4PH1

Current, Potential Difference & Resistance

Electricity · 1 question type

Practise

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.

What voltage is

  • Voltage (also called ) 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

Where charges gain and lose energy

  • 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 12 V cell connected to a bulb: each coulomb of charge gains 12 J of energy passing through the cell from the negative to the positive terminal, then transfers that 12 J to the bulb and wires as it passes round the circuit
Source: Voltage definition by Save My Exams

Measuring voltage

  • 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 , 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
A circuit with a battery, a resistor and a lamp in series; ammeters (A₁, A₂) are connected in series in the main loop while voltmeters (V₁, V₂, V₃) are connected in parallel across the battery, the resistor and the lamp
Source: Series Circuit Bulb and Resistor Both Meters by Wikimedia Commons

The energy-charge-voltage equation

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