Paper 3: Practical skills in physics

Describe how to find the voltage at which an LED just begins to emit light, and how to make the judgement as reliable as possible.

Results:

λ / nm	470	525	590	625	660
V / V	2.645	2.368	2.107	1.989	1.884

Figure 1 shows V against 1/λ. Use it to determine the Planck constant.

Evaluate the method, identifying the largest source of uncertainty and its likely effect on the result.

Describe how to calibrate a thermistor as a temperature sensor between 10 °C and 70 °C.

Results:

T / K	283	298	313	328	343
R / Ω	18 636	10 000	5696	3416	2142

The model is R = R₀e^(B/T). Show that the data fit the model and find B.

Explain why the thermistor is less sensitive at high temperatures, and how the resolution of the measuring instrument limits the temperature resolution.

Describe how to use a datalogger to record the discharge of a capacitor through a resistor.

The time for the pd to halve was 3.3 s with R = 10 kΩ. Calculate C and decide whether it matches the 470 μF ± 10% rating.

Repeated halving times were 3.2, 3.3, 3.4 and 3.3 s. Estimate the uncertainty in C.

Describe how to find the shortest resonant length of an air column closed at one end for a given tuning fork.

Results:

f / Hz	256	320	384	426	512
L / m	0.323	0.257	0.212	0.191	0.157

The model is L = v/(4f) − e, where e is the end correction. Use a suitable graph to find v and e.

Explain why plotting a graph is better than calculating v from a single reading, and how air temperature affects the results.

Plan an experiment to investigate how the damping of a pendulum depends on the area of a card attached to the bob.

For one card, the amplitude fell from 100 mm to 50 mm in 5 oscillations, and to 25 mm after 10 oscillations. (i) Show that the damping is exponential and calculate the ratio of successive amplitudes. (ii) Predict the amplitude after 20 oscillations.