Current, Potential Difference & Resistance Notes

Edexcel IGCSE Physics: Revision notes

Key facts

Current is the rate of flow of charge: $Q = It$ .
Potential difference is energy transferred per unit charge: $E = QV$ , and 1 V = 1 J/C.
Resistance links them: $V = IR$ .
In series, the current is the same and the potential difference is shared; the total resistance is the sum.
In parallel, the potential difference is the same across each branch; current is conserved at a junction.

Electrical units

Learn the unit for each electrical quantity, so that you can check equations and write answers correctly.

Charge is in coulombs, current in amperes, voltage in volts, resistance in ohms, energy in joules, power in watts and time in seconds.

Use these units in every calculation.

The ammeter measures current in amperes (A); the voltmeter measures potential difference in volts (V) across the lamp.

Unit Symbol Current ampere A Charge coulomb C Energy joule J Resistance ohm Ω Voltage (pd) volt V Power watt W

	Unit	Symbol
Current	ampere	A
Charge	coulomb	C
Energy	joule	J
Resistance	ohm	Ω
Voltage (pd)	volt	V
Power	watt	W

Unit

Current:: ampere
Charge:: coulomb
Energy:: joule
Resistance:: ohm
Voltage (pd):: volt
Power:: watt

Symbol

Current:: A
Charge:: C
Energy:: J
Resistance:: Ω
Voltage (pd):: V
Power:: W

What is the unit of resistance?

Current and charge

Current is the rate of flow of charge; in metals it is a flow of electrons.

Current is measured with an ammeter placed in series in the circuit. In a metal, the charge carriers are negatively charged electrons.

$Q$ is charge (C), $I$ is current (A) and $t$ is time (s).

A series circuit: the ammeter is placed in the loop.

Charge $Q = I \times t$

Worked example

A current of 2 A flows for 30 s. How much charge flows?

1
$Q = I \times t$ .
2
$Q = 2 \times 30$ .

A charge of 90 C flows in 30 s. What is the current?

Potential difference and energy

Potential difference is the energy transferred per unit of charge passed: 1 volt is 1 joule per coulomb.

Potential difference (voltage) is measured with a voltmeter connected in parallel across the component.

$E$ is energy transferred (J), $Q$ charge (C) and $V$ potential difference (V).

A voltmeter is connected in parallel across the lamp.

Energy transferred $E = Q \times V$

Worked example

A charge of 10 C passes through a component with 5 V across it. How much energy is transferred?

1
$E = Q \times V$ .
2
$E = 10 \times 5$ .

How should a voltmeter be connected?

Resistance and V = IR

Potential difference equals current times resistance, so a larger resistance gives a smaller current for the same voltage.

Resistance opposes the flow of current. Measure $V$ across a resistor and $I$ through it, then calculate $R = V \div I$ .

Use the graph: change $R$ and see how the gradient changes.

r = 2resistance R (Ω)

Potential difference against current for a resistor: the gradient is the resistance.

Potential difference $V = I \times R$

A 12 V supply drives a current of 3 A through a resistor. What is R?

Series and parallel rules

Series: same current, shared voltage, resistances add. Parallel: same voltage across each branch, and currents add at a junction.

Charge cannot be created or destroyed, so current is conserved at a junction: the current entering equals the current leaving.

In a series circuit the total resistance is the sum of the resistances.

A parallel circuit: each lamp has its own branch.

Series Parallel Current Same Shared between branches Potential difference Shared between components Same across each branch Total resistance Sum of the resistances Less than the smallest resistor

	Series	Parallel
Current	Same	Shared between branches
Potential difference	Shared between components	Same across each branch
Total resistance	Sum of the resistances	Less than the smallest resistor

Series

Current:: Same
Potential difference:: Shared between components
Total resistance:: Sum of the resistances

Parallel

Current:: Shared between branches
Potential difference:: Same across each branch
Total resistance:: Less than the smallest resistor

Worked example

Two 4 Ω resistors are in series with a 12 V supply. Find the current and the pd across each resistor.

1
Total resistance $= 4 + 4 = 8$ Ω.
2
$I = 12 \div 8 = 1.5$ A.
3
$V = IR = 1.5 \times 4 = 6$ V.

In a parallel circuit, the pd across each branch is

Try an exam question

Two resistors of 4 Ω and 8 Ω are connected in series to a 12 V supply. Calculate the current in the circuit and the potential difference across the 8 Ω resistor.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.