P1 Efficiency (page 172)

Question 1

True or False? Most Energy Transfers involve some Waste Energy?

Answer 1

True

Question 2

Regarding energy, useful devices are only useful because?

Answer 2

they can transfer energy from one store to another.

Question 3

Some of the input energy is usually wasted by?

Answer 3

being transferred to a useless energy store - usually a thermal energy store.

Question 4

The less energy that is ‘wasted’ in this energy store, is more or less efficient than the device is said to be?

Answer 4

the more efficient the device is said to be.

Question 5

how can you improve the efficiency of energy transfers?

Answer 5

by insulating objects, luricating them or making them more streamlined (see page 171 and 210).

Question 6

The efficiency for any energy transfer can be worked out using what equation?

Answer 6

Useful output energy transfer
Efficiency = __________________________________
Total input energy transfer

(you can give efficiency as a decimal or you can multiply your answer by 100 to get a percentage i.e. 0.75 or 75%)

Question 7

You might not know the energy inputs and outputs of a device, but you can still calculate its efficiency, how?

Answer 7

as long as you know the power input and output:

                          Useful power output Efficiency =      \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_
                            Total power input

Question 8

A blender is 70% effiient. It has a total input power of 600 W. Calculate the useful output power?

Answer 8

1) Change the efficiency from a percentage to a decimal
efficiency = 70% = 0.7

2) Rearrange the equation for useful power output
Useful power output = efficiency x total power input.

= 0.7 x 600

3) Stick in the numbers you’re given
= 420 W

Question 9

Is Useful Energy Output usually equal to Total Energy Input?

Answer 9

No

Question 10

Is any device 100% efficient? explain your answer

Answer 10

No, For any given example you can talk about the types of energy being input and output, but remember: NO device is 100% efficient and the wasted energy is usually transferred to useless thermal energy stores.

Apart from Electric heaters they are the exception to this.

Question 11

Why are Eletric heaters 100% efficient?

Answer 11

because all the energy in the electrostatic energy store is transferred to “useful” thermal stores.

Question 12

Ultimately, all energy endsup transferred to the thermal energy stores, give example?

Answer 12

If you use an electric drill, its energy transfers to lots of different energy stores, but quickly ends up all in thermal energy stores.

Question 13

A motor in a remote-controlled car transfers 300 J of energy into the car’s energy stores. 225 J are transferred to the car’s kinetic energy stores. Calculate the efficiency of the motor? (2 marks)

Answer 13

efficiency = useful output energy transfer ÷ total input energy transfer
= 225 ÷ 300 (1 mark) = 0.75 (1 mark)

Question 14

A machine has a useful power output of 900 W and a total power input of 1200 W. In a given time, 72 kJ of energy is transferred to the machine. Calculate the amount of energy usefully transferred by the machine in this time (4 marks)

Answer 14

efficiency = useful output energy transfer ÷ total power input

= 900 ÷ 1200 = 0.75 (1 mark)

Useful output energy transfer - efficiency x total input energy transfer (1 mark)
= 0.75 x 72 000 (1 mark) - 54 000 J (1 mark)