**Energy can be transferred between
energy stores in four principal ways**

**1.
By heating**
- the transfer
of thermal energy from a hotter material/object to a cooler
material/object.

You can only have a net heat
flower from a higher temperature to a lower temperature region.

**2.
By radiation of a wave** -
sound wave vibrations transfer energy and electromagnetic radiation.

There seven types of EM radiation
including microwaves, infrared and visible light.

Infrared radiation is referred to
as thermal radiation - its a means of net transfer of thermal energy
from a higher to a lower temperature material.

**3.
Flow of electrically** -
energy is transferred by electrical charge (electrons) moving around an
electrical circuit down
a potential difference i.e. moving from a higher to a lower potential
energy, releasing energy in the process e.g. in the form of heat or
light.

**See also **
Usefulness of electricity and energy transfer** for lots of
examples starting with electricity**

**4.
Mechanically** - anything
that is moved by a force acting on it involves a mechanical transfer of
energy - anything rotating, pushed, squashed or pulled etc.

**
The law of conservation of energy**

No matter the nature of an energy
store or an energy store transfer, the following law applies ...

**Energy can be (i )stored, (ii)
changed from one form to another and (iii) dissipated, but the total
energy of a closed system is constant and you cannot create or
destroy energy**

There is no net change in
total energy, no matter what energy transfers take place.

**Dissipated** usually
means wasted energy like heat spreading out to increase the
thermal energy store of the surroundings.

In the 19th century scientist began to
realise that mechanical work generated heat, and so concluded that there was
a connection between them e.g. heat is generated by friction when a
mechanical device does work.

Work done in joules = force in
newtons x distance through which force acts,

which means the same amount of work
transfers the same amount of energy, but not all usefully.

Electrical heating elements transfer
energy from an electrical energy store to the thermal energy store of
the material being heated e.g. oil in a heater, water in a kettle.

Moving objects have kinetic energy.
If you want to slow a moving object down you must reduce its kinetic
energy store e.g. car brakes by operating a force. In doing so work must
be done and heat from friction is released to increase the thermal
energy store of the surroundings.

Whenever you get an increase in
temperature of a system, energy must be transferred from one energy
store to another.

**A system** is the physical components
involved with the energy transfer conversion. e.g. your hand and winding up a
clock, a car engine, your muscles being used to lift a weight, a photocell, an
electrical appliance, objects colliding.

A **closed system** is a
collection of objects you treat on their own and energy can be
transferred between different energy stores of the system, but not
between energy stores outside of the system - in other words no energy
leaves or enters the defined closed system.

e.g. a hot drink standing on the
table - the system includes the surroundings too, because thermal energy
is readily transferred to the surrounding air - its an 'open' system..

However, if the hot drink is in a
well insulated thermos flask, then this can be considered a closed
system.

**When a system changes it involves an
energy transfer. Energy might be transferred into or out of a system between the
different components of a system or between different types of energy stores.**

You can transfer energy mechanically when a
force is applied to do work e.g. winding up a clock, dragging a heavy box across
the floor.

You can transfer energy electrically when
work is done by moving charge e.g. an electrical heater, electric motor.

When you raise the temperature of a material
(e.g. from a flame, electrical supply, electromagnetic radiation) you increase
the thermal energy store of that material.

When your vocal chords vibrate you are
transferring kinetic energy into sound energy

Know and understand that energy can be
transferred usefully from one form to another, or stored, or dissipated, but
**energy cannot be created or destroyed**.

**This is the law of
conservation of energy**.

Another way of expressing this law to say
**energy is never lost but transferred between different energy stores
often involving different objects or materials**.

However, energy is only useful
if it can be converted from one form to another.

**Be careful in using the term energy
loss!**

The phrase 'energy loss' is used in the
context of energy transfer when not all the energy is transferred into a
useful form e.g. some energy from a car fuel is 'lost' in the friction of
moving parts, i.e. some chemical energy ends up as heat or sound rather than
kinetic energy to move the car.

