PhysicsChapter 11: Electricity

Terminal Potential Difference

A real cell has internal resistance r. When it supplies current I, some emf is used inside the cell, so its terminal potential difference V is usually smaller than its emf ε.

Key equations

Key equation

V=ε-Ir

V: terminal potential difference (V)
ε: emf of the source (V)
I: current (A)
r: internal resistance (Ω)

Key equation

ε=V+Ir

Key equation

I=εR+r

Ir is the lost voltage inside a discharging cell.
Open circuit: I=0, therefore V=ε.
Greater current causes a greater internal voltage drop.
For an external resistance R, terminal voltage is also V=IR.

Quick solved example

For ε=12 V, r=0.50 Ω and I=2.0 A:

Key equation

V=ε-Ir=12-(2.0)(0.50)=11\ V

Did you understand this topic?

Your answer is saved with your learning record. It does not mark the topic as mastered.

Exam preparation

Exam Focus

Quick revision

Revision Zone

Review the key facts, formulas and checks before you move on.