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Internal resistance separates e.m.f. from terminal voltage Where does a battery’s power go? Power equations must use the component’s operating point Mechanical and electrical power measure the same rate A straight-line graph reveals a source’s properties Electrical circuits4.3 Analysing1.1.3 Power3.3.3 E.m.f. and potential difference4.2.2 Resistance4.2.3 Power in circuits4.2.5 Internal resistance4.3.2
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Where does a battery’s power go?

OCR A AS Physics · Two ideas, one explanation

The power supplied by a source is split between the external circuit and heating inside the source. Internal resistance makes the split depend on current.

εI = VI + I²r

Follow the reasoning

  1. Calculate the current using the external and internal resistances.
  2. Calculate power delivered to the load using terminal voltage.
  3. Compare it with the internal heating rate I²r.
Specification outcomes (3)
  • 4.3.2(c)(i) Use and apply E = I(R + r) and E = V + Ir for internal resistance.
  • 4.2.5(a) Use and apply power equations P = IV, P = I²R, and P = V² / R.
  • 4.2.5(b) Use energy transfer equation W = VIt.
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H156 v2.0 · May 2026

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