Internal resistance separates e.m.f. from terminal voltage
A real source transfers some energy internally when current flows. The terminal potential difference is therefore below its e.m.f. when the source supplies current.
Follow the reasoning
- Identify the e.m.f. and internal resistance of the source.
- Find the current in the complete circuit.
- Subtract the internal voltage drop to find terminal voltage.
Try it in a past paper 4
Open the question, then use its mark scheme to check your work.
Question 18
Calculate current for a cell with internal resistance.
Question 7
Connect a resistor network to internal resistance, drift speed, charge and dissipated energy.
Question 7
Find the internal resistance of two cells in series.
Question 6
Read a lamp characteristic and use circuit measurements to find internal resistance.
A curated collection checked against the question papers and mark schemes; not a complete past-paper catalogue. Links open official OCR PDFs in a new tab.
Put the ideas to work
More practice is on its way
No PhysicsUK question linked yet. New eligible questions appear here as the bank grows.
Specification outcomes (5)
- 4.2.2(b) Define electromotive force (e.m.f.) of a source such as a cell or power supply.
- 4.2.2(c) Distinguish between e.m.f. and p.d. in terms of energy transfer.
- 4.3.2(a) Define source of e.m.f. and internal resistance.
- 4.3.2(b) Define terminal potential difference and 'lost volts'.
- 4.3.2(c)(i) Use and apply E = I(R + r) and E = V + Ir for internal resistance.