Electric Circuits & Domestic Electricity

What is an electric current?

The rate of flow of electric charge, measured in amperes (A).

What is the equation linking charge, current and time?

Q = It

What is the unit of charge?

Coulomb (C).

What is the unit of current?

Ampere (A).

What is one ampere equal to?

One coulomb of charge passing a point every second.

What is potential difference?

The energy transferred per unit charge between two points in a circuit.

What is the unit of potential difference?

Volt (V).

What does 1 volt mean?

1 joule of energy is transferred for every coulomb of charge.

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

E = QV

What is resistance?

A measure of how much a component opposes the flow of current.

What is the unit of resistance?

Ohm (Ω).

What is Ohm's Law?

Current is directly proportional to potential difference if the temperature remains constant.

What is the equation linking voltage, current and resistance?

V = IR

What is electrical power?

The rate at which energy is transferred electrically.

What is the unit of power?

Watt (W).

What is one watt equal to?

One joule of energy transferred per second.

What is the equation linking power, voltage and current?

P = VI

What is the equation linking power, current and resistance?

P = I²R

What is the equation linking energy, power and time?

E = Pt

What is the equation linking energy, current, voltage and time?

E = IVt

What happens to the current in a series circuit?

It is the same at every point in the circuit.

What happens to the potential difference in a series circuit?

It is shared between the components.

What happens to the resistance in a series circuit?

The total resistance is the sum of the individual resistances.

State the equation for total resistance in series.

Rtotal = R1 + R2 + ...

Which resistor gets the largest share of the voltage in series?

The resistor with the greatest resistance.

What happens if one component is removed from a series circuit?

The whole circuit is broken and everything stops working.

What happens to the voltage across identical resistors in series?

The voltage is shared equally.

Why is voltage shared in a series circuit?

The battery's energy per unit charge is shared between the components.

What happens to the current in a parallel circuit?

It splits between the branches.

State the equation for current in a parallel circuit.

Itotal = I1 + I2 + ...

What happens to the voltage in a parallel circuit?

It is the same across every branch.

What happens to the total resistance when another branch is added in parallel?

The total resistance decreases.

Why does resistance decrease when components are added in parallel?

There are more paths for current to flow.

What happens if one branch is removed from a parallel circuit?

The other branches continue to work.

Why are household circuits connected in parallel?

Each appliance receives the full mains voltage and works independently.

What instrument measures current?

An ammeter.

How is an ammeter connected?

In series.

Why must an ammeter be connected in series?

So all the current flows through it.

What instrument measures potential difference?

A voltmeter.

How is a voltmeter connected?

In parallel across the component.

Why is a voltmeter connected in parallel?

To measure the potential difference across the component.

What is an ohmic conductor?

A conductor that obeys Ohm's Law.

What does the I-V graph of an ohmic conductor look like?

A straight line through the origin.

What does the straight-line I-V graph show?

Resistance is constant.

Why does a filament lamp not obey Ohm's Law?

Its resistance increases as it gets hotter.

What does the I-V graph of a filament lamp look like?

A curve that becomes less steep as voltage increases.

Why does the filament lamp graph flatten?

Increasing temperature increases resistance.

How does a diode behave?

It allows current to flow in one direction only.

What does the I-V graph of a diode look like?

Almost no current in reverse direction and a steep rise in forward direction.

What is a diode used for?

Rectifying alternating current into direct current.

How does a thermistor behave as temperature increases?

Its resistance decreases.

What are thermistors used for?

Temperature sensors and thermostats.

How does an LDR behave as light intensity increases?

Its resistance decreases.

What are LDRs used for?

Automatic lights and burglar alarms.

How does the resistance of a wire change as its length increases?

It increases.

What is the relationship between resistance and wire length?

Resistance is directly proportional to length.

Why should the current be kept low during the wire resistance experiment?

To stop the wire heating up and changing its resistance.

How is resistance calculated during the wire experiment?

R = V ÷ I

How is the resistance experiment set up?

Ammeter in series, voltmeter in parallel, crocodile clips to change wire length.

What graph is plotted in the wire resistance experiment?

Resistance against length.

What does the graph of resistance against length show?

A straight line through the origin.

What is the purpose of the National Grid?

To transfer electricity efficiently across the country.

Why is electricity transmitted at high voltage?

To reduce the current.

Why does reducing current reduce energy losses?

Less energy is wasted as heating in the cables.

What type of transformer is used at the power station?

A step-up transformer.

What does a step-up transformer do?

Increases voltage.

What type of transformer is used near homes?

A step-down transformer.

What does a step-down transformer do?

Reduces voltage to a safe level.

State the transformer equation.

Vp/Vs = Np/Ns

What does Vp represent?

Primary potential difference.

What does Vs represent?

Secondary potential difference.

What does Np represent?

Number of turns on the primary coil.

What does Ns represent?

Number of turns on the secondary coil.

What is the UK mains supply voltage?

About 230 V.

What type of current does the UK mains supply provide?

Alternating current (AC).

Why are appliances connected in parallel in homes?

so that each device receives the full supply voltage from the mains. This means each appliance can operate independently of the others. If one appliance is switched off or fails, the others continue to work because they are in separate branches of the circuit. In parallel circuits, the current splits between branches but the voltage across each branch remains the same.

What is the live wire?

The wire that carries the alternating potential difference.

What colour is the live wire?

Brown.

What is the neutral wire?

The wire that completes the circuit and is close to 0 V.

What colour is the neutral wire?

Blue.

What is the earth wire?

A safety wire connected to the metal case of an appliance.

What colour is the earth wire?

Green and yellow.

Why is the earth wire important?

It prevents the metal case becoming live if a fault occurs.

What does a fuse do?

It melts if the current becomes too large, breaking the circuit.

Why is a fuse important?

It protects the appliance and wiring from overheating and fire.

What determines the fuse rating used?

It should be slightly above the appliance's normal operating current.

What is a circuit breaker?

A safety device that automatically switches off the circuit if the current becomes too large.

What advantage does a circuit breaker have over a fuse?

It can be reset instead of replaced.

What causes an electric shock?

Current flowing through the body.

Why is water dangerous around electricity?

Water (especially with impurities) conducts electricity, increasing the risk of electric shock.

What two things increase the severity of an electric shock?

Higher current and longer exposure time.

Why are metal cases earthed?

So a fault current flows safely to earth instead of through a person.

What happens if the live wire touches the metal case of an earthed appliance?

A large current flows through the earth wire, causing the fuse to melt or the circuit breaker to trip.

What is meant by electrical energy transfer?

Electrical energy being transferred into other energy stores, such as thermal, light or kinetic.

What energy transfers occur in a kettle?

Electrical → thermal.

What energy transfers occur in a lamp?

Electrical → light and thermal.

What energy transfers occur in an electric motor?

Electrical → kinetic and thermal.

What is the relationship between power and energy?

Power is the rate of energy transfer.

If power stays constant, what happens to energy transferred as time increases?

It increases proportionally.