The method and the results are on the internal resistance lab. Do the analysis yourself first, then check the answers below.
Quick answer: The emf of a source is the energy it gives each coulomb of charge. These answers go with the Physics 11-12 lab on internal resistance: the graph, the calculations, the uncertainties and the conclusion explained, for all 8 analysis questions.
The sample readings follow V = ε − Ir with an emf near 1.5 V and an internal resistance near 0.56 Ω. The answers below use those readings, so your own results will differ slightly but should show the same straight line.
Level: Physics grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB B.5
The points lie close to a straight line with a negative gradient, so V decreases linearly as the current increases, as V = ε − Ir predicts.
Question 2 Emf
The line extrapolated to I = 0 meets the V axis at about 1.52 V ≈ 1.52 V. This is the emf, because with no current there are no lost volts.
Question 3 Internal resistance
Gradient = (1.18 − 1.46) / (0.60 − 0.10) = −0.28 / 0.50 = −0.56 V/A. The internal resistance is the size of the gradient: r = 0.56 Ω. (A least-squares fit of all six points gives r = 0.56 Ω and ε = 1.52 V.)
Question 4 Power in the cell
P = I²r = 0.60² × 0.56 = 0.20 W, which warms the cell.
Question 5 Short-circuit current
Isc = ε / r = 1.52 / 0.56 = 2.7 A. Testing it would heat the cell and leads, risk damaging them, and use up the cell quickly.
Question 6 Switching off between readings
The current heats the cell and changes its chemistry, which alters r and ε. Switching off lets it recover, so each reading is taken under the same conditions.
Question 7 Uncertainty
Percentage uncertainty = (0.01 / 1.18) × 100 = 0.8 %. The uncertainty in the p.d. is small compared with the change of 0.28 V over the range.
Question 8 Reliability of r
Any two of: take more readings and repeat each; use a datalogger with a faster response; use a fresh cell for every run; use a smaller current range to reduce heating; add a fixed protective resistor. Any two sensible changes.
