topic 2 - electricity

What is electric current?

The rate of flow of electric charge.

What is the equation for electric current?

Current = charge flow ÷ time; I = Q ÷ t.

What does each symbol mean in I = Q ÷ t?

I = current in amperes (A), Q = charge in coulombs (C), t = time in seconds (s).

What is one ampere?

One coulomb of charge flowing past a point per second.

What is the equation linking charge, current and time?

Q = I × t.

What is potential difference (voltage)?

The energy transferred per unit charge passing through a component.

What is the equation for potential difference?

V = E ÷ Q.

What do the symbols mean in V = E ÷ Q?

V = potential difference in volts (V), E = energy transferred in joules (J), Q = charge in coulombs (C).

What is one volt?

One joule of energy transferred per coulomb of charge.

What is resistance?

A measure of how difficult it is for current to flow through a component.

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

V = I × R.

What do the symbols mean in V = IR?

V = potential difference in volts (V), I = current in amperes (A), R = resistance in ohms (Ω).

How do you calculate resistance?

R = V ÷ I.

How do you calculate current using Ohm's law?

I = V ÷ R.

What happens to current if potential difference increases and resistance stays constant?

Current increases in direct proportion to potential difference.

What happens to current if resistance increases and potential difference stays constant?

Current decreases.

What is a direct current (d.c.)?

A current that flows in one direction only.

What is an alternating current (a.c.)?

A current that repeatedly changes direction.

What is the difference between current and charge?

Current is the rate of flow of charge; charge is the quantity of electricity transferred.

How is current measured?

Using an ammeter connected in series with the component.

How is potential difference measured?

Using a voltmeter connected in parallel across the component.

Why must an ammeter be connected in series?

The same current must flow through the ammeter and the component being measured.

Why must a voltmeter be connected in parallel?

It measures the potential difference between two points across a component.

What is a series circuit?

A circuit with only one loop, so there is only one path for current.

What is the current in a series circuit?

The current is the same at every point in the circuit.

How is potential difference shared in a series circuit?

The potential differences across the components add up to the supply potential difference.

How does adding resistors in series affect total resistance?

Total resistance increases because the resistances add together.

What is the total resistance of resistors in series?

Rtotal = R1 + R2 + R3 + …

What happens if one component breaks in a series circuit?

The circuit is broken and current stops everywhere.

What is a parallel circuit?

A circuit with two or more branches, providing multiple paths for current.

What is the potential difference across branches in parallel?

The potential difference is the same across each branch.

How is current shared in a parallel circuit?

The total current equals the sum of the currents in the separate branches.

How does adding another branch in parallel affect total resistance?

Total resistance decreases.

What happens if one branch breaks in a parallel circuit?

Current can still flow through the other branches.

What is the I–V characteristic of an ohmic conductor at constant temperature?

Current is directly proportional to potential difference; its I–V graph is a straight line through the origin.

What happens to the resistance of a filament lamp as its temperature increases?

Resistance increases because the metal ions vibrate more, making it harder for electrons to flow.

What shape is the I–V graph for a filament lamp?

A curve that becomes less steep as potential difference and current increase, because resistance increases as the filament heats up.

What is the I–V characteristic of a diode?

It allows current mainly in one direction; little current flows until the forward potential difference is high enough.

What is the resistance of a diode in reverse bias?

It is very high, so almost no current flows.

What is a variable resistor?

A component whose resistance can be adjusted to change the current or potential difference in a circuit.

What is a thermistor?

A resistor whose resistance changes with temperature.

How does the resistance of a typical NTC thermistor change as temperature increases?

Its resistance decreases as temperature increases.

What is an LDR?

A light-dependent resistor whose resistance changes with light intensity.

How does the resistance of an LDR change as light intensity increases?

Its resistance decreases as light intensity increases.

What is the purpose of a resistor in a circuit?

To limit current or produce a potential difference.

What is the difference between a conductor and an insulator?

A conductor allows charge to flow easily; an insulator resists the flow of charge.

What is the UK mains electricity supply?

An alternating potential difference of about 230 V at a frequency of 50 Hz.

What is the frequency of UK mains electricity?

50 Hz, meaning the current completes 50 cycles each second.

What is the live wire?

The wire that alternates between positive and negative potential relative to earth and supplies the potential difference.

What is the neutral wire?

The wire that completes the circuit; it is approximately at earth potential.

What is the earth wire?

A safety wire connected to the metal casing of an appliance, providing a low-resistance path for fault current.

What are the colours of the UK live, neutral and earth wires?

