P2 More on Power (page 188)

Question 1

What does it mean when an electrical charge goes through a change in potential difference?

Answer 1

It means that energy is transferred.

Question 2

Why is Energy supplied to the charge at the Power Source?

Answer 2

to ‘raise’ it through a potential.

The charge gives up this energy when it ‘falls’ through any potential drop in components elsewhere in the circuit.

Look at diagram 1 on page 188.

Question 3

What is the formula for ‘Energy Transferred’?

Answer 3

Energy transferred (J) E = QV

Q means charge flow (C), V means Potetial difference (V).

see diagram 2 on page 188.

Question 4

Why is it that a battery with a bigger pd will supply mre energy to the circuit for every coulomb of charge which flows round it?

Answer 4

because the charge is raised up “higher” at the start.

Question 5

The motor in an electric toothbrush is attached to a 3 V battery. 140 C of charge passes through the circuit as it is used. Calculate the energy transferred?

Answer 5

E = QV = 140 x 3 = 420 J

The energy is transferred to the kinetic energy store of the motor, as well as to the thermal energy store of the surroundings.

Question 6

As well as energy transferred in a given time, the power of an appliance can be found with what formula?

Answer 6

Power (W) = Potential difference (V) x Current (A)

P = VI

Question 7

A 1.0 kW hair dryer is connected to a 230 V supply. Calculate the current through the hair dryer. Give your answer to two significant figures?

Answer 7

1) Rearrange the equation for current
I = P ÷ V

2) Make sure your units are correct
1.0 kW = 1000 W

3) Then just stick in the numbers that you have.
1 = 1000 ÷ 230 = 4.34… = 4.3 A (to 2 sf.)

Question 8

How do you find the power if you don’t know the potential difference?

Answer 8

To do this, stick V = IR from page 180 into P = VI, which gives you:

P = I²R Resistance (Ω)

Question 9

Calculate the energy transferred from a 200 V source as 10 000 C of charge passes (2 marks)

Answer 9

E = Q x V = 10 000 x 200 (1 mark)
= 2 000 000 J (1 mark)

Question 10

An appliance is connected to a 12 V source. A current of 4.0 A flows through it. Calculate the power of the appliance (2 marks)

Answer 10

P = V x I = 12 x 4.0 (1 mark) = 48 W (1 mark)

Question 11

An appliance has a power of 2300 W and has a current of 10.0 A flowing through it. Calculate the resistance of the appliance? (3 marks)

Answer 11

R = P ÷ I² (1 mark) = 2300 ÷ 10.0² (1 mark)
= 23 Ω (1 mark).