Current, Potential Difference and Resistance

Electric current

Current is the rate of flow of electric charge around a circuit. It is measured in amperes (A) with an ammeter placed in series. Charge flows only in a complete (closed) circuit.

Charge equation:

charge = current × time      (Q = I × t)

Charge (Q) in coulombs, current (I) in amps, time (t) in seconds.

Potential difference (voltage)

Potential difference (p.d.) is the energy transferred per unit charge as it passes through a component — it "pushes" the current round. Measured in volts (V) with a voltmeter placed in parallel across a component.

energy transferred = charge × potential difference   (E = Q × V)

Resistance

Resistance opposes the flow of current. Measured in ohms (Ω). Higher resistance = smaller current for the same voltage.

Ohm's law:

V = I × R      (potential difference = current × resistance)

Rearrange: I = V ÷ R and R = V ÷ I.

I–V characteristics

Graphs of current vs voltage reveal a component's behaviour:

• Fixed resistor (ohmic conductor): straight line through the origin — current is proportional to voltage (constant resistance) if temperature is constant.
• Filament lamp: curve that flattens — as current increases, the filament heats up, so resistance increases.
• Diode: only lets current flow one way; very high resistance in reverse.

Special components

• Thermistor: resistance decreases as temperature increases (used in temperature sensors).
• LDR (light-dependent resistor): resistance decreases as light increases (used in light sensors).

Worked example

A component has 12 V across it and a current of 3 A flows. What is its resistance?

R = V ÷ I = 12 ÷ 3 = 4 Ω

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Common mistakes

• Connecting a voltmeter in series or an ammeter in parallel — it's the opposite (ammeter in series, voltmeter in parallel).
• Forgetting a filament lamp's resistance increases as it heats up.
• Rearranging V = IR incorrectly — for resistance, R = V ÷ I.

Exam tips

• Learn how to connect ammeters (series) and voltmeters (parallel).
• Be able to sketch and explain the three I–V graphs (resistor, lamp, diode).
• Know that thermistors respond to temperature and LDRs to light.

Key facts to remember

• Current = flow of charge (Q = I × t), measured in amps (ammeter in series).
• Potential difference = energy per charge, in volts (voltmeter in parallel); E = Q × V.
• V = I × R (Ohm's law); fixed resistor gives a straight line; a filament lamp's resistance rises with heat; thermistor (temp) and LDR (light) change resistance.