. On many
occasions, Energy is wasted as Heat and Sound – the low-grade energies.
If a machine wastes little energy, we say that it is “efficient”.
Efficiency can be worked out with this equation:
𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =
𝑢𝑠𝑒𝑓𝑢𝑙 𝑜𝑢𝑡𝑝𝑢𝑡
× 100%
𝑡𝑜𝑡𝑎𝑙 𝑖𝑛𝑝𝑢𝑡
For example, if a bulb consumes 2000J of electricity and 1900J of these are
lost as Heat, then the bulb’s efficiency is:
Useful Energy = 2000J (input) – 1900J (wasted Heat) = 100J of Light
𝑈𝑠𝑒𝑓𝑢𝑙
× 100%
𝐼𝑛𝑝𝑢𝑡
100
𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =
× 100% = 𝟓% 𝐞𝐟𝐟𝐢𝐜𝐢𝐞𝐧𝐭
2000
𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =
Further Questions:
(1) A TV set consumes 250kJ of Electricity. If it gives out 75,000J of Light
and Sound, calculate the TV’s Efficiency.
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(2) A CFL lamp has an efficiency of 20%. If it gives off 800J of Light, how
much Electrical Energy was supplied to the lamp?
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Energy, Work and Power & Energy Sources
page 6
Energy Flow Diagrams – Sankey Diagrams
Sankey Diagrams are useful to illustrate Energy Conversions.
One should note that:
o Useful Energy Outputs should always be drawn as straight arrows
following in the arrow’s original path.
o Wasted energy outputs should be drawn pointing away from the arrow’s
original path.
o The thickness of the arrows indicates the amount of energy.
o Labels should be added to identify the types of energy involved, and,
where possible, a percentage or amount of energy should be written down.
For example:
A light bulb uses 200J of electricity. It gives out 10J of light. The rest is lost
as Heat.
A Sankey diagram for this bulb would look like this:
Heat
190J
(95%)
Electrical
200J
(100%)
Light
10J
(5%)
Sankey Diagram Practice
Draw Sankey diagrams for:
(a)
(b)
(c)
(d)
A candle (used to illuminate) – 20% efficient
Gas flame in oven – 90% efficient
A TV set – 45% efficient
A moving car – 25% KE, 15% Sound
Energy, Work and Power & Energy Sources
page 7
The Mechanical Energies: Potential and Kinetic
Along the Physics Course we will deal with each type of Energy and study it with more
detail. Right now, we will learn about two very important forms of Energy: Gravitational
Potential Energy and Kinetic Energy.
Gravitational Potential Energy (GPE or simply PE)
There are various forms of Potential Energy. A spring in tension has Elastic Potential
Energy, while an electrical charge might have Electrical Potential Energy. Now, all types
of Potential Energy can be regarded as energy obtained due to an “unstable” condition.
E.g. A spring develops Elastic PE when compressed. This is not an easy condition. In
fact, given the possibility, the spring jolts back to its original form.
Spring has Elastic PE. It is in an “unstable” condition.
At O-Level, we will study in depth only one type of Potential Energy, the Gravitational
PE. Therefore, when we mention Potential Energy (PE), we refer to the Gravitational
PE.
Look at the diagram below.
Stable,
so no PE
h
The pot has been
raised above
ground, and can
fall. It is
“unstable”, and so
it has PE.
All objects above ground possess PE. That is why they are able to fall! The Potential
Energy can be measured using the following equation:
Potential Energy = Mass x Gravitational
Field Strength
x Height
from ground
PE = mgh
Energy, Work and Power & Energy Sources
page 8
Kinetic Energy (KE)
Kinetic Energy is found in all moving objects such as moving cars, moving
aeroplanes, running athletes, etc.
The object’s KE depends on its velocity. The larger the velocity, the larger the
object’s KE.
Bodies which are at rest have zero KE.
Objects at
rest have
no KE!
Moving
objects
have KE!
