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Physics

17.4 - V = IR Equation & I-V Graphs

5 min

All 3 sections match your courseCambridge IGCSE Core Combined

The V = IR equation

I-V graphs for resistors

note 1 of 2

Formula linking p.d., current and resistance

$V = I R$

Where:

$V$ = Potential difference, in volts (V)
$I$ = Current, in amperes (A)
$R$ = Resistance, in ohms (Ω)

The greater the resistance of a component, the smaller the current that flows through it for a given potential difference across it.

Finding the resistance

The equation can be rearranged to find the resistance of a component from measured values of potential difference and current:

$R = \frac{V}{I}$

Watch out

Always check that current is in amps (A) before using the equation. If a question gives you a current in milliamps (mA), divide it by 1,000 to convert it to amps first.

note 2 of 2

Worked example - Current through a resistor

A 5.0 Ω resistor in a circuit has a potential difference of 9.0 V across it. Calculate the current through the resistor.

Step 1 - Identify the values

Potential difference: $V = 9.0$ V
Resistance: $R = 5.0 Ω$

Step 2 - Rearrange V = IR to make current the subject

$I = \frac{V}{R}$

Step 3 - Substitution and calculation

$I = \frac{9.0}{5.0} = 1.8 A$

The current through the resistor is 1.8 A.

end of notes

Key points to remember

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17.5 - Investigating ResistanceNext up

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