Work, Energy and Power

Question 1

State the equation for Gravitational Potential Energy and define G.P.E.

Answer 1

G.P.E = mgh

G.P.E. is the energy a mass has due to its position in a gravitational field

Question 2

Describe the change in G.P.E and K.E. for a falling object

Answer 2

As the object falls, its G.P.E decreases because it is converted to K.E. (in the form of velocity)
½mv² = mgΔh

Question 3

Define work done

Answer 3

Work Done (energy transferred) = Force X distance moved in the direction of the force

W=Fs

Question 4

Explain why the energy transferred in a falling object is equal to the work done.

Answer 4

Force (F) = Weight (mg)
Displacement (s) = change in height (h)
Therefore Fs = mgh

So W = mgh

Question 5

Define a machine

Answer 5

Something that makes the work easier

Question 6

Explain how a ramp/inclined plane acts as a machine

Answer 6

It allows a smaller force to be applied by ‘spreading it’ over a larger distance

Question 7

What does the area under the curve show in a Force vs. Displacement graph?

Answer 7

Work Done (Energy Transferred)

Question 8

Define Power and state the equation needed to calculate it

Answer 8

The rate of energy transfer

Power (J/s) = Work Done (J) / Time Taken (s)
P = W / t

Question 9

Show that P = Fv

Answer 9

Power = Work / Time
Power = (Force X Displacement) / Time
Displacement / Time = Velocity 
Therefore Power = Force X Velocity
P = Fv

Question 10

State the equation for Kinetic Energy

Answer 10

K.E. = ½mv²

Question 11

Give the 6 forms of energy

Answer 11

1) Potential energy
2) Kinetic energy
3) Thermal energy
4) Chemical/nuclear energy
5) Electrical energy
6) Elastic energy

Question 12

Define joules

Answer 12

A measurement of energy where 1 joule is equal to the energy needed to raise a 1N weight through a vertical height of 1m

Question 13

State the principle of conservation of energy

Answer 13

Energy cannot be created or destroyed

Question 14

Give the equation for the work done to stretch a spring to extension ΔL

Answer 14

Work done = ½FΔL

Question 15

Define kinetic energy

Answer 15

The energy of an object due to its motion

Question 16

Define potential energy

Answer 16

The energy of an object due to its position

Question 17

Define thermal energy

Answer 17

The energy of an object due to its temperature

Question 18

Define chemical/nuclear energy

Answer 18

The energy associated with chemical or nuclear reactions

Question 19

Define electrical energy

Answer 19

The energy of electrically charged objects

Question 20

Define elastic energy

Answer 20

The energy stored in an object when it is stretched or compressed

Question 21

Give the equation for the kinetic energy of a pendulum bob at height h, released from height h₀

Answer 21

kinetic energy = loss of potential energy from maximum height
½mv² = mg (h₀ - h)

Question 22

Give 2 ways in which energy can be transferred from one object to another

Answer 22

1) Work done - by a force due to one object making the other object move
2) Heat transfer - from a hot object to a cold object. Heat transfer can be due to conduction, convection or radiation

Question 23

Give the 3 ways heat can be transferred

Answer 23

Conduction
Convection
Radiation

Question 24

State the name for the name for the output power of an engine

Answer 24

Motive power

Question 25

Describe the resistive forces in relation to the motive force when an object moves at a constant velocity at constant height

Answer 25

The resistive forces (e.g. friction, drag, air resistance) are equal and opposite to the motive force

Question 26

Give the equation for the power of an engine in terms of its velocity

Answer 26

Work done per second = force x distance moved per second

P = Fv

Question 27

Describe the motion of an object when its output force exceeds the resistive forces on it

Answer 27

It speeds up

Question 28

Give the equation for the motive power of an object in terms of energy wasted

Answer 28

Motive power = energy wasted per second (due to resistive forces) + Gain of kinetic energy per second

Question 29

Give the equation for the output of a machine

Answer 29

P(out) = Work done by the machine / time taken
Output power, P(out) = Fv
where F = output force of machine and v = velocity of object

Question 30

Define useful energy

Answer 30

The energy transferred for a purpose

Question 31

Give the equation for efficiency of a machine

Answer 31

Efficiency = Useful energy transfered / energy supplied
= work done / energy supplied
= output power / input power

Question 32

Give the equation for percentage efficiency of a machine

Answer 32

% Efficiency = efficiency x 100%

Question 33

Give the average electrical power generated by a single wind turbine in a suitable location

Answer 33

2MW

Question 34

Give the equation for the kinetic energy per second of the wind passing through a wind turbine

Answer 34

Kinetic Energy per second = ½ρv²vA = ½ρv³A

where ρ is the density of air, v is the speed of the wind and A is the area of the cylinder of air passing through the turbine

Question 35

Describe the principle of how hydroelectricity and tidal power stations work

Answer 35

They make use of the potential energy released by water when it runs to a lower level

Question 36

Describe the difference between a tidal power station and a hydroelectric power station

Answer 36

Tidal power stations drop more water but over a smaller distance than a hydroelectric power station
Tidal power stations release more energy per day

Question 37

Explain why solar panels in space would absorb more solar energy than solar panels at the Earth’s surface

Answer 37

Because some solar energy is absorbed in the atmosphere

Question 38

Describe how a solar panels produces electricity

Answer 38

A solar panel produces electricity directly
A potential difference is produced across each cell when light is incident on the cell
This converts light energy into electrical energy

Question 39

Give 2 ways in which energy demands could be reduced

Answer 39

Better insulation in homes and buildings

More efficient machines

Question 40

Give 3 ways in which carbon emissions could be cut significantly

Answer 40

• Using more renewable sources of energy
• Carbon capture of carbon emissions from fossil-fuel power stations
• Road transport to be switched from fossil fuels