The Kinetic Energy of a body can be found by the equation:
Kinetic Energy = ½ x Mass x Square of the Velocity
KE
mv2 2
KE== ½
½ mv
Ex. 1 – Practice using PE=mgh
1) Calculate the PE of the following objects:
(a) A painting of mass 3.5kg hanging 180cm from the ground;
(b) An orange, of mass 200g, on a branch 2m high;
(c) 20 stones, of mass 15kg each, when lifted by a crane to a height of 8m.
25kg
2) Calculate the PE of the dog and the boy.
30kg
2m
3.5m
Ex. 2 – Practice using KE = ½mv2
Calculate the KE of the following objects:
(a) a tortoise of mass 8kg moving at 0.05m/s;
(b) a motorcycle of mass 400kg, moving at 25m/s;
(c) an aircraft, of mass 10,000kg, travelling at 350m/s.
Energy, Work and Power & Energy Sources
page 9
Relationship between PE and KE
Rising
Falling
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Energy, Work and Power & Energy Sources
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Ex. 3 – Practice using PE at top = KE at bottom
(1) A crane lifts a 500kg concrete block up a vertical height of 25m. Calculate:
(a) The PE of the block at the top;
(b) The steel cable breaks and the block falls freely. Calculate the velocity with which
the block hits the ground.
(2) A book, of mass 1.2kg, falls form a 2.5m high shelf. With what speed will it hit the
ground?
(3) If a bullet of mass 20g is fired vertically upwards at a speed of 200m/s, how high will it
reach?
(4) A child jumps off a springboard 10m high. At what speed will he enter the water’s
surface?
(5) A roller-coaster car of mass 500kg drops down a vertical height of 30m.
A
C
30m
B
(a) Assuming a Friction-less process, calculate the velocity of the car at B.
(b) If, in reality, the car loses 15% of its energy as Heat and Sound as it moves from A to B,
calculate
(i) the KE available to the car at B;
(ii) the maximum height the track can be at C.
Energy, Work and Power & Energy Sources
page 11
Work
Whenever an object is made to move against an opposing
Force, Energy must be transferred.
We say that “Work” is being done!
Diagram A shows Work being done against ___________________.
Diagram B shows Work being done against ___________________.
Work can be found by the formula:
Work = Force x Distance Moved
Work is only done if an applied force produces motion.
When Work is done, Energy is transferred to the object. For this reason, the unit of
Work is the same as that of Energy – the Joule (J).
NOTE:
When finding the Work done, the motion is to be considered to be at constant speed.
Therefore, the applied force and the opposing force are numerically equal, and so it is
sufficient to know the value of one of the forces!
Energy, Work and Power & Energy Sources
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Power
Power is a measure of the rate at which Work is being done. In other words, it
is a measure of a machine’s ability to change one type of Energy into another.
Look at this example.
A 60-Watt bulb shines brighter than a 20-Watt bulb. The reason for this is
that the 60-Watt bulb has greater Power. It can therefore transform
Electrical Energy into Light and Heat more rapidly than the 20-Watt bulb.
Power is found by the following equation:
Power = Work Done
Time Taken
The Unit of Power is the Watt (W)
Or Joules per Second (J/s)
Named after James Watt
(1736 – 1819), Scottish
scientist and developer of the
steam engine.
So, a 20-Watt bulb transforms 20 Joules of Electricity into Light and Heat each
second. That is why a high Power air-conditioner cools more rapidly than a low
power one, and so on.
Energy, Work and Power & Energy Sources
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Ex. 4 – Practice using w=Fs
(a) A lady pushes her pram with a force of 20N over a distance of 100m. Calculate the
work done by this lady.
(b) A crane is used to lift a 200kg load up a height of 5m. Calculate the work done.
(c) A lift carries 3 persons, of mass 65kg, 72kg and 86kg respectively. If the lift itself has a
mass of 100kg, calculate the work done by the lift’s motor as the lift rises by 12m.
(d) At the airport, a passenger 6300J of work in pushing a luggage over a distance of 180m.
(i) Calculate the force used to push this luggage.
(ii) On boarding the aircraft, the passenger places his 9kg luggage in the overhead
compartment. If he does 171J of work, how high is the luggage compartment?
