Energy

Question 1

What is the equation for calculating kinetic energy?

Answer 1

The kinetic energy of a moving object can be calculated using the equation:
kinetic energy = 0.5 ×mass ×speed2
Ek=1/2 m v2
2 = squared!

Question 2

What is the equation for calculating elastic potential energy?

Answer 2

elastic potential energy = 0.5 ×spring constant ×extension2
Ee=12 k e2
(assuming the limit of proportionality has not been exceeded)

Question 3

What is the equation for measuring potential gravitational energy (g.p.e.)

Answer 3

g.p.e. =mass ×gravitational field strength × height

Ep=m g h

Question 4

What is the earths gravitational field strength and how is it measured?

Answer 4

Gravitational field strength (g) is measured in newtons per kilogram (N/kg). The Earth’s gravitational field strength is 9.8 N/kg. This means that for each kg of mass, an object will experience 9.8 N of force.

Question 5

What is the letter used for a spring constant and how is it measured?

Answer 5

k is the letter and it is measured in newtons per metre N/m

Question 6

What letter is used for a spring extension and how is it measured?

Answer 6

e is the letter measured in metres

Question 7

What letter is used for mass and how is it measured?

Answer 7

m is used for mass, measured in kg

Question 8

What letter is used for height and how is it measured?

Answer 8

h for height measured in m

Question 9

What letters are used for potential, elastic and kinetic energy and how are they measured?

Answer 9

Ek = kinetic energy
Ep = potential energy
Ee = elastic energy
All energy is measured in Joules

Question 10

What is power and how is it measured?

Answer 10

Power is the rate of doing work or transferring energy. A machine that is more powerful than another machine transfers more energy each second.
It is measured in watts W

Question 11

What is meant by work done?

Answer 11

In science, work is only done by a force when an object moves. More work is done when:
• the force is bigger
• the object moves further – its displacement is bigger.
Sam’s car has broken down. He tries to push it but the car does not move. He is not doing any work though. Work is only done when a force moves. Sam gets some friends to help. Together, they can push with a larger force and the car moves. They are all doing work.

Question 12

What are fossil fuels, what type of energy do they contain?

Answer 12

• Fossil fuels are burnt to release the CHEMICAL energy stored in them.
• Fossil fuels were formed over millions of years and supplies
are running out.
• There are alternative energy resources which have advantages and disadvantages.

Question 13

How is thermal energy transferred?

Answer 13

• Thermal energy is transferred by conduction, convection and radiation.
• In conduction and convection energy is transferred by the movement of particles.
• Radiation is the only way energy can be transferred in a vacuum.

Question 14

What is the relationship between temperature and energy?

Answer 14

• Temperature tells us how hot something is.
• When there is a difference in temperature between two
objects, energy is transferred from the hotter object to the
colder one.
• Energy transfer tends to reduce the temperature
difference.

Question 15

How can we control the transfer of energy?

Answer 15

Energy can be transferred usefully, stored or dissipated. The total amount of energy does not change. We can calculate the energy efficiency for any energy transfer. Some energy transfers are wasteful; we can try to reduce them.

Question 16

What is the equation for work done?

Answer 16

The equation that links work, force and distance is:
work done = force × distance moved along the line of action of the force
where work (W) is in joules, force (F) in newtons and distance in metres.
W = F ×s
When a person climbs stairs or jumps in the air the force moved is their weight.
Example: Dev climbs a flight of stairs rising a vertical height of 5 m. He weighs 600 N. Calculate the work Dev does by lifting his weight up the stairs.
Work done = force × distance moved along the line of action of the force.
Work done = 600 N × 5 m = 3000 J.

Question 17

You can get calculations which combine formulae so you have to use the information from one to work out the next. Here is an example.

Answer 17

When work is done on an object there can also be a change in its kinetic energy. We can use this to calculate the force needed to stop a car when the distance it travels while coming to rest is known.
Example: A car of mass 1000 kg does an emergency stop when travelling at 15 m/s. It stops in a distance of 20 m. Calculate the braking force.
Work out the kinetic energy of the car first
E k o f c a r = 1/2 m v 2
= 1/2× 1000 kg × (15 m/S)2

= 112 500 J.
Work has to be done to reduce the kinetic energy of the car and bring it to a stop. The force that does the work is the friction force between the brakes and the wheel. Therefore the work done by the braking force is 112 500 J.
W = F × s, where F is the braking force and s is the distance moved during braking. We can rearrange this to make F the subject of the equation.
F = W/s
= 112500J/20 m
= 5625 N.
Work done against the frictional forces acting on an object causes a rise in the temperature of the object. The temperature of the brakes increases.

Question 18

What are the formulae to calculate power?