Examples - from **a suitable
energy source ==> useful form of energy** (plus waste in most cases)

**
Energy is usually transferred by
radiation, electricity flow, heating or doing work in a mechanical sense.**

**When a gun fires** chemical energy
is converted into heat energy, sound energy, light energy and mainly kinetic
energy. When the bullet embeds itself into some material the kinetic energy
of movement is converted into some sound energy, but mainly heat energy.
Some of the energy is wasted but most of the chemical energy stored in
cartridge is converted to the useful kinetic energy of the bullet.

**Photovoltaic
solar panels** convert light energy into electrical energy. If this
electrical energy is used to charge up a battery then you increase
chemical energy store of the battery.

**
We
use a large number of electrical devices in the home eg**

TV converts electrical energy
into useful light and sound, but some waste heat

A charged mobile phone battery
converts chemical energy into electrical energy, which in turn is converted
into useful light and sound energy.

In the charging process you are
increasing the energy store of the mobile phone battery.

**
Wind
turbines convert kinetic energy into electrical energy**

The kinetic energy store of the
wind is decreased, whereas that of the turbine blades is increased,
initially an energy transfer involving the same type of energy
store.

**The start of making a cup of tea
is a system.** You use an electrical heating element in the kettle
base to boil water by transferring energy to water via the conversion of
electrical energy to thermal energy. The water increases in temperature and
therefore its thermal energy store is increased. The water and heating
element constitute a system.

Whenever an electrical current
flows in a circuit, work is done against the resistance of the wire.
This is all to do with the behaviour of the electrons moving due to
an electrical potential difference.

It doesn't seem the same as say,
pushing a lever to operate the machine, but you are applying a force
to move the lever against a mechanical resistance. In fact the work
done or energy transferred is equal to the force applied x the
distance through which the force operates (see calculations further
down the page).

When the car of a '**big dipper**'
fun ride is raised to the top of a loop using an electric motor you
are converting electrical energy mechanically into the car's kinetic
energy store.

As the car increases in
height its gravitational potential energy store (GPE) increases.

When the car rolls down the
other side its GPE store decreases and its kinetic energy store
increases.

**See also **
Conservation of energy,
more on energy transfers-conversions, efficiency

TOP OF PAGE

**
Introduction to
work done calculations, power and wasted energy**

If an object begins to move or changes
how it moves (change in speed or direction) then a force must be involved.

**If a force acts on an object, then work
is being done on the object increasing its energy store** - this must involve
an energy transfer.

If an object exerts a force or
transfers energy in some way, work is done by this object and energy is
transferred from its energy store.

If there is no friction and a
force is applied to an object, the work done on the object will
equal the energy transferred to the object's kinetic energy store.

However, if an object is already
moving and experiences a resistive force of friction slowing it
down, the energy transferred from its kinetic energy store is equal
to the work done against the object's motion.

The energy can be transferred usefully
i.e. doing useful work, but some energy might (often) be wasted - often
dissipated to the surroundings as heat due to friction.

The **energy transferred**,
usefully or wastefully, is often referred to, and equal to, **work done**.

When dealing with the action of a
resultant force acting through a linear distance in the direction of the
force, you can calculate the work done with a very simple formula.

**
****When ever you or a machine moves
something work is done**

**
Work done (J) = energy
transferred = force (N) x distance (m)**

**W (J) = F (N) x d (m)**

The distance equals the length of
the line of action through which the force acts.

(work done might be denoted
by **E or W** for the energy of work done, but remember the unit
for power is the watt, W, so take care!)

If you apply a force of one
newton through a linear distance of one metre, you do one joule of
work.

one joule of work done = a force of one newton x
one metre distance,
**
1 J = 1 Nm**

Imagine a force of 1 newton
acting through a distance of 1 metre, 1 joule of work is done =
1 joule of energy transferred.

(note that newton metres (Nm) =
joules (J) = work done = energy transferred)

**The distance must be along the
line of action of the force.**

You need an energy source to keep
applying any force for any length of time.

Whenever work is done e.g.
mechanically or electrically, energy is transferred from one energy
store to another.

