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Physics · 6. Electricity and magnetism

Charge, current, potential difference and circuits

Build circuit reasoning from charge and energy conservation rather than isolated formulas.

40 minutesContent version: 2.1
Start the interactive pathwayOpen the virtual experiment

Driving question

How are current and energy distributed in series and parallel circuits?

Curriculum coverage

Vietnam 2018 and IGCSE electricity and magnetism.

Model scope and limitations

Resistors are treated as approximately ohmic at constant temperature; wires and sources are ideal unless stated.

Learning objectives

  • Explain I = Q/t and V = W/Q.
  • Apply Ohm’s law under appropriate conditions.
  • Reason about current and voltage in series/parallel circuits.

Prerequisite knowledge

  • Charge and energy.
  • Ratios and units A, V, Ω.

Core knowledge

Current

Current is charge flow rate; charge is not consumed by a resistor.

Potential difference

Potential difference is energy transferred per coulomb between two points.

Circuit rules

Series: same current; parallel: same potential difference across branches.

Worked example

Two 4 Ω and 8 Ω resistors in series across 12 V: R = 12 Ω, I = 1 A; voltage drops are 4 V and 8 V.

Misconceptions to avoid

  • Current does not get used up after each resistor in a series loop.
  • Ohm’s law does not apply to every component at every temperature.

Virtual experiment procedure: Direct-current circuits

  1. Use one resistor, vary V and record I at at least five points.
  2. Plot I–V, identify the linear region and gradient.
  3. Compare two-resistor series and parallel circuits on the same supply.

Safety and cautions

  • In a real lab use low voltage, disconnect before rewiring and avoid short circuits.

Evidence to collect

  • A circuit diagram with standard symbols and a V–I table.
  • A check of total potential drops around a loop.
Open the virtual experiment

Diagnostic check

1. How is an ammeter connected?

  1. In series
  2. In parallel
  3. Outside the circuit
Show answer and explanation

Correct answer: In series

An ammeter must lie in the measured current path.

2. A voltmeter measures a quantity between what?

  1. Two points
  2. One point only
  3. Two currents
Show answer and explanation

Correct answer: Two points

Potential difference is defined between two points.

3. 6 C passes in 2 s. What is I?

  1. 3 A
  2. 12 A
  3. 0.33 A
Show answer and explanation

Correct answer: 3 A

I = Q/t = 6/2 = 3 A.

Application practice

1. V = 12 V and R = 24 Ω. What is I?

  1. 0.5 A
  2. 2 A
  3. 288 A
Show answer and explanation

Correct answer: 0.5 A

I = V/R = 0.5 A.

2. What is the equivalent resistance of 3 Ω and 6 Ω in series?

  1. 9 Ω
  2. 2 Ω
  3. 18 Ω
Show answer and explanation

Correct answer: 9 Ω

Series: R = R₁ + R₂ = 9 Ω.

3. Which quantity is common to parallel branches?

  1. Potential difference
  2. Always the same current
  3. Always the same resistance
Show answer and explanation

Correct answer: Potential difference

Each branch connects to the same two nodes, so V is common.

Academic references

  1. OpenStax Physics — peer-reviewed high-school physics
  2. BIPM — SI Brochure, 9th edition

This lesson is maintained against the cited sources below. Simulations are learning models and do not replace supervised physical-laboratory safety procedures.

Content publisher
WebLab STEM
Technical maintainer
Đức Tiến — Control & Automation Engineer