EMF and Internal Resistance of a Cell: Virtual Required Practical
A virtual e.m.f. and internal resistance practical: voltmeter across the cell or across the cell and ammeter, protective resistor, meter ranges, the rheostat before closing the switch, and opening the switch between readings. Plot V against I, read ε from the intercept and r from the gradient, and see why a false origin changes what the I intercept means.
About this simulation
- What
- An interactive Physics simulation of EMF and Internal Resistance of a Cell: Virtual Required Practical.
- Who
- Designed for AP, IB, and A‑Level Physics students.
- How
- Runs in any modern browser — drag, adjust, and explore in real time.
Updated 2026-10-11
Key Concepts
Terminal p.d.
V = ε − Ir: the terminal p.d. falls linearly as the current rises. On a V–I graph the intercept is ε and the gradient is −r.
Voltmeter across the cell
The standard circuit. The voltmeter draws a tiny current, so ε and r come out very slightly low — a negligible error.
Ammeter inside the voltmeter's span
ε is right, but the 'internal resistance' measured is r + R_A — a large error for a cell whose r is about the same as R_A.
Protective resistor
A fixed resistor in series limits the current and makes V change more. If it is inside the voltmeter's span, subtract it from the gradient.
What This EMF and Internal Resistance Lab Teaches
Measuring the e.m.f. and internal resistance of a cell is AQA A-level required practical 8 and a standard experiment in the Chinese textbook. The physics is V = ε − Ir; the marks are in the circuit and the analysis: where the voltmeter goes, why a protective resistor is used, which ranges to choose and how to read the V–I graph.
Here you build the circuit: meter arrangement, cell, protective resistor and ranges. The meters have real resistances and reading resolutions, and a cell left delivering current polarises. Vary the rheostat, record V and I, plot the graph and find ε and r.
The analysis sets your values beside the true ones and the values expected for your circuit, so you can see why putting the ammeter inside the voltmeter's span inflates r, why the standard circuit's error is tiny, and what the I intercept means with a false origin.
