Topic 18: Energy
Homework 8: Work Done & Power 1
Task 1: Quick questions
Interleaving questions (Topic 20: Particle model of matter);
1. What causes the pressure in a gas?
2. How can you increase the pressure in a gas?
3. What is the name given to the random motion of particles in a gas?
4. What are the units used for pressure?
5. What is the equation for Boyle’s law?
Current topic;
6. What is work done the same as?
7. What is the equation that links work done, distance moved along the line of action and force
applied?
8. What is the equation that links energy transferred, power and time?
9. What are the units of power?
10. Define the term power
11. If you want to make a device more powerful, what two variables could you alter?
12. In the example of a car, the engine has to do mechanical work to overcome which resistive
force?
13. A lightbulb is labelled with the value 40W- what does this tell you about the lightbulb in
terms of energy transfers?
14. Object A and object B both do the same amount of work, but object A has a greater power –
what does this tell you about object A compared to object B?
1
15. How would you work out the power wasted by a device?
Task 2: Exam practice
Q1.
The diagram shows a climber part way up a cliff.
(a) Complete the sentence.
When the climber moves up the cliff, the climber
gains gravitational ______________________ energy.
(1)
(b) The climber weighs 660 N.
(i) Calculate the work the climber must do against gravity, to climb to the
top of the cliff.
_______________________________________________________
_______________________________________________________
Work done = _________________________ J
(2)
2
(ii) It takes the climber 800 seconds to climb to the top of the cliff.
During this time the energy transferred to the climber equals the work
done by the climber.
Calculate the power of the climber during the climb.
_______________________________________________________
_______________________________________________________
Power = _________________________ W
(2)
(Total 5 marks)
Q2.
A student finds some information about energy-saving light bulbs.
(a) A 30W light bulb uses 600J of electrical energy in a certain period of time.
In that time, it produces 450 J of light energy. The rest of the energy is
wasted.
(i) Calculate the energy wasted by the light bulb in this period of time.
_______________________________________________________
Wasted energy = _________________ J
(1)
(ii) What happens to the energy wasted by the light bulb?
_______________________________________________________
_______________________________________________________
(1)
(iii) Calculate the efficiency of this light bulb.
_______________________________________________________
_______________________________________________________
Efficiency = ______________________________
(2)
(iv) Calculate the period of time, in seconds, during which the 600 J is
provided to the 30 W light bulb.
_______________________________________________________
_______________________________________________________
Time = _____________ s
(2)
3
(b) A company that makes light bulbs provides information about some of their
products.
The table shows some of this information.
Power in watts Lifetime in hours Cost of bulb in £
Filament bulb 60 1250 2.00
LED bulb 12 50 000 16.00
A homeowner is thinking about replacing his filament bulbs with LED bulbs.
A 12 W LED bulb gives the same light output as a 60 W filament
bulb.
Suggest reasons why the homeowner is likely to choose LED bulbs.
Use the information given in the table.
_______________________________________________________
_______________________________________________________
_______________________________________________________
_______________________________________________________
(2)
(Total 8 marks)
Q3.
Figure 1 shows a lift inside a building.
Figure 1
4
(a) The motor in the lift does 120 000 J of work in 8.0 seconds.
Calculate the power output of the motor in the lift.
Use the equation:
___________________________________________________________
___________________________________________________________
___________________________________________________________
___________________________________________________________
Power output = _____________________ W
(2)
(b) The power input to the motor is greater than the power output.
Tick two reasons why.
Energy is transferred in heating the
surroundings.
Friction causes energy to be transferred in non-
useful ways.
The motor is connected to the mains electricity
supply.
The motor is more than 100% efficient.
There are only four people in the lift.
(2)
(Total 4 marks)
Q4.
Figure 1 shows a cyclist riding along a straight, level road at a constant speed.
Figure 1
5
(a) Complete the sentences.
As the cyclist rides along the road, the _______________________
energy store
in the cyclist’s body decreases.
The speed of the cyclist is constant when the work done by the cyclist is
_______________________ the work done against air resistance.
(2)
Figure 2 shows how the speed changes as the power output of the cyclist
changes.
Figure 2
(b) Write down the equation that links power, time and work done.
___________________________________________________________
(1)
(c) Calculate the work done by the cyclist when his power output is 200 W for
1800 seconds.
___________________________________________________________
___________________________________________________________
___________________________________________________________
___________________________________________________________
Work done = ______________________ J
(3)
(Total 6 marks)
