P2 Power of Electrical Appliances (page 187)

Question 1

How is Energy transferred?

Answer 1

between stores electrically (like you saw on page 167) by electrical appliances.

Question 2

Energy is transferred from?

Answer 2

Cells and other sources

Question 3

You know from page 167 that a moving charge transfers energy, why?

Answer 3

because the charge does work against the resistance of the circuit.

Question 4

What does ‘work done’ the same as?

Answer 4

‘work done’ is the same as energy transferred - see page 203.

Question 5

Electrical appliances are designed to do what with energy?

Answer 5

transfer energy to components in the circuit when a current flows.

Question 6

How do Kettles transfer energy?

Answer 6

electrically from the mains ac supply to the thermal energy store of the heating element inside the kettle.

Question 7

How do hand held fans transfer energy?

Answer 7

Energy is transferred electrically from the battery of a handheld fan to a kinetic energy store of the fan’s motor.

Question 8

Do all appliances transfer all energy completely?

Answer 8

No.

Question 9

The higher the current of electricity, the MORE or LESS energy is transferred to the thermal energy stores of the components (and then the surroundings)?

Answer 9

the higher the current, the MORE energy is transferred.

(you can calculate the efficiency of any electrical appliance (see page 172).

Question 10

The total energy transferred by an appliance depends on what?

Answer 10

how long the appliance is on for and its power.

Question 11

What is the power of an appliance?

Answer 11

it is the energy that it transfers per second.

so the more energy it transfers in a given time, the higher its power.

Question 12

What is the equation for the amount of energy transferred by electrical work given?

Answer 12

Energy transferred (J) = Power (W) x Time (s)

Question 13

A 600 W microwave is used for 5 minutes. How long (in minutes) would a 750 W microwave take to do the same amout of work?

Answer 13

E = Pt = 600 x (5 x 60)
= 180 000 J

t = E ÷ P
= 180 000 ÷ 750 = 240 s

240 ÷ 60 = 4 minutes

So the 750 W microwave would take 4 minutes to do the same amount of work. (remember that the time must be in seconds).

workings out:

1) calculate the energy transferred by the 600 W microwave in 5 minutes

2) rearrange E - Pt and sub in the energy you calculated and the power of the 750 W microwave.

3) Convert the time back to minutes.

Question 14

Appliances are often given a power rating, what do this mean?

Answer 14

they’re labelled with a maximum safe power that they can operate at. You can usually take this to be their maximum operating power.

Question 15

What do the power rating tell you?

Answer 15

it tells you the maximum amount of energy transferred between stores per second when the appliance is in use.

Question 16

Why is a power rating useful for customers?

Answer 16

it helps customers choose between models - the lower the rating, the less electricity an appliance uses in a given time, and so the cheaper it is to run.

Question 17

Do a higher power appliance mean it transfers more energy usefully?

Answer 17

no it doesn’t necessarily mean that it transfers more energy usefully. An appliance may be more powerul than another, but less efficient, meaning that it might still only transfer the same amount of energy (or even less) to useful stores ( see page 170).

Question 18

An appliance transfers 6000 J of energy in 30 seconds. Calculate its power (2 marks)

Answer 18

P = E ÷ t = 6000 ÷ 30 (1 mark)
= 200 W ( mark)

Question 19

Calculate the difference in the amount of energy transferred by a 250 W TV and a 375 W TV when they are both used for two hours (4 marks)

Answer 19

E = P x t (1 mark)
= 250 x (2 x 60 x 60)
= 1 800 000 J (1 mark)
E = 375 x (2 x 60 x 60) = 2 700 000 J (1 mark)
so change in energy is
2 700 000 - 1 800 000 = 900 000 J (1 mark)