GCSE AQA Physics - Electricity

Electrical current

The flow of electrical charge.

Size of current

The rate of flow of charge.

Potential difference

The energy transferred per unit of charge.

Resistance

Opposition to flow of electrical current.

The greater the resistance across a component (2)

The smaller the current that flows for a given potential difference.

What is this component?

Diode

What is this component?

Thermistor

What is this component?

Light Dependent Resistor (LDR)

What is this component?

Fuse

What is this component?

Light-emitting diode

Ammeter

Measures the current.

How must the ammeter be placed?

In series

Voltmeter

Measure the potential difference.

Relationship between length of wire and resistance

Resistance is directly proportional to length.

Ohm’s law

Potential difference across a conductor is directly proportional to the current flowing through it, and resistance is constant.

Ohmic conductors

A conductor that follows ohm’s law, having a constant resistance.

What component’s IV characteristic is this?

Ohmic conductor/variable resistor

What component’s IV characteristic is this?

Filament lamp

What component’s IV characteristic is this?

Diode

LDR (Light Dependent Resistor)

A resistor that is dependent on light intensity.

How is an LDR affected by light?

• In bright light, resistance falls

• In darkness, resistance is the highest

Thermistor

A temperature dependent resistor.

Explain why a cooling fan does not run when temperature is low in a sensing circuit

• The thermistor has a high resistance

• So the thermistor has high potential difference

• And so potential difference across the cooling fan is low

Explain why a cooling fan runs when temperature is high in a sensing circuit

• The thermistor has a low resistance

• So the thermistor has a low potential difference

• And so potential difference across the cooling fan is high

How come a light sensing circuit has the opposite effect to a temperature sensing circuit?

The LDR is connected in parallel to the lamp, so in the darkness resistance for the LDR and the lamp will be highest, and therefore potential difference as well.

Current in series circuits

the same flows through all components

Potential difference in series circuits

shared between components

Resistance in series circuits

adds up

Current in parallel circuits

shared between branches

Potential difference in parallel circuits

the same across each branch

resistance in parallel circuits

less than the resistance of the smallest resistor

Alternating current (AC)

Current that is constantly changing direction.

UK mains supply voltage

230V

UK mains supply frequency

50Hz

Direct current

Current that is always flowing in the same direction.

How is AC created?

By alternating voltage, in which the positive and negative ends continuously reverse.

Live wire

Brown wire that provides the alternating potential difference from the mains supply.

Neutral wire

Blue wire that completes the circuit from the appliance back to the supply. It is around 0V.

Earth Wire

green and yellow - Only carries current when there’s a fault to stop the appliance casing from becoming live.

How can the live wire give you an electric shock? (3)

• Your body is at 0V

• If you touch the live wire a large potential difference is produced and current flows through your body

• This causes an electric shock

Why is any link between the live wire and earth dangerous?

If the link creates a low resistance path, a huge current will flow which could start a fire.

Power

The rate of energy transfer.

Power rating

The maximum safe power an appliance can operate at.

Electrical appliances

Designed to bring about energy transfers.

The national grid

A giant system of cables and transformers that distributes electrical power from the power stations to the consumers.

Why do power stations usually run below their maximum power output?

So they can spare electricity to deal with a higher demand.

How do power stations produce enough electricity for everyone when they need it?

By predicting when demand will change. For example when people wake up in the morning, or during a sporting event.

Why does the national grid use a high potential difference and a low current?

To decrease the energy lost by heating.

Transformer

A device which can change the potential difference of an ac supply.

Step-up transformer

Increases potential difference.

Step-down transformer

Decreases potential difference.

power in primary coil =

power in secondary coil

Static charge

An electrical charge which is not free to move, which usually forms on insulators.

How is a static charge created? (3)

• Two insulating materials are rubbed together

• Electrons will be transferred from one material to the other

• This leaves a positive static charge on one and an equal negative on the other

How does a spark occur? (2)

• Two objects have a high potential difference between them

• This causes electrons to jump across the gap between them

Like charges

Repel

Opposite charges

Attract

Electric field

A region in which an electrically charged object will feel an electrostatic force.

Direction electric field lines

Positive to negative

Explain sparks with electric fields (4)

• There is a high potential difference between two objects

• This causes a strong electric field between the objects

• The strong electric field causes electrons in air particles to be removed

• The air is now much more conductive, allowing a current to flow through it.