(2) Energy Transfers

Question 1

Equations for power

Answer 1

Power= current x potential difference

Power= current² x resistance

Power= watts Current= amps Potential difference= volts Resistance= ohms

Question 2

Equation for energy using power and time

Answer 2

Energy transferred= power x time
(E=Pxt)
Energy= Joules Power= Watts Time= seconds

Question 3

Equation for energy using charge and P.D

Answer 3

Energy transferred= charge x P.D
(E=QxV)
Energy= Joules Charge= coulombs Potential difference= Volts

Question 4

How is the power transfer in any device related to potential difference and current?

Answer 4

If either potential difference or current increases, the power increases

Question 5

What 2 main factors does the amount of energy transferred by and appliance depend on?

Answer 5

1. How long it’s being used for
2. The power of the appliance

Question 6

Describe the energy transfers in a battery powered motor

Answer 6

The battery converts chemical energy into electrical energy
- motor converts electrical energy into kinetic energy as well as thermal waste energy (due to friction)

Question 7

Describe the energy transfers in a battery powered torch

Answer 7

The battery converts chemical energy into electrical energy
- the bulb converts electrical energy into light energy and thermal waste energy

Question 8

What 3 things determine the power of a circuit device?

Answer 8

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

Question 9

What is the National Grid?

Answer 9

The national grid is a system of cables and transformers linking power stations to consumers

Question 10

What is the National Grids purpose?

Answer 10

To distribute electricity across the country

Question 11

Name the 2 types of transformers used in the national grid

Answer 11

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

Question 12

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

Answer 12

• they are used when connecting power stations to transmission cables
• they increase the potential difference and reduce the current (voltage is increased from about 25,000v to 400,000v causing the current to decrease, less current means less energy is lost through heating the wire)

Question 13

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

Answer 13

• they are used to connect transmission cables to domestic buildings like houses
• they reduce the potential difference from the transmission voltage to a safer one (230v)

Question 14

Why do transmission lines transfer electricity at high voltages?

Answer 14

• high voltage means low current
• lower the current, the less energy is wasted as heat
  (This makes the National Grid more efficient overall)