Live = brown; neutral = blue; earth = green and yellow.

Why is the live wire dangerous?

It is at a high alternating potential difference relative to earth, so touching it can cause current to pass through the body.

What is the purpose of a fuse?

It melts and breaks the circuit if the current becomes too large.

How does an earth wire reduce the risk of electric shock?

If the live wire touches a metal casing, the earth wire provides a low-resistance path for a large fault current, helping the fuse to melt and disconnect the supply.

Why are appliances often double-insulated?

They have insulating casing or construction that prevents the user touching live electrical parts, so an earth wire is not required.

What is the equation for electrical power?

P = V × I.

What do the symbols mean in P = VI?

P = power in watts (W), V = potential difference in volts (V), I = current in amperes (A).

What is the equation for power using current and resistance?

P = I² × R.

What is the equation for power using potential difference and resistance?

P = V² ÷ R.

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

E = P × t.

What is the equation for energy transferred electrically?

E = V × Q.

How can electrical energy be calculated using voltage, current and time?

E = V × I × t.

What are the units of electrical energy?

Joules (J); electricity bills commonly use kilowatt-hours (kWh).

What is one kilowatt-hour (kWh)?

The energy transferred by a 1 kW appliance operating for 1 hour.

How do you convert kilowatts to watts?

Multiply by 1,000.

How do you convert hours to seconds?

Multiply by 3,600.

How do you calculate energy in kWh?

Energy (kWh) = power (kW) × time (hours).

How do you calculate the cost of electricity?

Cost = energy used in kWh × cost per kWh.

Why does a high-power appliance transfer more energy each second?

Power is the rate of energy transfer, so a higher power means more energy transferred per second.

What is the relationship between charge flow and energy transferred?

E = Q × V, so energy transferred increases with charge flow and potential difference.

What is the required practical for resistance?

Investigate how the resistance of a wire depends on its length, keeping other factors such as material, thickness and temperature constant.

How do you investigate the resistance of a wire?

Set up a circuit with a power supply, ammeter and test wire in series, and a voltmeter across the wire. Measure current and potential difference, then calculate R = V ÷ I.

What is the independent variable in the resistance-of-a-wire practical?

The length of the wire.

What is the dependent variable in the resistance-of-a-wire practical?

The resistance calculated from potential difference divided by current.

What variables should be controlled in the resistance-of-a-wire practical?

The wire material, thickness, temperature and potential difference or current conditions as appropriate.

Why should the current be switched off between readings in the wire practical?

To reduce heating of the wire, because temperature changes affect resistance.

How do you improve reliability in the resistance practical?

Repeat readings for each length and calculate a mean; identify and investigate anomalous results.

What is the expected relationship between wire length and resistance?

For the same material and thickness at constant temperature, resistance increases as wire length increases.

Why does a longer wire have a greater resistance?

Electrons travel through more of the wire and undergo more collisions with vibrating ions.

How does increasing the thickness of a wire affect its resistance?

A thicker wire has a lower resistance because there is more space for charge to flow.

How does temperature affect the resistance of a metal wire?

As temperature increases, resistance increases because ions vibrate more and obstruct electron flow.

What is a circuit diagram?

A diagram using standard electrical symbols to represent components and their connections.

What is the function of a cell or battery?

It provides a potential difference that transfers energy to charges in the circuit.

What is the function of a switch?

It opens or closes the circuit.

What is the function of a lamp?

It transfers electrical energy into light and thermal energy.

What is the function of a motor?

It transfers electrical energy into kinetic energy and thermal energy.

What is the function of a buzzer?

It transfers electrical energy into sound and thermal energy.

What is the function of a voltmeter?

It measures potential difference across a component.

What is the function of an ammeter?

It measures current through a component.

What happens to energy in a resistor?

Electrical energy is transferred mainly to thermal energy.

How can you reduce wasted energy in electrical devices?

Use efficient components and reduce unwanted heating, for example by reducing resistance where high current is needed.

What is the difference between power and energy?

Energy is the amount transferred, measured in joules; power is the rate of transfer, measured in watts.

How do you rearrange Q = It to calculate time?

t = Q ÷ I.

How do you rearrange V = IR to calculate potential difference?

V = I × R.

How do you rearrange E = Pt to calculate time?

t = E ÷ P.

How do you rearrange E = VQ to calculate charge?

Q = E ÷ V.

How do you rearrange P = VI to calculate current?

I = P ÷ V.

How do you rearrange P = VI to calculate potential difference?

V = P ÷ I.

How do you rearrange E = VIt to calculate current?

I = E ÷ (V × t).