Physics · Class 12

EMF and terminal voltage

EMF is energy supplied per unit charge inside the source. Internal resistance causes terminal voltage to drop when a load draws current.

Open the interactive experiment →

EMF and terminal voltage

மூலத்தின் உள்ள்மின்தடையில் Ir மின்னழுத்த இழப்பு உண்டு. மின்னோட்டம் செல்லும்போது முனை மின்னழுத்தம் EMF-ஐவிடக் குறையும்.

Key notes, relationships and model limits

Key relationship

I=ε/(R+r); Vterminal=ε−Ir.

Remember

EMF is energy supplied per unit charge inside the source. Internal resistance causes terminal voltage to drop when a load draws current.

Common trap

A discharging cell’s terminal voltage is below its EMF by Ir. Open-circuit EMF and loaded terminal voltage are different readings.

Model limits

The cell EMF and internal resistance stay fixed while it supplies a positive resistive load. This does not model battery chemistry, charging or changing charge capacity.

What you will learn

Use the controls and plotted relation to explain emf and terminal voltage.

Try the experiment

Try both scenarios, change one control, and predict the result before checking the graph and readouts.

Think of it this way

Change one input while keeping the others fixed, as you would in a controlled experiment.

Worked example

With ε=1.5 V, r=0.5 Ω and R=5 Ω, I≈0.273 A and V≈1.364 V.

Why this matters

The model connects emf and terminal voltage with an observable reading. Check the equation and assumptions before applying it.

Before you start

SI units · algebra · graph reading

Check your understanding

When a cell delivers current, its terminal voltage is?

EMF plus Ir.
EMF minus the internal drop Ir.
Always equal to EMF, even under load.

Read the explanation

EMF minus the internal drop Ir.

Course connection: Class XII Chapter 3. Consult the roadmap for CBSE, NEET, JEE Main and Advanced scope.