Voltage and Resistance

Explain potential difference and resistance, then use V = IR to analyse simple circuits.

About 3 min read
  • Define potential difference and resistance
  • Use V = IR in calculations
  • Describe how to measure the resistance of a component
  • Interpret a current and voltage graph for a resistor
Before you start

You know that current is the rate of flow of charge, and that an ammeter is connected in series to measure it.

Potential difference

A cell gives energy to the charges in a circuit. Components transfer that energy as the charges pass through them. Potential difference, also called voltage, measures the energy transferred per unit of charge. We measure it with a voltmeter connected in parallel across a component.

Definition: potential difference

The energy transferred per coulomb of charge between two points, measured in volts (V).

Resistance

Resistance describes how strongly a component opposes current. For the same potential difference, a higher resistance gives a smaller current.

Definition: resistance

The ratio of potential difference to current, measured in ohms (Ω).

Resistance equation

V = I × R

At constant temperature, the current through a fixed resistor is directly proportional to the potential difference. Its current and voltage graph is therefore a straight line through the origin.

Find resistance from a measurement

A resistor has 6.0 V across it and a current of 0.30 A through it. Find its resistance.

Maths
V = IR
R = V ÷ I
R = 6.0 ÷ 0.30 = 20 Ω

Answer: 20 Ω

Meter positions

Put the ammeter in series with the component and the voltmeter in parallel across it.

Exam focus
Say it like this“Potential difference is the energy transferred per unit charge.”
Not like this“Voltage is the speed of electricity.” — Use the definition based on energy and charge.
Common mistakeConnecting a voltmeter in series, or forgetting to convert mA to A before using V = IR.

Practice

A 12 V supply is connected to a 4 Ω resistor. Find the current.
I = V ÷ R = 12 ÷ 4 = 3 A.
A current of 2 A flows through a 15 Ω resistor. Find the potential difference.
V = IR = 2 × 15 = 30 V.
A current of 200 mA flows with 5 V across a component. Find the resistance.
200 mA = 0.2 A, so R = 5 ÷ 0.2 = 25 Ω.
Going further
A student doubles the voltage across a filament lamp and the current less than doubles. Explain what happened to its resistance.
R = V ÷ I increased. The lamp became hotter, so the metal ions vibrated more and opposed the flow of charge more.

Quick check

Voltage and resistance quick check

What is the unit of resistance?

One volt per ampere equals one unit, shown by the Ω symbol.

How is a voltmeter connected?

It compares energy per charge between two points on either side of the component.

If V stays fixed and R doubles, what happens to I?

From I = V ÷ R, a larger denominator makes the current smaller in proportion.
Key takeaways
  • Potential difference is energy transferred per unit charge.
  • Resistance is potential difference divided by current.
  • Use V = IR with volts, amps and ohms.
  • Connect ammeters in series and voltmeters in parallel.
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