**See**
work calculation questions

**Power is the rate at which energy
is transferred or 'used up' - work done!**

In other words **power is the
rate of doing work**.

The unit of power is the watt, **
W**.

**
power (W) = energy
transferred (J) / time taken (s)**

**
power (W) = work done (J) / time taken (s)**

**P (W) = E (J) / t (s)**

{and don't forget: **work done
(J) = force (N) x distance (m)**, which you need to solve some
power problems}

A power rating of **one watt means
one joule of energy is transferred per second**,

or, **1 joule of work is done in 1
second**.

The joule is the unit of energy
and the **watt is the unit of power (W)**.

one watt = one joule of energy is
transferred per second,
**1 W = 1 J/s**

A 1 kW electric heater converts
1000 J of electrical energy ==> heat energy every second

(**1 kW = 1000 W =
1000 J/s**)

**See**
work calculation questions

**The power of a machine or any
other device is the rate at which it transfers energy.**

A powerful machine doesn't
necessary mean a bigger force is generated, it just means a lot
of energy is transferred in a short time (or a lot of work done
in a short time).

The more work done/energy
transferred in a given time, the greater the power.

The shorter the time taken to
transfer energy/do work, the greater the power generated.

Conversely, by using gears
and a low powered electric motor, you can generate a relatively
big force. So, don't confuse power and force - use the terms
appropriately.

**See **
Turning
forces and moments including gears

However, no mechanical device or
electrical appliance is 'perfect' and energy is lost e.g. by thermal
energy by friction or sound from unwanted vibrations, heat loss from
circuit wires etc., but it is still part of the total work done - it
just isn't all useful!

Therefore you should know and understand that when energy is transferred
**only part of it may be usefully transferred **as useful energy or
useful work, the rest is ‘**wasted’** to the surroundings

You should know and understand that wasted energy is eventually
transferred to the surroundings, which will become **warmer** AND ...

... the wasted energy
becomes increasingly spread out and so becomes **less useful**.

**For more on wasted energy see:**

Conservation of energy,
energy transfers-conversions, efficiency - calculations and
Sankey diagrams

TOP OF PAGE

**
More examples of e****nergy
stores, energy transfers and doing work**

Doing work in a mechanical sense is just
another set of examples of energy transfer e.g. from one energy store to
another.

This usually involves doing useful work,
but the final energy store destination might have no further use.

**For example when you burn fuel in a
car engine the energy changes are as follows.**

fuel + oxygen (chemical energy store)
==> hot gases (thermal energy store) ==> car moves (kinetic energy
store)

In the process some of the original
chemical energy store of the system is lost through friction and sound.

BUT, all the energy from the
chemical energy store ends up as thermal energy to warm up the surroundings. Therefore the
thermal energy store of the environment is increased but its now of no
use whatsoever!

The waste of energy increases (but
necessarily!) when you apply the brakes. Mechanical work is being done in the
process - **work is being done against the force of friction**. The friction effect of the brake pad on the brake disc converts
kinetic energy into thermal energy. The kinetic energy store of the car decreases
(fortunately!) and the thermal energy store of the brake pad + disc
increases. Eventually as the braking system cools down the heat energy
is transferred to increase the thermal energy store of the surroundings
- wasted or degraded energy!

**If a vehicles crashes into a
stationary object**, the contact force causes energy to be
mechanically transferred from the vehicle's kinetic energy store to
**elastic potential energy store** of the crushed vehicle parts,
the **thermal energy stores** of the vehicle, object crashed into
and the surrounding air (including some sound energy too - which
also ends up as thermal energy!).

**See also **
Reaction times, stopping distances, safety
aspects. calculations including F=ma
gcse physics notes

**When you lift an object up you are
doing work against the effects of gravity.**

The chemical energy stored in your
muscles is decreased as some of it used to lift the object-weight which
on gaining height increases its gravitational potential energy store.

