1.11 - Efficiency

Question 1

What is the law of conservation of energy?

Answer 1

Energy cannot be created or destroyed. It can only be transferred usefully to other objects or forms, or be dissipated to the surroundings as waste energy

Question 2

Define efficiency

Answer 2

The proportion of the energy supplied that is transferred into the useful energy output

Question 3

Define useful and wasted energy.

Answer 3

Useful energy is energy transferred to where it is intended to arrive
Wasted energy is the energy that is not usefully transferred

Question 4

Which of the following are correct formulas for energy efficiency? Select 2 options

A: Eff = Useful Power Output/Total Power Input
B: Eff = Total Energy Input/Total Energy Output
C: Eff = Useful Energy Output/Total Energy Input

Answer 4

A: Eff = Useful Power Output/Total Power Input & C: Eff = Useful Energy Output/Total Energy Input

Question 5

Efficiency is usually given as a decimal, such as 0.6.

How would we convert this to percentage efficiency?

A: Multiply by 100
B: Divide by 100
C: Square it

Answer 5

A: Multiply by 100

Question 6

When provided with an input of 240 W, a set of Christmas lights emit 36 W of light radiation.

What is the percentage efficiency of the Christmas lights?

A: 10 %
B: 5 %
C: 15 %

Answer 6

C: 15 %

Question 7

The government has recommended that the sale of filament lamps should be banned.

Suggest why this is.

A: It would give shops new products to sell
B: Filament lamps look bad
C: Filament lamps are inefficient

Answer 7

C: Filament lamps are inefficient

Question 8

How could a bicycle be made more efficient?

A: Add some lights
B: Add some oil to the chain
C: Cycle faster

Answer 8

B: Add some oil to the chain

Oil acts as a lubricant and so reduces the friction between the chain and the cogs.

Question 9

During the course of a day, an electric motor usefully transfers 1600 kJ. The energy supplied to the electric motor is 4000 kJ

What is the efficiency of the motor?

A: 2.5
B: 2400
C: 0.6
D: 0.4

Answer 9

D: 0.4

Question 10

Rearrange the formula for calculating efficiency to make useful power output the subject.

Answer 10

Useful power output = Efficiency x Total power input

Question 11

What is the equation for waste power?

Answer 11

Waste power = total power input - useful power output

Question 12

The efficiency of a 25 W energy-saving lamp is 0.6.

How much energy does it waste each second?

A: 150 J
B: 15 J
C: 25 J
D: 10 J

Answer 12

D: 10 J

Question 13

No device is 100% efficient.

A speaker has an input power of 30 W, but only 10 W is converted to sound energy.

Suggest what happens to the other 20 W.

A: It is lost as light energy
B: It is used for electricity
C: It is lost as thermal energy

Answer 13

C: It is lost as thermal energy

Question 14

For a light bulb, what is the useful energy output

Answer 14

Light energy

Question 15

For a light bulb, what is the main wasted energy output?

Answer 15

Heat energy

Question 16

When calculating efficiency, the answer should always be between _ and _

Answer 16

When calculating efficiency, the answer should always be between 0 and 1

Question 17

Explain how oiling a bike chain improves the efficiency of a bike. (4 marks)

Answer 17

1 mark - Oil acts as a lubricant
1 mark - This reduces friction between the bike chain and cogs
1 mark - Thus, less work needs to be done to overcome friction
1 mark - This means that less energy is wasted/dispersed as heat