Energy

Question 1

(1) The hosepipe is painted black because blackened surfaces are

Answer 1

good absorbers of radiation

Question 2

(2) At first, the temperature of the water in the pipe increases.
After a while, the temperature becomes constant.
Suggest two changes to the box which would increase the constant
temperature reached.

Answer 2

cover box with transparent
material (1)
• use of reflector (1)
• method to increase energy
supplied (1)
• method to reduce energy loss
(1)
• paint (box) black/dull/matt
(1)

Question 3

(3) Explain why the water reaches a constant temperature.

Answer 3

pipe / water absorbs heat (1)
• pipe radiates heat (1)
• radiation (rate) increases
with temperature(1)
• (at constant temperature)
absorption rate = radiation
rate (1)

Question 4

energy incident
on the heater
10 000 J
energy reflected
by the heater
6 000 J
Calculate the power absorbed by the heater.

Answer 4

4000 (1)

4000)/200 (1

Question 5

(2) Explain the meaning of the term 15% efficient.

Answer 5

15 % of power /energy (1)

• is transferred usefully (1)

Question 6

(2) Draw a labelled energy flow diagram to show what happens to 100 J of
electrical energy supplied to the lamp.

Answer 6

two qualitative output labels
(1)
light (energy) and thermal
/heat (energy)
• a quantitative output label
(1)
15 J for light/used /useful
or on narrower arrow (of
otherwise unlabelled Sankey
diagram)

Question 7

(2) Many people choose to buy expensive low-energy lamps instead of cheaper
filament lamps.
Give two reasons for this

Answer 7

Energy, e.g. (low-energy
lamps) are more efficient /
waste less energy / produce
less heat (1)
• Economy, e.g. (low-energy
lamps) use less electrical
energy /cost less to run /
have a lower power (rating)
(1)
• Environment, e.g. using
(low-energy lamps) reduces
CO2 emissions / saves fossil
fuel (1)
• Practical, e.g. (low-energy
lamps) last longer / need
replacing less often / (can
be) easier to obtain (1)

Question 8

(3) When a filament lamp is in use, the temperature of the wire filament remains at
2500 °C.
Explain why this temperature remains constant.

Answer 8

energy gain is from power
supply(1)
• energy loss is by
radiation(1)
• the loss and the gain are
equal /at the same rate(1)

Question 9

(1) The solar heating panels are painted black because

Answer 9

black is a good absorber of heat

Question 10

(3) On one sunny day no hot water is used in the house.
The water in the panels reaches a constant temperature even though the
water is still absorbing energy from the Sun.
Explain why the temperature of the water in the panels becomes constant.

Answer 10

• {energy / heat / radiation}
is lost (1)
• (heat lost) = heat gained /
absorbed (1)
• rate (of heat loss) = rate
(of heat gained) (1)

Question 11

(1) Which energy transfer takes place in a solar cell?

Answer 11

light to electrical

Question 12

(2) A large solar farm has 21 700 solar panels and generates 5.0 MW of power.
1.0 MW = 1.0 106
W
Calculate the average power each panel produces.

Answer 12

substitution (1)
5 000 000 / 21 700
evaluation (1)
230 W

Question 13

(2) The solar farm receives 25 MW of power from the Sun to generate 5 MW of
electrical power.
Calculate the efficiency of the solar farm.

Answer 13

substitution (1)
5 x 100 / 25
evaluation (1)
20(%)

Question 14

(1) State one example of a non-useful energy transfer in the motor.

Answer 14

(transfer of energy to) thermal

1

Question 15

(1) energy supplied= 1400 kJ
useful energy = 1300 kJ
Calculate the amount of energy wasted in one second in the generator.

Answer 15

1400 – 1300 (= 100) (kJ)

1

Question 16

(2) energy supplied= 1400 kJ
useful energy = 1300 kJ
Calculate the efficiency of the generator.

Answer 16

Substitution (1)
1300 / 1400 x 100
Evaluation (1)
93(%) or 0.93

Question 17

(1) The electric motors which drive the wheels are painted black.
Suggest why the motors are painted black.

Answer 17

black is a good thermal radiator
(1)
(helps to) prevent motors
overheating (1)

Question 18

(1) The students read the statement: ‘All the energy supplied to the motor eventually
ends up as thermal energy in the surroundings.’
This statement best describes the idea of

Answer 18

conservation of energy

Question 19

(1) input energy 200 J, energy transferred to the weight

50 J,. How much energy is wasted?

Answer 19

150 (J)

Question 20

(2) Calculate the efficiency of the motor

Answer 20

substitution (1)
50 ÷ 200 (x 100%)

evaluation (1)
25 (%)

Question 21

(1) The case of the motor is painted black.

Give a scientific reason why the case of the motor is painted black.

Answer 21

(black) is a good {(thermal)

emitter / radiator}

Question 22

(1) The output power of a solar panel is the rate of transfer of

Answer 22

energy

Question 23

(3) A homeowner fits a solar panel to her roof.
The cost of the solar panel is £4800.
The solar panel supplies an average of 800 kW h of electrical energy to the
National Grid each year.
The homeowner is paid 40p for each kW h of energy supplied to the National Grid.
Calculate the payback time for the solar panels by selling energy to the National
Grid.

Answer 23

substitution (1)
800 x 0.4 / 800 x 40

evaluation of payment (1)
(£)320 / 32000 (p)

evaluation of payback time (1)
15 (years)