In fact the work you do (J) equals
the weight of the object (N) x vertical distance lifted (m)

**When you kick a ball up in the air,
three energy stores should be fairly obvious to you.**

chemical energy store of muscles ==>
kinetic energy store of ball ==> gravitational potential energy store of
ball

AND, when the ball falls (ignoring a
bit of lost sound energy)

gravitational potential energy store
of ball ==>`kinetic energy store of ball ==> thermal energy store of the
ground

**For more examples see**
Conservation of energy,
energy transfers-conversions, efficiency - calculations,
Sankey diagrams

TOP OF PAGE

**
Examples of s****imple
mechanical work done calculation questions**

**
and**** Examples
of simple power calculation questions**

**
**

**
How to solve 'work done' problems.**

The formula for work done is quite
simple:

**work done in joules = force in newtons x
distance in metres** along the line through which the force acts

**work done (J) = force (N) x distance
through which force acts (m)**

**E ** **= F x d**

In real situations of energy
transfers involving mechanical work, I'm afraid friction effects
accompany the useful work done.

Moving parts rubbing against each other
causes friction and rise in temperature - kinetic energy store ==>
thermal energy store of machine and surroundings.

Note that the force can be the weight
of an object acting in a vertical direction **... see the page on ...**

Mass and the effect of gravity force on it - weight, (mention of work done,
GPE and circular motion)

**How to solve power problems** - just a few
simple 'mechanical' examples here..

Power is the rate at which energy is
transferred, that is the rate at which work is done.

**power in watts = (work done = energy
transferred) ÷ time taken**

**P (W) = E (J) / t (s)**

Power **P** in Watts, Energy
**E**
in Joules, time** t** in seconds

The power rating of 1 watt is equal to a work rate of 1 joule
of energy transferred per second

**See also**
FORCES 3. Calculating resultant forces using vector
diagrams and work done gcse physics revision notes

**
Examples of problem solving using
the 'work done' and 'work rate = power' formulae**

**In every case watch the units** e.g.
W/kW, J/kJ/MJ, min/secs etc. so take care!

**
Q1**
If you drag a heavy box with a force of 200 N across a floor for 3 m,

(a) what
work is done?

work done = 200 x 3 = **
600 J**

(b) If you do the job in 5 seconds,
what was your power rating in doing this task?

power (W) = work done (J) / time
taken (s)

your power = 600 / 5 =
**120 W**
(just over the power of 100 W light bulb!)

**
Q2** (a) If a machine part
does 500 J of work moving linearly 2.5 m, what force was applied by the
machine?

work done = force x distance,
rearranging, **force (N) = work done (J) ÷ distance (m)**

force = 500 ÷ 2.6 = **
200 N**

(b) If an electric motor transfers
12.0 kJ of useful energy in 3.0 minutes.

Calculate the power output of the
motor.

work done = energy transferred =
12 x 1000 = 12,000 J, time taken = 3 x 60 = 180 s

power = work done / time = 12,000
/ 180 =
**66.7 W** (3 sf)

(c) An appliance has a power rating
of 1.2 kW.

How many kJ of energy is
transferred in 8.0 minutes?

1.2 kW = 1200 watts, time = 8
x 60 = 480 seconds

P = E / t, rearranging gives
**E = P x t**

energy transferred = 1200 x
480 = 576 000 J = **
576 kJ**

**
Q3** A machine applies a
force of 200 N through a distance of 2.5 m in 2.0 seconds.

What is the power of the machine?

work done = force x distance = 200 x
2.5 = 500 ?

power = work done / time taken = 500
/ 2.0 = 250 J/s = **250 W**

**
Q4** A 500 kg express
skyscraper lift moves non-stop up a total height of 100 m.

(a) If the force of gravity = 9.8
N/kg, calculate the weight of the lift.

weight = mass x g

= 500 x 9.8 = **
4900 N**

(b) Calculate the work done by the
lift motor.

work = force x distance

the
force exerted by the lift motor must be at least equal to the weight
of the lift due to gravity

therefore work done = 4900 x 100
= **490000 J** ≡
__490 kJ__ ≡
0.490 MJ

(c) If the lift ascent time is 20
seconds, what is the power of the lift motor?

power = work done / time taken

power = 490000 / 20 =
**24500 W** ≡
**
24.5 kW**

(d) What assumptions has been made
for calculations (b) and (c)?

