2.4 Energy transfers

Question 1

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

Answer 1

P = IV

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 = QV

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

● How long the appliance is being used for

● The power of the appliance

Question 5

Describe the energy transfers in a battery powered torch

Answer 5

The battery converts chemical energy into electrical energy. The bulb converts electrical energy into light as well as waste energy in the form of heat energy.

Question 6

Describe the energy transfers in a battery powered motor

Answer 6

The battery converts chemical energy into electrical energy. The 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

● The potential difference across the 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 two 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 the potential difference and decrease the 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 buildings (like houses). They decrease the potential difference and increase the current.

Question 12

Why do transmission lines transfer electricity at a high potential difference?

Answer 12

A high potential difference 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 difference need to be decreased between transmission lines and houses?

Answer 13

● A lower potential difference is safer for domestic use
and reduces the likelihood of severe electrocution.

● Appliances are designed for 230V.