Energy and Efficiency

Calculating Efficiency.

Efficiency (%) = Useful Energy Out x 100 Total Energy In

Questions

1) A light bulb takes in 30J of energy per second. It transfers 3J as useful light energy and 27J as heat energy. Calculate the efficiency.

2) A kettle takes in 2000J of energy per second. It transfers 1500J as useful heat energy and 500J is wasted as sound energy. Calculate the efficiency of the kettle.

Remember: In the exam they may not ask for efficiency as a percentage!!!

3) Calculate the efficiency of a hair dryer which takes in 3000J of energy per second and transfers 600J as useful heat energy. Express your answer as a decimal and not as a percentage.

4) Calculate the efficiency of a TV which takes in 5000J of energy per second and transfers 1000J as useful light energy and 1500J as useful sound energy. The remaining 2500J is wasted as heat energy. Express your answer as a decimal and not a percentage.

Extended Questions (Includes Work, Energy & Power)

1. Calculate the efficiency of a light bulb that gives of 40J of light from 200J of electrical energy.

2. Calculate the percentage efficiency of a motor that does of 60J of work from 240J of electrical energy.

3. Calculate the efficiency of a radio that gives of 30J of sound from 360J of electrical energy.

4. Calculate the efficiency of a runner who produces of 500J of kinetic energy from 2500J of chemical energy.

5. Calculate the percentage efficiency of a student who takes 2 hours to do 30 minutes of homework.

6. What is the useful light output from a bulb of efficiency 0.25 when supplied with 600J of energy?

7. What is the useful work output from a motor of efficiency 0.60 when supplied with 1200J of energy?
8. What is the useful sound output from a radio of efficiency 50% when supplied with 20J of energy?

9. What is the useful work done by a runner of efficiency 0.30 when supplied with 200J of energy?

10. How long does it take a student of efficiency 40% to do a 30 minute homework?

11. A light bulb is rated as 40W (a) How much electrical energy does this bulb use in one second? (b) Calculate the efficiency of the bulb if it gives out 5J of light in one second. (c) What is the percentage efficiency of the bulb? (d) What is the useful power output of the light bulb? (e) How much light energy should the bulb give out in 3 minutes? (f) How much heat energy should the bulb give out in 3 minutes?

12. An electric motor pulls a mass of 4kg up by 3 metres in 5 seconds when supplied with 360J of electrical energy. (a) What is the weight of the mass? (b) How much work does the motor do? (c) What is the power of the motor? (d) What is the percentage efficiency of the motor? (e) What is the power supplied to the motor?

13. A runner of mass 120kg maintains a speed of 5 m/s while using chemical energy from food at a rate of 4500J per second. Calculate: (a) The runner’s kinetic energy (b) The input power to the runner from food (c) The efficiency of the runner

Equations to use:

efficiency = useful output ÷ total input

efficiency = useful energy output ÷ total energy input efficiency = work output ÷ total energy input efficiency = useful power output ÷ total power input percentage efficiency = efficiency x 100

weight = mass x gravitational field strength (which is 10 N/kg) work = force x distance moved kinetic energy = ½ x mass x (speed)2 gravitational potential energy = mass x gravitational field strength x height power = energy change / time taken OR = work done / time taken