Physics ELECTRICITY

Charge (Q) unit: (c)

a measure of the total current that flowed within a certain period of time

Current (I) unit: (A)

The rate of flow of charge

Voltage (V) unit: (V)

the energy transferred per unit of charge

Resistance (R) unit: (Ω)

opposition to flow of charge - More resistance = less current - Less resistance = more current

Series circuit

Circuit with only one path for current

Parallel circuit

circuit with more than one route for current

Direct current

A type of current where charge only flows in one direction

Alternating current

A type of current where the charge is always changing direction

Earthing

Connecting something to the ground for safety. Commonly done by the earth wire.

Double insulation

A device that has two layers of insulation so that the outside cannot be affected by internal problems. Allows for no Earth wire.

Fuse

A wire in a cylinder used as a safety device. The wire will melt if the current goes above the 'rating of the fuse'. Breaks the circuit.

Circuit Breaker

An alternative to a fuse that does the same thing but can be reset.

Live wire + uses

A brown wire supplied with a voltage of 230V above ground. It's attached to the fuse in series. It's the wire that supplies electrical energy.

Neutral wire + uses

A blue wire that is supplied with just above 0V that completes the circuit between a device and the main energy supply.

Earth wire + uses

A green and yellow wire at 0V that is connected to the Earth. It is a safety feature that devices with double insulation could function without.

Mainstream fuse sizes

3A, 5A and 13A

How do the fuse and Earth wire work together as a safety feature? (example question)

If a fault occurs and the live wire touches the outer metal casing:

- current flows along the live wire, through the metal casing and down to the earth wire.

- The route has low resistance and the current flowing down the live wire is large.

- The wire in the fuse will heat up and melt.

- The circuit breaks so there is no current in the live wire.

Power formula

Power = current x voltage

P = I x V

Energy transferred (fuse) formula

Energy transferred = current x voltage x time

E = I x V x t

Current formula

Charge = current x time

Q = I x t

Voltage formula

Voltage = current x resistance

V = I x R

Explain how Ohms law affects a fixed resistor or wire

- It obeys Ohms law

- the current in these devices is directly proportional to the voltage

- As long as the temperature remains the same

- Straight line through the origin of the graph

Explain how Ohms law affects a filament lamp

- It does not obey Ohms law

- Resistance increases with voltage because the wire continues to get hotter

- This causes the ions to vibrate more

- This causes more collisions with delocalised electrons

- which means more resistance

- As resistance increases, current decreases

- Therefore voltage and current are not directly proportional

- line through the origin but curves on the graph

Explain how Ohm's law affects a light emitting diode

- Does not obey Ohm's law

- In forward bias, as the voltage increases, the current only increases after 0.6V

- The diode has low resistance

- In reverse bias, the current is always at 0 because the diode has high resistance.

State how voltage changes in a series circuit than in a parallel circuit

The sum of all voltage in a series equals the voltage of the power supply. The voltage is the same in parallel.

State how current changes in a series circuit than in a parallel circuit

In a series circuit, the current stays the same all the way round. In a parallel circuit, the current divides between junctions and joins back up at the end.

Advantage of a parallel circuit

Lamps can work independently

Advantage of a series circuit

Lamps can all be turned off at once

How does the fuse protect the circuit?

Flow of current higher than the fuse can support. Wire inside fuse will melt to stop the flow of energy and prevent the appliance from overheating.

Describe the difference between alternating current and direct current.

Alternating current changes direction while direct current only goes one way.

Describe how an earth wire acts as a safety feature.

If casing becomes live, Earth wire connects casing to the ground. Provides low resistance route to the ground. Large current can go through the wire. The wire in the fuse will then melt and switch everything off.

Describe energy change that takes place in lamps

Electrical change to light up and heat up lamps.

(Thermistor) circuit is moved from cold to hot room. How does this affect the brightness of lamp?

Thermistor resistance decreases when temp increases. Current increases therefore brightness increases.