ASPhysicsOCRRevision Notes4. Electrons, Waves & PhotonsEMF & Internal ResistanceCalculating E.m.f

Calculating E.m.f (OCR AS Physics): Revision Note

Exam code: H156

Written by: Ashika

Reviewed by: Caroline Carroll

Updated on 2 July 2026

Calculating E.m.f

The e.m.f ε is defined by the equation:

EMF equation, downloadable AS & A Level Physics revision notes

E.m.f can therefore be defined as the total, or maximum, voltage available to the circuit

Worked Example

A battery of e.m.f 7.3 V and internal resistance r of 0.3 Ω is connected in series with a resistor of resistance 9.5 Ω.

WE - internal resistance question image, downloadable AS & A Level Physics revision notes

Determine:

a) The current in the circuit

b) Lost volts from the battery

Answer:

Worked example internal resistance (2), downloadable AS & A Level Physics revision notes

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Author: Ashika

Expertise: Physics Content Creator

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.

Reviewer: Caroline Carroll

Expertise: Head of Content Delivery

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about delivering high-quality resources to help students achieve their full potential.