Both (b) and (c) calculations ignore
the extra work done in overcoming any forces of friction.

**Q5** Part of a machine
requires a continuous force of 500 N from a motor to move it in a
linear direction.

(a) How much work is done in moving
it a distance of 50 m?

work done = force x distance =
500 x 50 =
**25000 J** (25 kJ)

(b) If the power of the machine
is 5.0 kW, how long will it take to move the machine part the 50 m
distance?

power = work done / time taken

time taken = work done / power

time = 25000 / 5000 =
**5.0 s** (stand clear!)

Q6 A machine has to move a conveyor
belt at a rate of 30 m/min.

The system is designed to use 6
kJ/min of electrical energy to move the conveyor belt along.

What is the minimum force the motor
must produce to move the conveyor belt along?

(i) power = rate of energy transfer =
energy transferred (J) / time taken (s)

power = 6000 / 60 = 100 W (100
J/s)

(ii) 30 m/min ≡
30 / 60 = 0.5 m/s

so in one
second, the energy transferred is 100 J and the distance moved is
0.5 m.

work = force x distance

force = work / distance = 100 /
0.5 = **200 N**

(iii) If you are smart, you can solve
the problem directly, because both bits of data involved '**per minute**'.

So you can just say force = work
/ distance = 6000 / 30 = **
200 N** !!!

BUT, always work logically in
your own comfort zone, the data might not always be so kind!

**Q7** A toy model car has
a clockwork motor, whose spring can store 8.75 J of elastic potential
energy.

On release the clockwork motor can
deliver a continuous force of 2.5 N.

How far will the car travel in one
go?

energy store = total work done =
force x distance

distance = energy store / force =
8.75 / 2.5 = **
3.5
m**

**Q8** A sliding object
moving across a very rough surface has 5.0 J in its kinetic energy store.

If the object experiences a constant
resistive force of friction of 25 N, calculate in cm how far the object
travels before coming to a halt.

energy transferred = kinetic
energy = resistive force x distance to come to a halt

E = KE = F x d, 5.0 = 25 x
d, d = 5.0/25 =
0.20 m = 20 cm

**Q9 **A small electric
motor uses 120 J of electrical energy in 3.0 minutes.

Calculate the power of the electric
motor.

Energy transferred = 120 J, time = 3
x 60 = 180 s

P = E / t = 120/180
=
**0.67 W **(2 sf)

**Q10** A 45.5 kg mass is
hoisted up vertically 15.5 m.

If the gravitational field force is
9.8 N/kg, calculate the work done to the nearest joule.

weight = force = mass x gravity =
45.5 x 9.8 = 445.9 N

This force acts through the
vertical height the object is lifted.

work done = force x vertical
distance = 445.9 x 15.5 = 6911.45

**
work done = 6911 J**

**Q11** A small electric
motor operating a water pump for a garden ornament does 6.5 kJ of work in
3.0 minutes.

Calculate the power of the pump.

Work done = 6.5 x 1000 = 6500 J

Time taken = 3 x 60 = 180 s

Power = work done / time taken =
6500 / 180 = 36.11 =
**36 W** (2 sf).

**
Q12** An electrical appliance transfers
8000 J of energy in 1 minute and 40 seconds.

Calculate the power of the appliance.

power = energy transferred / time =
8000 / 100 = **
80 W**

**
Q13** A person weighing 600
N strides up 20 steps, each one 20 cm high, in 10 seconds.

(a) Calculate the work done.

Each step is 20 cm = 0.20 m,
total height = 20 x 0.20 = 4.0 m

Work done (J) = force (N) x
distance (m)

Work done = 600 x 4.0 = **
2400 J**

(b) calculate the average power the
person generates in the process of ascending the 20 steps.

power (W) = work done (J) / time
taken (s)

power = 2400 / 10 = **
240 W**

(c) If the person stands still on the
20th step, what is the person's power?

