Current, Potential Difference and Resistance

The three quantities

Quantity	Symbol	Unit	Measured with
Current	I	ampere (A)	ammeter (in series)
Potential difference	V	volt (V)	voltmeter (in parallel)
Resistance	R	ohm (Ω)	(calculated)

Ohm's law

V = I × R

Rearranged: I = V / R, R = V / I.

For an ohmic conductor at constant temperature, current is directly proportional to potential difference (straight-line graph through the origin).

Charge

Q = I × t     (charge = current × time)   in coulombs (C)

Worked example

A 12 V supply across a 4 Ω resistor:

I = V / R = 12 / 4 = 3 A

Component graphs (I–V)

• Resistor (fixed temp): straight line (ohmic).
• Filament lamp: curve — resistance rises as it heats up.
• Diode: only conducts one way.

Exam tip

Ammeters go in series, voltmeters in parallel. As a filament lamp heats up, resistance increases, so the I–V graph curves.