Energy Flashcards
(1) The hosepipe is painted black because blackened surfaces are
good absorbers of radiation
(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.
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)
(3) Explain why the water reaches a constant temperature.
pipe / water absorbs heat (1) • pipe radiates heat (1) • radiation (rate) increases with temperature(1) • (at constant temperature) absorption rate = radiation rate (1)
energy incident on the heater 10 000 J energy reflected by the heater 6 000 J Calculate the power absorbed by the heater.
4000 (1)
4000)/200 (1
(2) Explain the meaning of the term 15% efficient.
15 % of power /energy (1)
• is transferred usefully (1)
(2) Draw a labelled energy flow diagram to show what happens to 100 J of
electrical energy supplied to the lamp.
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)
(2) Many people choose to buy expensive low-energy lamps instead of cheaper
filament lamps.
Give two reasons for this
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)
(3) When a filament lamp is in use, the temperature of the wire filament remains at
2500 °C.
Explain why this temperature remains constant.
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)
(1) The solar heating panels are painted black because
black is a good absorber of heat
(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.
• {energy / heat / radiation}
is lost (1)
• (heat lost) = heat gained /
absorbed (1)
• rate (of heat loss) = rate
(of heat gained) (1)(1) Which energy transfer takes place in a solar cell?
light to electrical
(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.
substitution (1)
5 000 000 / 21 700
evaluation (1)
230 W
(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.
substitution (1)
5 x 100 / 25
evaluation (1)
20(%)
(1) State one example of a non-useful energy transfer in the motor.
(transfer of energy to) thermal
1
(1) energy supplied= 1400 kJ
useful energy = 1300 kJ
Calculate the amount of energy wasted in one second in the generator.
1400 – 1300 (= 100) (kJ)
1
(2) energy supplied= 1400 kJ
useful energy = 1300 kJ
Calculate the efficiency of the generator.
Substitution (1)
1300 / 1400 x 100
Evaluation (1)
93(%) or 0.93
(1) The electric motors which drive the wheels are painted black.
Suggest why the motors are painted black.
black is a good thermal radiator
(1)
(helps to) prevent motors
overheating (1)
(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
conservation of energy
(1) input energy 200 J, energy transferred to the weight
50 J,. How much energy is wasted?
150 (J)
(2) Calculate the efficiency of the motor
substitution (1)
50 ÷ 200 (x 100%)
evaluation (1)
25 (%)
(1) The case of the motor is painted black.
Give a scientific reason why the case of the motor is painted black.
(black) is a good {(thermal)
emitter / radiator}
(1) The output power of a solar panel is the rate of transfer of
energy
(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.
substitution (1)
800 x 0.4 / 800 x 40
evaluation of payment (1)
(£)320 / 32000 (p)
evaluation of payback time (1)
15 (years)
