OCR A Level Physics B (Advancing) Practical skills in physics June 2023 question paper H557/03

A cell can be made by inserting a zinc cathode and a copper anode into a lemon. A high resistance voltmeter is used in the circuit shown in Fig. 1.1 to measure the p.d. across the lemon when different resistors are placed across the cell.

Table 1 below shows the data recorded.

resistance / k	potential difference / mV	current / A
3.3	540	164
6.6	666	101
10.0	738
13.3	765
16.6	784	47
20.0	812	41

Calculate the current and complete the missing rows in Table 1.

Plot the missing points on the graph in Fig. 1.2 and draw the line of best fit.

Find the gradient of the line of best fit and hence calculate the lemon’s internal resistance, in ohms, . Show your working.

A student suggests that two lemon cells in series could be used to run a 1.5 V, 0.45 W filament lamp. Explain why this is not correct.

This question is about an experiment to determine the value of the Planck constant h.

A student increases the potential difference across a blue LED until it just starts to glow. The student measures the value of this pd using a multimeter. This is known as the ‘striking voltage’. The measurement is repeated, and a dot-plot is drawn, as shown in Fig. 2.1.

Calculate the mean value of striking voltage and its absolute uncertainty.

Mean striking voltage =........ ±........ V

Describe one significant cause of uncertainty in this measurement and suggest a method which would reduce the uncertainty.

The student then repeats the measurement of striking voltage for LEDs of different colours. She refers to the manufacturer’s data to find the value of wavelength for each LED and plots a graph of 1/wavelength (on the x-axis) against striking voltage (on the y-axis). The graph is shown in Fig. 2.2.

The gradient of this line of best fit = 8.90 × 10^–7 V m.

Using the value for uncertainty found in (a)(i), add vertical error bars to all the plotted

points and draw a steepest acceptable line through the error bars.

Hence calculate the absolute uncertainty of the gradient value.

Absolute uncertainty of gradient = ±........ V m

Show that the Planck constant, h = gradient ×

The gradient of the line of best fit is 8.90 × 10^–7 V m. (ii) Use this value to calculate a value of the Planck constant. Include a value for absolute uncertainty in your answer.

Planck constant =........ ±........ J s

This question is about the discharge of a capacitor.

Values of V as the capacitor discharges through the 4.7 k resistor are recorded at 10 s intervals in Table 3.

Capacitor discharge is an example of exponential decay and follows the equation

Complete the ln V column in Table 3 and then perform a mathematical test on the data to show that it follows exponential decay.

You may use the extra column in the table for your working.

Show that the value of the time constant, is approximately 25 s.

Calculate the value of the capacitance.

There is a general rule of thumb used by electronic engineers that the time taken for a capacitor to discharge completely is 5.

Justify why this rule of thumb is useful to engineers.

This question is about inducing an emf in a coil of wire.

Fig. 4.1 shows a coil of wire perpendicular to a uniform magnetic field.

The coil has 10 turns of radius 0.020 m. The field strength is 5.0 mT.

Show that the flux linkage of the coil is about 6.3 × 10^–5 Wb.

The coil is rotated at a constant rate as shown in Fig. 4.2. An emf is produced across the coil.

An oscilloscope trace of the emf produced is shown in Fig. 4.3.

Suggest and explain why the trace has the shape shown on Fig. 4.3.

The horizontal divisions on the trace representing time are set at 0.625 s/div. Using the trace shown in Fig. 4.3, calculate the angular frequency, , of the coil.

Angular frequency, =........ rad s^–1

The angle , that the normal to the plane of the coil makes with the field lines varies with time as the coil rotates. Angle is shown in Fig. 4.4.

By considering , show that the flux in the coil can be given by = BA cos t.

The original trace of the coil is shown in Fig. 4.5.

The number of turns of the coil is changed to 20. Nothing else is changed.

Add to Fig. 4.5 a drawing of the trace produced by the 20-turn coil.

The driving force spinning the coil is removed. Fig. 4.6 shows how the flux linkage varies with time for three whole revolutions.

Using the graph in Fig. 4.6, estimate the emf induced in the coil at the instant the flux linkage is zero at 0.23 s. Show all your working.

* It is suggested that the peak emf generated by the coil falls by the same fraction in each cycle.

Explain how this suggestion can be tested by using data from Fig. 4.6.

Carry out your test on the data and state your conclusions.