Current, Resistance and Potential Difference

Section 1 of 9Three key quantities

Quantity	Meaning	Unit
Current, I	Rate of flow of electric charge: how many electrons pass a point each second	ampere (A)
Potential difference, V	Energy transferred per unit of charge between two points; the push that drives the current	volt (V)
Resistance, R	Opposition to current, caused by electrons colliding with the lattice of metal ions	ohm (Ω); 1 Ω = 1 V/A

Section 2 of 9Why wires resist

1Electrons flow

• Electrons flow A potential difference makes free electrons drift through the metal.

2Collisions

• Collisions They collide with the vibrating positive ions in the lattice.

3Energy transferred

• Energy transferred The ions vibrate more, so the wire's thermal store increases and the wire warms.

4Current limited

• Current limited The more resistance, the lower the current for a given potential difference.

Section 3 of 9Using V = I × R

1

List what is known, e.g. current 0.50 A and resistance 12 Ω.

2

Write the equation: potential difference (V) = current (A) × resistance (Ω).

3

Substitute: V = 0.50 × 12 = 6.0 V.

4

To find current use I = V ÷ R, and to find resistance use R = V ÷ I; check the unit matches the quantity.

Section 4 of 9Worked examples

Question	Working	Answer
A 4.0 Ω resistor carries 3.0 A. Find the potential difference.	V = I × R = 3.0 × 4.0	12 V
A lamp has 6.0 V across it and a resistance of 20 Ω. Find the current.	I = V ÷ R = 6.0 ÷ 20	0.30 A
A component has 9.0 V across it and carries 0.15 A. Find the resistance.	R = V ÷ I = 9.0 ÷ 0.15	60 Ω
A current of 250 mA flows through a 48 Ω resistor. Find the potential difference.	Convert 250 mA = 0.25 A; V = 0.25 × 48	12 V

Section 5 of 9Relationships

V = I × R

Higher resistance

• Higher resistance With the same potential difference the current is smaller.

Lower resistance

• Lower resistance The current is larger for the same potential difference.

Conductors and insulators

• Conductors and insulators Good conductors have low resistance; insulators have very high resistance.

Exam assumption

• Exam assumption Unless told otherwise, the resistance of connecting wires and cells is treated as negligible.

Section 6 of 9Measuring the quantities in a circuit

Ammeter

• Ammeter Measures current and is connected in series with the component, so that all of the current flows through it.
  • It has very low resistance.

Voltmeter

• Voltmeter Measures potential difference and is connected in parallel across the component.
  • It has very high resistance so that hardly any current flows through it.

Finding resistance

• Finding resistance Record the voltmeter and ammeter readings together and then calculate R = V ÷ I.
  • Repeating at several settings and averaging improves reliability.

Section 7 of 9Charge, current and time

1Charge flows

• Charge flows Charge is measured in coulombs (C); a current of 1 A means 1 C passes a point each second.

2Equation

• Equation Q = I × t, where charge Q is in coulombs, current I in amperes and time t in seconds.

3Example

• Example A current of 2.0 A flowing for 30 s transfers Q = 2.0 × 30 = 60 C of charge.

Section 8 of 9Key terms

Term	Meaning
Current	The rate of flow of electrical charge, measured in amperes.
Potential difference	The energy transferred per unit charge between two points, measured in volts; also called voltage.
Resistance	A measure of how much a component opposes the flow of current, in ohms.
Ohm	The unit of resistance; one volt per ampere.
Lattice	The regular arrangement of positive ions in a metal.
Ammeter	A meter placed in series that measures the current in a circuit.

Section 9 of 9Exam technique

1

Saying that current is 'used up' in a component. Charge keeps flowing; energy is transferred.

2

Mixing up potential difference (across a component) with current (through it).

3

Forgetting units, or giving resistance in volts or amps.

4

Thinking higher resistance means higher current. It means lower current.

5

Forgetting to convert milliamps to amps before using V = I × R.