Voltage and Energy — Current, Potential Difference & Resistance | IGCSE Physics

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 16% of your exam marks.

stable

Very High

Stable16%

Ohm's Law calculations and I-V characteristic graphs are among the most reliably tested question types.

Voltage (also called potential difference) between two points in a circuit is the energy transferred per unit charge that passes between those two points
Voltage is measured in volts, symbol V
One volt is the voltage between two points when one joule of energy is transferred for every coulomb of charge that moves between them, so 1 V = 1 J/C
"Potential difference" emphasises that voltage is always a difference between two ends; like temperature difference or height difference, it only makes sense as a comparison between two points

A charge gains energy when it is pushed through the cell by the chemical reaction inside; this is the cell doing work on the charge
A charge loses energy when it passes through a component such as a bulb, a heater or a motor; this is the charge doing work on the surroundings, transferring its energy to light, internal energy of the surroundings, or kinetic energy
Around a full loop, the total energy gained at the cell exactly equals the total energy lost in the rest of the circuit. This is conservation of energy applied to electricity
Numerical intuition: a 9 V battery gives each coulomb that crosses it a 9 J energy boost; if that same coulomb then traverses a single 9 V bulb on the way back round, it hands the full 9 J to the bulb's filament

A voltmeter is the instrument that measures voltage
A voltmeter must be wired in parallel with the component whose voltage is being measured, so its two leads sit on either side of the component, reading the voltage across it
An ideal voltmeter has infinite resistance, so no current is diverted through it and the rest of the circuit is undisturbed
Practical tip: build the rest of the circuit first, get it working, then clip the voltmeter across the chosen component. This avoids confusing yourself with the voltmeter mid-build

E = Q × V

where:
- E = energy transferred (J)
- Q = charge that has moved (C)
- V = voltage between the two points (V)
Rearrangements:
- V = E / Q (voltage is energy per coulomb, straight from the definition)
- Q = E / V

Example — a 9.0 V cell drives 1500 C of charge around a circuit containing a single resistor. Calculate the total energy delivered to the resistor.