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Voltage and Resistance

Using V = I × R and reading simple circuit measurements.

  • Define and explain Voltage and Resistance in your own words
  • Use key terms such as voltage accurately
  • Apply what you have learned to new examples and questions
  • Avoid the common mistakes learners make with this topic

This lesson focuses on Voltage and Resistance: using V = I × R and reading simple circuit measurements.

Definition: Voltage and Resistance

Using V = I × R and reading simple circuit measurements.

Key ideas

Voltage, current and resistance are linked by V = IR

Voltage drives current through resistance like pressure drives water through a pipe. Doubling the voltage doubles the current through a fixed resistor. Ohmic conductors give a straight-line current–voltage graph through the origin — this is Ohm's law.

Resistance opposes the current

Resistance, measured in ohms (Ω), tells you how hard it is to push current through a component. Long thin wires and poor conductors have high resistance; short thick copper wires have low resistance. Ohmic conductors (like a wire at constant temperature) obey V = IR exactly, giving a straight-line I–V graph — but filament bulbs and diodes do not, because their resistance changes as they heat up.

Key term — voltage: The energy transferred per unit charge, measured in volts (V) — the 'push' driving the current.

Worked example: Voltage and Resistance

A 6 Ω resistor has 2 A flowing through it. Calculate the voltage across it.

Use V = IR: V = 2 × 6 = 12 V.

Answer: 12 V across the resistor.

Common mistakes
  • Saying current is 'used up' in a circuit Current is the same all round a series loop; it is energy that is transferred to the components.
  • Adding resistances in parallel like series In parallel, 1/R = 1/R₁ + 1/R₂ — two 4 Ω resistors in parallel give 2 Ω, not 8 Ω.

Practice

A bulb has 2 A through it and 12 V across it. Calculate its resistance.
R = V ÷ I.

12 ÷ 2 = 6 Ω.

A current of 0.5 A flows for 20 s. How much charge passes?
Q = It.

0.5 × 20 = 10 C.

Two 6 Ω resistors are in series across a 12 V battery. What is the current?
Add the resistances first.

Total R = 12 Ω, so I = 12 ÷ 12 = 1 A.

A kettle draws 5 A from the 230 V mains. Calculate its power.
P = IV.

5 × 230 = 1150 W.

Quick check

Voltage and Resistance — quick check

Which of these best defines "voltage"?

The energy transferred per unit charge, measured in volts (V) — the 'push' driving the current.

How much energy does a 100 W bulb transfer in 60 s?

100 × 60 = 6000 J.
Key takeaways
  • Voltage and Resistance: using V = I × R and reading simple circuit measurements.
  • Voltage, current and resistance are linked by V = IR: Voltage drives current through resistance like pressure drives water through a pipe.
  • resistance: How much a component opposes current, measured in ohms (Ω).
  • Watch out for: saying current is 'used up' in a circuit
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