P6.2. Electricity

What do circuit diagrams use to represent electrical components?

Standard circuit symbols.

What should you be able to do with circuit diagrams?

Draw and interpret them.

What is needed for electrical charge to flow through a closed circuit?

A source of potential difference.

What is electric current?

A flow of electrical charge.

What does the size of an electric current represent?

The rate of flow of electrical charge.

What is the equation linking charge flow, current and time?

Q = It.

What is charge flow measured in?

Coulombs (C).

What is current measured in?

Amperes (A).

What is time measured in?

Seconds (s).

What happens to current at any point in a single closed loop?

It has the same value at every point.

What does the current through a component depend on?

The resistance of the component and the potential difference across it.

What happens to current when resistance increases for a given potential difference?

The current decreases.

What is the equation linking potential difference, current and resistance?

V = IR.

What is potential difference measured in?

Volts (V).

What is resistance measured in?

Ohms (Ω).

What is the required practical for resistance?

Use circuit diagrams to set up and check appropriate circuits to investigate factors affecting the resistance of electrical circuits.

What should you investigate in the resistance practical?

Factors affecting resistance, including the length of a wire at constant temperature and combinations of resistors in series and parallel.

What are the two ways of joining electrical components?

In series and in parallel.

What is a series circuit?

A circuit where components are connected in a single loop.

What is the current through components connected in series?

The same current flows through each component.

What happens to the total potential difference in a series circuit?

It is shared between the components.

What is the equation for the total resistance of two resistors in series?

Rtotal = R₁ + R₂.

What happens to total resistance when resistors are added in series?

It increases.

What is a parallel circuit?

A circuit where components are connected on separate branches.

What is the potential difference across components connected in parallel?

It is the same across each component.

What happens to total current in a parallel circuit?

It is the sum of the currents through the separate branches.

How does adding resistors in parallel affect total resistance?

It decreases.

How does the total resistance of two resistors in parallel compare with the smallest individual resistance?

It is less than the resistance of the smallest individual resistor.

Do you need to calculate the total resistance of two resistors joined in parallel?

No.

What should you be able to do with DC series circuits?

Design and use them for measurement and testing purposes.

What should you be able to calculate in DC series circuits?

Currents, potential differences and resistances.

What is direct potential difference?

A potential difference that is constant in direction.

What is alternating potential difference?

A potential difference that repeatedly changes direction.

What type of supply is UK mains electricity?

An alternating current (AC) supply.

What is the frequency of UK mains electricity?

50 Hz.

What is the potential difference of UK mains electricity?

About 230 V.

What type of cable is normally used to connect mains appliances?

Three-core cable.

What are the three wires in a three-core cable?

Live, neutral and earth.

What colour is the live wire?

Brown.

What colour is the neutral wire?

Blue.

What colour is the earth wire?

Green and yellow stripes.

What does the live wire carry?

The alternating potential difference from the supply.

What does the neutral wire do?

It completes the circuit.

What does the earth wire do?

It is a safety wire to stop the appliance becoming live.

What is the potential difference between the live wire and earth?

About 230 V.

What is the potential difference of the neutral wire relative to earth?

At or close to 0 V.

What is the potential difference of the earth wire?

0 V.

When does the earth wire carry current?

Only when there is a fault.

Why can a live wire be dangerous even when a switch is open?

The live wire may still be at a dangerous potential difference.

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

It can cause a dangerous current to flow.

What is electrical power?

The rate at which energy is transferred in a circuit device.

What is the equation linking power, potential difference and current?

P = VI.

What is the equation linking power, current and resistance?

P = I²R.

What is power measured in?

Watts (W).

What happens when charge flows in a circuit?

Work is done and energy is transferred.

What does the amount of energy an appliance transfers depend on?

How long it is switched on for and its power.

How can domestic appliances transfer energy?

From batteries or AC mains to stores such as the kinetic energy store of an electric motor or the thermal energy store of a heating device.

What is the equation linking energy transferred, power and time?

E = Pt.

What is the equation linking energy transferred, charge flow and potential difference?

E = QV.

What is energy transferred measured in?

Joules (J).

How is the power of a circuit device related to potential difference and current?

P = VI.

How is the power of a circuit device related to energy transferred over time?

Power is the energy transferred per unit time.

What should you be able to describe about domestic appliance power ratings?

How their power ratings relate to changes in stored energy when they are in use.

What is the National Grid?

A system of cables and transformers linking power stations to consumers.

How is electrical power transferred from power stations to consumers?

Using the National Grid.

What does a step-up transformer do?

It increases the potential difference from the power station to the transmission cables.

What does a step-down transformer do?

It decreases the potential difference to a much lower value for domestic use.

Why is the National Grid an efficient way to transfer energy?

Electricity is transmitted at a high potential difference, which means a lower current is needed for a given power, reducing energy losses in the cables.

What equation links the primary and secondary sides of a transformer?

VpIp = VsIs.

What does Vp represent?

The potential difference across the primary coil.

What does Ip represent?

The current in the primary coil.

What does Vs represent?

The potential difference across the secondary coil.

What does Is represent?

The current in the secondary coil.

What should you be able to do with the transformer equation?

What happens to the current when a transformer increases the potential difference? The current decreases, so that power is approximately conserved.

Why does transmitting electricity at a high potential difference reduce energy losses? A higher potential difference means a lower current is needed for the same power, reducing energy wasted as heating in the cables.