Ex. 5 – Practice using P=w/t or P=E/t
(a) A bulb consumes 36kJ when it is switched on for 10 minutes. Calculate the bulb’s
Power.
(b) Grace, of mass 40kg, is able to run up a flight of stairs 9m high.
(i) Calculate the work done by Grace;
(ii) Calculate her Power if she is able to do so in 24s.
(c) If a TV is rated 250W, calculate the Energy it consumes in 1hr.
(d) A crane lifts a 200kg load over a height of 12m in 5 seconds. Find the crane’s power.
(e) An electric kettle is rated at 2kW. If it consumes 480kJ to boil some water, calculate the
time taken for this operation.
Energy, Work and Power & Energy Sources
page 14
Sources of Energy – Introduction
In today’s technological world, we need enormous amounts of
energy for transportation, industry, communication and entertainment. Up till
now, most of this energy1 derives from fossil fuels2 like coal and oil.
This is done either directly – by burning such fuels to run engines (such as
petrol in a car); or indirectly, by using the electricity generated by coal or oildriven power stations (such as using a PC whose electricity is coming from an oil
driven power-station).
The problem with fossil fuels is that they give off pollution when burnt.
Moreover, fossil fuels are finite – someday we will use up all the available fossil
fuel sources! This is happening since fossil fuels are a non-renewable source.
Therefore we must make sure to use the remaining fossil fuel sources carefully
(by building more efficient machines) and at the same time find alternative
energy sources to sustain and eventually replace our dependency on fossil fuels.
Non-Renewable vs Renewable Sources
 Non-Renewable3 Source:
 Source not easily replaced
 Using it entails the destruction of the Source
 Renewable4 Source:
 Source is naturally replenished
(e.g. each day the Sun shines)
 Source can be sustained
(e.g. If crops are used as fuel, one can grow more crops)
 Using it does not entail the destruction of the Source
(e.g. The same water which powers one hydroelectric turbine can
be used to do the same further downstream)
1
In 2008, 90% of Global Energy needs originated from fossil fuels.
Fuels derived from the decomposed remains of prehistoric organisms. Mainly categorised as oil (and its
derivatives like Diesel, petrol, etc), coal and natural gas (methane).
3
Mainly taken to be Fossil Fuels and Nuclear Power.
4 Do not confuse “renewable” with “non-pollutant”. It is true that most renewable sources are also clean, but
there are some renewable which do damage the environment (e.g. biomass)
Energy, Work and Power & Energy Sources
page 15
2
Producing Electricity
Batteries convert chemical energy to electricity. They do not store electricity!
Using batteries only to power houses or factories is impossible. One would need
enormous amounts of battery packs, and they
themselves would need to be replaced or re-charged!
Thankfully, scientists have learnt how to produce
electricity in great abundance using electromagnetism.
With the use of a dynamo, basically a coil turning round
a magnet, (similar in principle to those mounted on
bicycles) we can produce electricity “on-demand”. A
dynamo converts mechanical energy into electricity.
Fossil-fuel and Nuclear Power Stations generate electricity in this way. Heat is
used to create high-pressure steam which rotates turbines. The rotating
turbine shaft powers a generator (dynamo) to produce electricity.
Wind turbines and Hydro-electric stations use the moving wind
or water to power their turbines. Then electricity is produced
in a similar way to a power-station.
The discovery of photo-electricity in the early 1900’s led to the
development of the Photo-Voltaic (P-V) panels. These produce
electricity by converting solar radiation into electricity.
Research and Development in P-V cells over the last decades
have resulted in increased yield and a greater share of the
overall global electricity production.
Energy, Work and Power & Energy Sources
page 16
Fossil Fuels:
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ADVANTAGES:
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Nuclear Power:
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ADVANTAGES:
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Energy, Work and Power & Energy Sources
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Solar Power:
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ADVANTAGES:
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Wind Power:
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ADVANTAGES:
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Energy, Work and Power & Energy Sources
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Hydro-electric Power:
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ADVANTAGES:
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Energy, Work and Power & Energy Sources
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Biomass:
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