Mini PhysicsPart of Mini Education

Interactive practice

Internal Resistance Explorer

Connect terminal p.d., lost volts, signed-current graphs, and power balance for a real source.

  • A Level
  • Explore first
  • 10 min
BetaNewReviewed Jul 2026Back to A Level Core

Focus: Define current direction first; the graph then keeps one equation, V = ε − rI.

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Interactive stageLive model

Study lensDistinguish e.m.f. from terminal potential difference in a real-source model.10 min activity

Try this

Define current direction first; the graph then keeps one equation, V = ε − rI.

Learn to

  • Distinguish e.m.f. from terminal potential difference in a real-source model.
  • Use the correct terminal-p.d. sign for discharging and charging.
  • Interpret the intercept and signed gradient of a terminal-p.d.–current graph.

Exam transfer

  • Internal resistance
  • Terminal p.d.
  • V–I graph

Governing idea

With current defined positive when it leaves the positive terminal, V = ε − rI. During charging I is negative; using charging-current magnitude Icharge gives V = ε + Icharge r.

Model boundary

The source has constant e.m.f. and internal resistance, the external load is purely resistive, and wire resistance is negligible. Real values can vary with temperature, state of charge, and current.

Avoid this trap

E.m.f. is not always the terminal p.d. During discharge V < ε; during charging V > ε; only at I = 0 does V = ε in this model.

Practice run

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Source setup

Current direction and source values
6.0 V
0.80 Ω
5.0 Ω
Learning options
Guidance

Source checkpoint

Explore the model, then start a prediction.

What to notice:

    What you'll train

    Learning objectives

    • Distinguish e.m.f. from terminal potential difference in a real-source model.
    • Use the correct terminal-p.d. sign for discharging and charging.
    • Interpret the intercept and signed gradient of a terminal-p.d.–current graph.
    • Check discharge and charge power balances, including internal heating.

    Use this when

    • A Level current electricity
    • Students who mix up charging and discharging signs
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