Work, Power, Efficiency

Question 1

Define work

Answer 1

When a force acts on an object, causing it to move, then work is done.

Question 2

Write out the word equation for work

Answer 2

Work = force x distance moved in direction of force
or
Work = force x displacement

Question 3

Write out the symbol equation for work

Answer 3

W = F x s

Question 4

Prove
The increase in gravitational potential energy equals what in terms of work done.
In an instant of walking up some stairs or the roller coaster car being raised
e.g. mass = 50kg

height = 20m

Answer 4

The increase in gravitational potential energy equals the work done
mass
Ep = mgh
Ep = 50 x 10 x 20
Ep = 10000J

W= F x s Force = weight (the force needed to walk up stairs is the weight) = m x g
W= (50 x 10) x 20
W= 10000

Question 5

Units for:
Work
Force
Displacement

Answer 5

Work in Joules = J
Force in Newtons = N
Displacement in metres = m

Question 6

Give examples that illustrate the definition of power

Answer 6

Two electric motors can lift the same weight through the same height but one does it faster
than the other. The work done is the same but the one which took the least amount of time has the larger power output

Question 7

Fill in the gap

The more powerful an appliance is, the _____ the rate at which it transfers ________

Answer 7

The more powerful an appliance is, the faster the rate at which it transfers energy

Question 8

Define power

Answer 8

Power is the work done or energy used in one second

Question 9

Energy and work are the same thing

Answer 9

Energy and work are the same thing

Question 10

Write out the word equation which links power work done and time

Answer 10

Power = work done / time

Question 11

Write out the word equation which links power energy and time

Answer 11

Power = energy / time

Question 12

Write down the word equation for power

Answer 12

Power = work done / time
Power = energy / time

Question 13

Write down the symbol equation for power

Answer 13

P = W/t or P=E/t

Question 14

Units for:
Power
Work done
Energy
Time

Answer 14

Power = Joules per second - J/s or Watts - W
Work done = Joules, J
Energy = Joules, J
Time - Seconds, s

Question 15

what does 50 watts mean

Answer 15

50 joules of electrical energy transferred per second

Question 16

What does 1 joule per second equal to

Answer 16

1 joule per second is equal to a power of 1 watt

Question 17

What is a 1 joule

Answer 17

1 Joule is the amount of energy needed to move 1N, 1 metre

Question 18

Define efficiency

Answer 18

The efficiency is the percentage of the input energy which is converted into a useful form

Question 19

Write down the word equation for efficiency

Answer 19

Efficiency = useful energy out / total energy in x100

It is multiplied by 100 to convert the answer to a percentage.
A decimal efficiency is fine

Question 20

Write down the word equation for efficiency

Answer 20

efficiency = useful power out/total power in x100

Question 21

What does this statement mean?
Fridge A is more efficient than Fridge B

Answer 21

It means that Fridge A converts more of it’s input energy into a useful form

Question 22

Describe how friction causes devices wastes energy

Answer 22

Friction causes devices to waste energy as Friction between the moving parts causes heating

Question 23

Describe how the resistance wires causes devices wastes energy

Answer 23

The resistance of a wire causes the wire to get hot when a current passes through it

Question 24

Describe how air resistance causes devices wastes energy

Answer 24

Air resistance causes a force on a moving object that opposes its motion
Energy transferred from the object to the surroundings by this force is wasted.

Question 25

Describe how sound causes devices wastes energy

Answer 25

Sound created by machinery causes energy transfer to the surroundings

Question 26

How to increase the efficiency of a motor and reduce the problem of friction

Answer 26

To increase efficiency, lubricate the moving parts of the motor to reduce friction

Question 27

How to increase the efficiency of a circuit and reduce the problem of the resistance of wires

Answer 27

To increase efficiency, in circuits, use wires with as little electrical resistance as possible

Question 28

How to increase the efficiency of a moving object and reduce the problem of air resistance

Answer 28

To increase efficiency, streamline the shapes of moving objects to reduce air resistance

Question 29

How to increase the efficiency of machinery and reduce the problem of sound

Answer 29

To increase efficiency, cut out noise (for example, tighten loose parts to reduce vibration)

Question 30

Explain why the percentage efficiency of a machine can never be more than 100%

Answer 30

efficiency = useful energy delivered/ input energy supplied
A machine can never be more than 100% efficient because the useful energy delivered can never be greater than the total energy supplied.

Question 31

Explain why the efficiency of a machine can never be equal to 100% (4 marks)

Answer 31

A machine can never be 100% efficient since the useful energy delivered is always less than the total energy supplied

due to electric currents in circuit wires/components

and friction between moving parts,

energy is dissipated by transferring thermal energy to the surroundings

Question 32

Fridge A is more efficient is more efficient than Fridge B
What does this statement mean?

Answer 32

This statement means:

Fridge A converts more of the input energy into a useful form