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

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

Answer 2

1. step-up transformers
2. step-down transformers

Question 2

state an equation linking energy transferred, power and time.

Answer 2

E = P t
energy (joules), power (watts), time (seconds)

Question 3

describe the energy transfers in a battery powered torch.

Answer 3

• battery converts chemical energy into electrical energy
• bulb converts electrical energy into light as well as waste energy in the form of heating

Question 3

state an equation linking energy transferred, power and time.

Answer 3

E = Q V
energy (joules), charge (coulombs), potential difference (volts)

Question 3

describe the energy transfers in a battery powered motor.

Answer 3

• 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 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 4

what three things determine the power of a circuit device?

Answer 4

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

Question 5

what is the purpose of the National Grid?

Answer 5

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

Question 6

where are step-up transformers found in the National Grid? what do they do?

Answer 6

• step-up transformers are used when connecting power stations to transmission cables
• they increase the potential difference

Question 6

where are step-down transformers found in the National Grid? what do they do?

Answer 6

• step-down transformers are used in connecting transmission cables to domestic buildings (like houses)
• they decrease the potential difference

Question 7

why do transmission lines transfer electricity at high potentials?

Answer 7

• 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 8

why does the potential need to be decreased between trnasmission lines and houses?

Answer 8

• lower potentials are safer for domestic use and reduce the likelihood of severe electrocution
• appliances are designed for 230V