Answer 18

Power = work done in Joules/ time in secs or energy transferred in Joules/time in secs

P = W/t. Or P= E/t

An electric kettle is rated at 2 kW (2000 W). How much energy is transferred in 30 s?
2000 = E/30 so E = 2000 x 30 = 60000 Joules

Question 19

What is specific heat capacity and what letter do scientists use to depict it?

Answer 19

specific heat capacity (c) is a measure of how much energy is required to raise the temperature of 1 kg of a substance by 1 °C.

Question 20

What equation is used to calculate the amount of energy stored in a system?

Answer 20

change in thermal energy = mass × specific heat capacity × change in temperature
In maths, the symbol Δ means the difference
or change in a quantity.
ΔE means a change in energy and Δ𝛉 means a change in temperature.
ΔE = mcΔ𝛉
where ΔE is in J, m is in kg, c is in J/kg°C and Δ𝛉 is in °C.
Example: Calculate the change in thermal energy when 2 kg of water is heated from 20 °C to 80 °C.
The specific heat capacity of water is 4200J/kg
ΔE = 2 x 4200 x (80-20) = 540000J or 540KJ

Question 21

What is meant by a system?

Answer 21

A system is an object or group of objects. The total energy in a closed system is always constant. Energy is never created or destroyed.

Question 22

What is wasted energy? Give an example using friction

Answer 22

Work done against the frictional forces acting on a moving object cause energy to be transferred from the object. This energy transfer raises the temperature of the surroundings by such a small amount that it is of no use. This is wasted energy – energy is transferred to a store where it cannot be used. Some energy is always dissipated when it is transferred.

Question 23

What is conservation of energy?

Answer 23

Energy cannot be created or destroyed, only transferred from one store to another. In a closed system (one in which no energy can enter or leave) the total amount of energy put into the system equals the total amount of energy output. We say that energy is conserved. This is the law of conservation
of energy. However, only some of the energy output is useful to us. The rest is dissipated as wasted energy. This affects the efficiency of a machine.

Question 24

How is energy efficiency calculated?

Answer 24

Efficiency is an indication of how much of the energy supplied to a device is transferred as a useful output. If all of the energy supplied was transferred usefully the transfer would be 100% efficient.

Energy efficiency = useful output energy (power)transfer /total input energy (power) transfer

It can be energy or power but not a mixture

Question 25

Describe the energy stores and system in a ball when you throw it in the air and catch it again

Answer 25

When the ball leaves your hand it has a store of kinetic energy
At it’s highest point the ball has a store of gravitational potential energy
Just before you catch it it has a store of kinetic energy

Question 26

Describe the energy stores and system in a kettle when you turn it on.

Answer 26

The electrical energy in the power supply is converted into thermal energy in the water

Question 27

Describe the energy stores and system when you burn coal

Answer 27

Chemical energy stored in the coal is transferred into thermal energy

Question 28

Describe the energy stores and system when a car brakes

Answer 28

The moving car has a store of kinetic energy, the brakes exert a frictional force on the brakes making them hot. The kinetic energy has been converted into thermal energy

Question 29

Describe the energy stores and system when you throw an object

Answer 29

The chemical potential energy in our muscles decreases and the store of kinetic energy in the object increases.

Question 30

Identify the different types of energy stores

Answer 30

kinetic
chemical
internal (or thermal)
gravitational potential
magnetic
elastic potential
nuclear

Question 31

What is meant by the term conservation of energy

Answer 31

It means that energy cannot be created or destroyed

Question 32

What is dissipation of energy? give a practical example

Answer 32

When energy is transferred from one system to another some can be wasted
Eg when a light bulb converts electrical energy into light energy some of the energy is dissipated as heat energy which is lost into the surrounding area

Question 33

Identify dome ways in which homes can reduce unwanted dissipation of heat energy

Answer 33

1. Walls - the thicker the walls the less heat loss - walls can be insulated inside or outside
   Modern houses are built with a cavity of air between 2 layers of brick. Foam insulation traps air which has a lower thermal conductivity.
2. Windows - double glazing when a layer of gas it trapped between the 2 sheets of glass reducing energy loss
3. Loft insulation reduces energy loss through the roof
4. Carpets reduce energy loss through the floor.

Question 34

Explain efficiency

Answer 34

Efficiency is the proportion of energy that is usefully transferred
It doesn’t have any units because it is always expressed as a percentage or decimal

Efficiency = useful output energy
———————————
total power input

Question 35

How can friction be reduced

Answer 35

1. Using wheels
2. Using lubricants

Question 36

How can air resistance be reduced

Answer 36

Traveling slowly
Streamlining

Question 37

What are renewable energy sources?

Answer 37

Energy sources which will not run out
Examples are solar, tidal (wave) wind and thermal from the earth

Question 38

What are non renewable energy sources?

Answer 38

Energy sources which will eventually run out such as fossil fuels like oil and coal