4.2.4 Energy transfers

Question 1

State two equations for the power of a circuit. Give appropriate units.

Answer 1

P = I V
P = I^2 R
Power (Watts), Current (Amperes)
Potential Difference (Volts), Resistance
(Ohms)

Question 2

State an equation linking energy transferred, power and time. Give appropriate units.

Answer 2

E=Pt
Energy (Joules), Power (Watts), Time (Seconds)

Question 3

State an equation linking energy transferred, charge flow and potential difference. Give appropriate units.

Answer 3

E= Q V
Energy (Joules), Charge (Coulombs), Potential Difference (Volts)

Question 4

What two main factors does the amount of energy transferred by an appliance depend on?

Answer 4

1. How long the appliance is being used for
2. The power of the appliance

Question 5

Describe the energy transfers in a battery powered torch.

Answer 5

● Battery converts chemical energy into electrical energy
● Bulb converts electrical energy into light as well as waste energy in the form of heating

Question 6

Describe the energy transfers in a battery powered motor.

Answer 6

● Battery converts chemical energy into electrical energy
● Motor converts electrical energy into kinetic energy as well as waste energy
in the form of heating due to friction

Question 7

What three things determine the power of a circuit device?

Answer 7

1. The potential difference across the circuit
2. The current through the circuit
3. The amount of energy transferred in a given time

Question 8

What is the purpose of the National Grid?

Answer 8

To link power stations to consumers so that they have access to a source of electricity.

Question 9

What are the two types of transformers used in the National Grid?

Answer 9

1. Step-Up Transformers
2. Step-Down Transformers

Question 10

Where are step-up transformers found in the National Grid? What do they do?

Answer 10

● Step-Up Transformers are used when connecting power stations to
transmission cables
● They increase the potential difference

Question 11

Where are step-down transformers found in the National Grid? What do they do?

Answer 11

● Step-Down Transformers are used in connecting transmission cables to
domestic buildings (like houses)
● They decrease the potential difference

Question 12

Why do transmission lines transfer electricity at high potentials?

Answer 12

● A high potential, results in a low current
● The lower the current, the less energy
that is wasted as heat
● Therefore it is more efficient

Question 13

Why does the potential need to be decreased between transmission lines and houses?

Answer 13

● Lower potentials are safer for domestic use and reduces the likelihood of
severe electrocution
● Appliances are designed for 230V