PHYSICS LAB/ELECTRIC CIRCUITS
OFFLINE READY
03THE UNKNOWN BATTERY

Decode an unknown battery.

Change the load. Collect evidence. Find the emf and internal resistance.

LIVE APPARATUS

Your circuit bench

SIMULATED READINGS
Battery circuit with a variable resistor, voltmeter, ammeter and switch The ammeter is in series with the battery and variable resistor. The voltmeter is connected across the battery terminals. Adjust the load using the slider below. IDEAL METERS · CONSTANT TEMPERATURE DC VOLTAGE 0.000 V TERMINAL VOLTAGE +− DC CURRENT 0.000 A CIRCUIT CURRENT +− UNKNOWN ε = ? V r = ? Ω +− VARIABLE RESISTOR LOW RHIGH R
Ammeter in seriesVoltmeter across terminals
6.0 Ω
2 ΩDrag to move the contact20 Ω
Current is flowingU = 0.000 V · I = 0.000 A
THE EVIDENCE

Terminal voltage vs current

0 points
Start with one readingYour recorded U–I pairs appear here.
Recorded readings
FIND THE RELATIONSHIP
U = ε − rI

Record at least two different currents, then turn on the best fit.

YOUR LAB NOTEBOOK

Recorded readings

#Load R / ΩI / AU / VSwitchRemove reading
No readings yet. Try a range of loads from 2 Ω to 20 Ω.

Simulated ideal DC readings, rounded to 0.001 V and 0.001 A. The graph and fit use these recorded values.

MAKE YOUR PREDICTION

What is inside?

Use the intercept and gradient of your graph to estimate the two hidden properties.

Unlocks after two different current readings.
The model & how to investigate

A short investigation

  1. Close the switch. Change R and record several well-spaced U–I pairs.
  2. Fit U against I. What do the vertical intercept and negative gradient tell you?
  3. Predict ε and r, then reveal the battery. An open-switch reading also tests your emf estimate.

What is being modelled?

A DC source with constant emf and internal resistance, an adjustable external resistor, ideal wires and ideal meters. Temperature and battery charge remain constant. There is no random measurement noise.

ε = U + Ir  →  U = ε − Ir

For a closed circuit, U = IR, so I = ε/(R + r). With the switch open, I = 0 and U = ε. Meter rounding can make fitted values differ slightly from the model values.