Ohm's law for a complete circuit – internal resistance
PhysicsElectricity & CircuitsAges 16–17Virtual lab
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Electricity & Circuits
A closed circuit made of a source (EMF E, internal resistance r) and a rheostat R. Drag the slider directly on the circuit diagram to see I = E/(R + r), terminal voltage U = IR = E − Ir, power P = I²R, and efficiency H = U/E change; the P(R) graph shows the external power peaks when R = r. The EMF lab screen is the practical: read a voltmeter and an ammeter with realistic errors and plot V against I to find ε and r. The Potentiometer screen compares two EMFs and measures internal resistance with a potentiometer wire.
Lesson: Source, EMF, internal resistance; Ohm's law for a complete circuit; power and efficiency of a source; practical: EMF and internal resistance of a cell; potentiometer
What it shows
A real source such as a battery has an EMF and an internal resistance r. The current is I = E/(R + r), so the terminal p.d. falls as more current is drawn: V = ε − Ir. The external power I²R peaks when R = r, where the efficiency is only 50 %. The EMF lab adds meter errors, so a straight-line fit of V against I is needed: the intercept is ε and the gradient is −r. The potentiometer measures a p.d. with no current drawn from the cell at balance, so it compares EMFs directly.
How to use
On Circuit, set EMF E and Internal resistance r and drag the rheostat on the diagram. On EMF lab, choose the Cell under test, press Close switch, vary Load resistance R and press Record data for each setting, then tick Show best-fit line. On Potentiometer, choose the Experiment, drag the jockey along the wire or use Jockey position l to the null point, and record the balancing lengths.
Parameters you can change
- EMF E 1–24 V
- Internal resistance r 0.1–5 Ω
- External resistance R 0.1–20 Ω
- Screen Circuit, EMF lab, Potentiometer
- Cell under test (EMF lab) Cell A, Battery B (two cells), Cell C (well used), E and r of the Circuit screen
- Potentiometer experiment Compare two EMFs, Internal resistance
Questions to explore
- Why does the terminal p.d. of a cell fall as the current it supplies rises?
- On a graph of V against I, what do the intercept and the gradient tell you about the cell?
- Why does a potentiometer at balance measure the EMF rather than the terminal p.d.?
