Energy transfers

Question 1

State 2 equations for the power of a circuit. Give appropriate units

Answer 1

P = I V
P = I² R
Power (watts), current (amps), potential difference (volts), resistance (ohms).

Question 2

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

Answer 2

E = P t

Energy (joules), power (watts), time (seconds).

Question 3

State and equation liking 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 2 main factors does the amount of energy transferred by an appliance depend on?

Answer 4

• How long the appliance is being used for.

- The power of the appliance.

Question 5

Describe the energy transfer 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 3 things determine the power of a circuit device?

Answer 7

• The potential difference across a circuit.
• The current through the circuit.
• 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 2 types of transformers used in the national grid?

Answer 9

• Step up transformers.

- 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 potential difference and lower current.

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 building like house.
• They decrease the potential difference and increase the current.

Question 12

Why do transmission lines transfer electricity at high potential differences?

Answer 12

• A high potential difference results in a low current.
• The lower the current, the less energy that is waste as heat.
• Therefore, it is more efficient.

Question 13

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

Answer 13

• Lower potential differences are safer for domestic uses and reduces the likelihood of severe electrocution.
• Appliances are designed for 230V.