Questions

For each question show all your working clearly.

1.      A lot of energy is wasted in a car. For every
100 J of chemical energy in the petrol, only
25 J are transferred to useful kinetic energy.
The rest just heats up the engine and the air.

a)      Calculate the efficiency. 25%

2.       The diagram shows the energy transfers for a
Bunsen burner heating a beaker of water.
What is its efficiency as a water heater? 40%

3.       In a solar cell, for every 80 J of solar energy
shining on it, only 4 J is transferred to useful
energy (as electricity).

a)      What happens to the other 76 J? reflected, lost as low grade hear

b)      What is its efficiency? 5%

c)       

4.       A pulley system lifts a load and gives it
6000 J of potential energy. The person
pulling on the rope gives it 8000 J of energy.
What is the efficiency?  75%

5.       An electric kettle has a power rating of 2 kW
and is switched on for 100 seconds.
While heating up, it loses 60 000 J to the
surroundings.

a)      How much energy is supplied to the kettle?
(1 kW = 1000 W = 1000 joules per second)200kJ

b)      How much is given to the water? 60kJ

c)      What is the efficiency of heating water? 70%

6.       An electric motor on a building site has a
power rating of 400 W and lifts a load of
bricks weighing 600 N through a height of
10 m in 20 seconds.

a)      How much energy is needed to lift the bricks? 6000J

b)      How much energy is supplied to the motor
in 20 seconds? 8000J

c)      What is the efficiency of the motor in
doing this job? 75%

Efficiency

1. An energy efficient light bulb is rated at 20W. It produces 5W of light. Calculate its efficiency.

5/20 x 100 = 25%

2. Christine transfers 40 000J of chemical energy during a race. She transfers 32 000J of heat energy to the surroundings during the race. Calculate her efficiency 32/40 x 100 = 80% (fallen into the trap!)

3. Geraint does 1600J of work turning the pedals on his bike. 1577.6J is transferred to the rear sprockets. How much heat is lost and what is the efficiency of Geraint’s bike. 1577.6/1600 x100 = 98.6%

4. The coal in Thomas’ boiler contains 40 kJ of energy. He does 2576J of useful work as he puffs along a branch line. Calculate Thomas’ efficiency.

2.576/40 x100 = 6.44%

5. The fuel in Diesel’s tank contains 60kJ of chemical energy. He does 20.1kJ of work on the mainline. Calculate Diesel’s efficiency
6. 20.1/60 x100 = 33.5%

7. Jeremy is the proud owner of an electric car. The electric battery in Jeremy’s car can develop 3kW. If the engine develops 2.658kW calculate its efficiency. 2.685/3 x100 = 88.6%

8. Jeremy dreams of a Ferrari Enzo which can develop a maximum of 700 bhp. (1 bhp = 750 Watts). Sadly for Jeremy petrol cars are not very efficient. Typically, only about 30% of the energy that is available from the combustion of the petrol actually ends up overcoming friction to move the car forwards. Calculate the maximum wattage of the chemical energy in the fuel burnt. 700 bhp x 750W = 5.25 x 10⁵ W

5.25 x 10⁵ x 100 /output = 30%

5.25 x 10⁷ / output = 30

Output = 5.25 x 10⁷ / 30 = 1.75 x 10⁶  W

Of the 70% of energy is that is not usefully converted, 55% heats the cooling water that surrounds the engine block 0.55 x 1.75 x10⁶ W = 9.625 x 10⁵ W whilst  15% may be in the hot exhaust gases. 2.625 x 10⁵ W. Calculate the power lost by the radiator and the exhaust.

To make car engines more efficient the fuel has to burn at a higher temperature and the exhaust must be kept cooler. Where should Jeremy move to improve the efficiency of his car? Pluto