Since no force is acting through
a distance, in a mechanical sense, the
**power is zero**, **but ....**

(d) When standing still, is your body
still doing work? Explain your answer!

**YES**. Your body must be
still releasing energy through respiration to maintain the tension
in your muscles to keep you upright, otherwise you would flop down
in a heap!

**
Q14** (you might not
have dealt with all aspects of Q14 yet)

Imagine a car of 1000 kg travelling
at 20 m/s doing an emergency stop in a distance of 25 m - the braking
distance.

Calculate the average braking force
produced by the driver when pressing on the brake pedal.

To solve this question you to use
several formulae.

(a) Calculate the kinetic energy of
the car.

KE = 0.5 mv^{2} = 0.5 x
1000 x 202 = **
200 000 J**

(b) What work must be done to bring
the car to a halt? Explain your answer.

If the kinetic energy of the car
is 200 000 J, then
200 000 J
of work must be done to bring the KE of the car to zero i.e
zero velocity.

(c) Calculate the average braking
force required.

Work (J) = force (N) x distance
(m)

work = 200 000 J and the braking
distance was 25 m

force = work / distance = 200 000
/ 25 = **
8000 N average braking force.**

**
Q15** A large car is
travelling at 120 km/hour and the engine provides a constant driving force
of 1500 N.

(a) How far does the car travel in 1
second?

1 km = 1000 m, 1 hour = 60 x 60 =
3600 s

speed (m/s) = distance (m) / time
(s)

speed of vehicle = 120 x 1000 /
3600 = 33.33 m/s

so, **
in one second the vehicle
moves 33.3 m** (3 sf)

(b) From your answer to (b) calculate
the power of the engine.

You have to do a little 'think'
**connecting two equations**.

**work = force x distance**
and **power = energy transferred / time**

i.e. energy transferred = work
done = force x distance

**power (W) = **work done /
time = **force (N) x distance (m) / time (s)**

power = 1500 x 33.33 / 1 =
50 000 W
or 50 kW
(3 sf)

In other words the distance /
time in the equation is quite simply the speed of 33.3 m/s, get it
OK?

Q16

**See also**
FORCES 3. Calculating resultant forces using vector
diagrams and work done gcse physics revision notes

**See also**
electrical energy used, power and cost of electricity calculations gcse physics revision notes

TOP OF PAGE

Energy resources, and
transfers, work done and
electrical power supply revision notes index

Types of energy & stores - examples compared/explained, calculations of
mechanical work done and power

Chemical *
Elastic
potential energy *
Electrical
& electrostatic
*
Gravitational potential
energy

Kinetic
energy store *
Nuclear
energy store *
Thermal
energy stores *
Light energy *
Sound energy

Conservation of energy,
energy transfers-conversions, efficiency - calculations and
Sankey diagrams gcse physics

Energy resources: uses, general survey & trends,
comparing renewables, non-renewables, generating electricity

Renewable energy (1) Wind power and
solar power, advantages and disadvantages gcse physics revision
notes

Renewable energy (2) Hydroelectric power and
geothermal power,
advantages and disadvantages
gcse physics

Renewable energy (3) Wave power and tidal barrage power,
advantages and disadvantages
gcse physics

**See also **
Renewable energy - biofuels & alternative fuels,
hydrogen, biogas, biodiesel gcse chemistry notes

Greenhouse
effect, global warming, climate change,
carbon footprint from fossil fuel burning gcse chemistry

The absorption and emission of radiation by
materials - temperature & surface factors including global warming

The Usefulness of Electricity gcse
physics electricity revision notes

**and **
The 'National Grid' power supply, mention of small
scale supplies, transformers gcse
physics notes

aqa gcse 9-1
physics: Know that a system is an object or group of objects. There
are changes in the way energy is stored when a system changes. For
example: an object projected upwards, a moving object hitting an
obstacle, an object accelerated by a constant force, a vehicle
slowing down, bringing water to a boil in an electric kettle.,
Throughout this section on Energy you should be able to : describe
all the changes involved in the way energy is stored when a system
changes and calculate the changes in energy involved when a system
is changed by: heating