What is electromotive force and how is it different to potential difference?

The electromotive force or emf of a cell in a circuit is, for a start, not a force at all: it has the unit of volts. A good definition is ‘the power supplied by the cell per unit current through the cell’ or, equivalently, ‘the energy supplied by the cell per unit charge through the cell’ (IB examiners love to ask for this by the way). When the internal resistance of a cell is zero (or negligible), the emf is exactly the same as the potential difference (or voltage) between the terminals of the cell. But this changes when the internal resistance is significant – this is the resistance inside the actual cell which will ‘use up’ some of the emf before the current even leaves the cell. Since V = IR, you can find the potential difference across this internal resistance by multiplying the resistance by the current through it. Taking away this value from the emf will give you the remaining potential difference between terminals of the cell.

Tom B. A Level Maths tutor, IB Maths tutor, GCSE Maths tutor, A Level...

2 years ago

Answered by Tom, an IB Physics tutor with MyTutor

Still stuck? Get one-to-one help from a personally interviewed subject specialist


£22 /hr

Jonas P.

Degree: MPhys Theoretical Physics (Bachelors) - Edinburgh University

Subjects offered: Physics, Maths


“I am currently a student at the University of Edinburgh, doing my 4th year of the MPhys Theoretical Physics programme. I've managed to score an average grade of 81% in year 3, and I tend to consistently do well in my exams, which I be...”

£20 /hr

Ayokansola A.

Degree: Civil and Structural Engineering with a Year in Industry (Masters) - Sheffield University

Subjects offered: Physics, Spanish+ 3 more


“About Me: Hello! I am a Civil Engineering student at the University of Sheffield. I have always had a natural incline towards all things Maths and Science, so without a doubt, my genuine passion for the subjects’ I am offering will be...”

MyTutor guarantee

£20 /hr

Szymon K.

Degree: Mechanical Engineering (Masters) - Manchester University

Subjects offered: Physics, Maths+ 1 more

Further Mathematics

“About me: I am a student pursuing my degree in Mechanical Engineering at the University of Manchester. Ever since I can remember I have always been passionate about passing my knowledge to others. As a tutor I taught people of differen...”

About the author

Tom B.

Currently unavailable: for regular students

Degree: Maths and Physics (Masters) - Durham University

Subjects offered: Physics, Maths


“Friendly and patient tutor studying Maths and Physics, devoted to getting students to achieve their full potential.”

You may also like...

Posts by Tom

What is electromotive force and how is it different to potential difference?

When do you use 'n choose k' and where does the formula come from?

Other IB Physics questions

How would I write the binary number 11001 in decimal form?

Why are some nuclei stable while others are unstable?

What is Olbers' paradox?

How can an object in circular motion be accelerating when it's at the same speed?

View IB Physics tutors


We use cookies to improve our service. By continuing to use this website, we'll assume that you're OK with this. Dismiss