The method and the results are on the electrical power lab. Do the analysis yourself first, then check the answers below.
Quick answer: In a series circuit the current is the same everywhere and the voltages add up to the supply; in a parallel circuit every branch has the full supply voltage and the branch currents add up. These answers go with the Physics 9-10 lab on electrical power: the graph, the calculations, the uncertainties and the conclusion explained, for all 7 analysis questions.
Model answers use the sample table. Your own results will differ slightly, but the straight-line graph through the origin and a gradient close to the resistor value should appear in every good data set.
Level: Physics grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: GCSE P2, HS-PS3-3
Question 1 Planning the readings
Independent variable: the supply voltage (which sets the current). Dependent variable: the power (calculated from V and I). Controls: the same 10 Ω resistor and the same temperature or room conditions, with the switch closed only briefly.
Question 2 Check one row
P = V I = 4.0 × 0.39 = 1.56 W. I²R = 0.39² × 10 = 0.1521 × 10 = 1.52 W. Difference = (1.56 − 1.52) ÷ 1.52 × 100 = about 2.6 %, within the measurement uncertainty.
Question 3 Plot and gradient
The points lie on a straight line through the origin, so P is proportional to I². Gradient = (3.54 − 0.10) ÷ (0.348 − 0.010) = 3.44 ÷ 0.338 = 10.2 Ω. The unit is W ÷ A² = Ω.
Question 4 Resistance from V ÷ I
V ÷ I = 10.0, 10.0, 10.3, 10.3, 10.2 and 10.2 Ω. The mean is 10.2 Ω, the same as the gradient and close to the labelled 10 Ω.
Question 5 Heating effect
The resistor transfers electrical energy as heat. Leaving the current on lets it warm up, and in a metal the resistance rises with temperature, so later readings would not match earlier ones. At 6.0 V it dissipates about 3.5 W, so the resistance may rise slightly, which fits the slight upward drift in V ÷ I.
Question 6 Uncertainty
At 0.10 A: 0.01 ÷ 0.10 × 100 = 10 %. At 0.59 A: 0.01 ÷ 0.59 × 100 = 1.7 %. The large-current readings are more reliable. The uncertainty in I² is double that in I.
Question 7 Improve the lab
Use an ammeter with a finer resolution (0.001 A) and take more readings, especially at higher currents. Repeat each measurement and average. These reduce random and resolution uncertainty.
