How does energy get into an electric circuit?

Energy is supplied to an electric circuit by a power source.

How do you work out how much energy has been transferred by an appliance in Kilowatt-hours?

Example: A 2kW heater runs for 4 hours, what is the energy transferred in kWh

To work out the energy in kilowatt hours you multiply the power in kilowatts by the time in hours

Example: Energy in kWh = 2 x 4 = 8kWh

What does the word efficiency mean?

Efficiency is the proportion (percent) of the energy that you put into an appliance that comes out in a useful form

What do the symbols mean in this equation

P = E/t

Example calculation: what is the power of Torch which converts 10J of energy in 2 seconds?

P = Power (W - Watts)

E = Energy (J - Joules)

t = time (s - seconds)

A Torch that converts 10J of energy in 2 seconds has a power of 10/2 = 5W

What do the symbols mean in this equation

E = P x t

Example: How much energy is transferred by a 20W lightbulb that is left on for 3 seconds?

E = P x t

E = Energy (J - Joules)

P = Power (W - Watts)

t = time (s - seconds)

When a 20W light bulb is left on for 3 seconds the energy transferred is 20 x 3 = 48000J

What are the main factors that affect how much an appliance costs when you use it?

The main factors that affect the cost of an appliance are:

The power of the appliance - the more powerful it is the more energy it uses

The time it is used for - the more time it is used for the more energy it uses

The electricity tariff - the more each unit of electricity costs, the higher the bill

Describe the mains electricity supply in the UK

The mains electricity supply in the UK is an alternating current supply.

It has a potential difference of 230V and a frequency of 50Hz (it goes up and down 50 times each second)

What is the difference between an AC and a DC power supply

AC means alternating current. In an AC circuit the current moves backwards and forwards.

DC means direct current. In a DC circuit the current always flows one way.

What are the colours of the three different wires in a plug?

What does each wire do?

Brown = live wire. This takes electricity to the component

Blue = Neutral wire. This takes electricity away from the component

Green/Yellow = Earth wire. Electricity only travels through this if the case comes into contact with the live wire

What are the common hazards that can be found on electricl wiring?

The common hazards are

- loose wiring
- broken wire sheathes exposing the insides of the wires.
- Missing cable grip meansing the wires could be pulled out easily
- Water near electrical appliances

Explain what a short circuit in an appliance is and why it could cause a problem.

A short circuit occurs when a live wire touches the metal casing of an appliance, this means that the electric current bypasses the appliance and flows through the case.

This is dangerous if a person touches the case they could be electrocuted

What is the definition of power?

What is it measured in?

Power is how quickly the energy is transferred from one form into another

Power is measured in Watts

1 Watt = 1 Joule per second

What factors affect the power that is transferred through an appliance

The power transferred by an appliance depends on the current and the potential difference. It can be found from this formula

P = IV

P = Power (W - Watts)

I = current (A - Amps)

V = Potential difference (V - Volts)

When there is a higher potential difference, each coulomb of charge that passes through an appliance transfers more energy, so the energy is transfered faster

When there is a higher current more coulombs of charge pass through the appliance each second. This means the energy is transferred faster so power goes up.

Explain why different appliances have different sized fuses

What are the problems that could occur if the wrong fuse was put into an appliance?

Different appliances need to have different sized fuses because they have different currents running through them.

If you put in a fuse that is too small the fuse will blow even in normal operation. If you put in a fuse that is too large it will blow in normal operation.

Explain how to select which fuse to use for an appliance

For example what fuse would you use for an appliance that draws 3.6 A of current in normal operation

Fuses come in 3A, 5A and 13A sizes. to select the right size of fuse to use you need to pick the smallest fuse that is larger than the normal current of the appliance

For a current that normally draws 3.6A of current you would choose a 5A fuse, because this is the smallest fuse that is larger than the current.

How does energy leave an electric circuit?

Energy is transferred away from a circuit by a resistor or another component with resistance.

What are the safety features of a plug?

The earth wire will take the current away from the case if there is a problem.

When there is a short circuit the current gets very high. This causes the fuse to blow which breaks the circuit and stops any more current.

What is the casing of a plug made of and why?

The casing is made of plastic.

This is becuase it is an insulator, protecting the user from being electrocuted, and is strong and rigid, preventing it from being damaged.

What are the pins of a plug made from and why?

The pins are made of brass.

This is becuase brass conducts electricity. Although it is not as good as copper at conducting electricity it is preferred for the pins becuase it is strong and does not bend

What are the wires in a plug made of and why?

The wires are made of copper. Copper conducts electricity very well and is flexible.

What are the wire sheathes in a plug made of and why?

The sheath is made of rubber. This is becuase it is flexible and is an insulator.

Complete this diagram to show the different parts of a plug