AQA A Level Physics Paper 3 Section A: Practical skills and data analysis November 2021 question paper 7408/3A

Figure 1 shows apparatus used to investigate the inverse-square law for gamma radiation.

A sealed source that emits gamma radiation is held in a socket attached to clamp B. The vertical distance between the open end of the source and the bench is 138 mm. A radiation detector, positioned vertically above the source, is attached to clamp T.

A student is told not to move the stands closer together.

Describe a procedure for the student to find the value of d, the vertical distance between the open end of the source and the radiation detector.

In your answer, annotate Figure 1 to show how a set-square can be used in this procedure.

Before the source was brought into the room, a background count C_b was recorded.

With the source and detector in the positions shown in Figure 1, d = 530 mm. Separate counts C₁, C₂ and C₃ are recorded.

R_C is the mean count rate corrected for background radiation.

Show that when d = 530 mm, R_C is about 0.3 s⁻¹.

The apparatus is adjusted so that d = 380 mm. Counts are made that show R_C = 0.76 s⁻¹.

The student predicts that:

where k is a constant.

Explain whether the values of R_C in Questions 01.2 and 01.3 support the student’s prediction.

Describe a safe procedure to reduce d. Give a reason for your procedure.

The student determines R_C for further values of d. The values of d change by the same amount ∆ d between each measurement. Figure 2 shows these data.

Determine ∆ d.

Explain how the student could confirm whether Figure 2 supports the prediction:

No calculation is required.

When a gamma photon is detected by the detector, another photon cannot be detected for a time t_d called the dead time.

It can be shown that:

where R₁ is the measured count rate R₂ is the count rate when R₁ is corrected for dead time error.

The distance between the source and the detector is adjusted so that d is very small

On average, two of the gamma photons that enter the detector every second are not detected.

Calculate t_d for this detector.

A student says that if 100 gamma photons enter a detector in one second and t_d is 0.01 s, all the photons should be detected.

Explain, with reference to the nature of radioactive decay, why this idea is not correct.

A light-emitting diode (LED) emits light over a narrow range of wavelengths.

Two LEDs L_G and L_R are adjusted to give the same maximum light intensity. L_G emits green light and L_R emits red light.

Figure 3 shows how the light output of the LEDs varies with the wavelength λ.

Figure 3

Light from L_R is incident normally on a plane diffraction grating.

Determine N, the number of lines per metre on the grating.

Suggest one possible disadvantage of using the fifth-order maximum to determine N

Figure 4 shows part of the current−voltage characteristics for L_R and L_G.

When the linear part of the characteristic is extrapolated, the point at which it meets the horizontal axis gives the activation voltage V_A for the LED.

Determine, using Figure 4, V_A for L_R.

It can be shown that:

where h = the Planck constant.

Deduce a value for the Planck constant based on the data given about the LEDs.

Figure 5 shows a circuit with L_R connected to a resistor of resistance R.

The power supply has emf 6.10 V and negligible internal resistance. The current in L_R must not exceed 21.0 mA.

Deduce the minimum value of R.

An analogue voltmeter has a resistance that is much less than that of a modern digital voltmeter. Analogue meters can be damaged if the full-scale reading is exceeded. Figure 6 shows a dual-range analogue voltmeter with a zero error.

The voltmeter is set to the more sensitive range and then used in a circuit.

What is the potential difference (pd) between the terminals of the voltmeter when a full-scale reading is indicated?

Tick () one box.

Explain the use of the mirror when reading the meter.

A student corrects the zero error on the meter and then assembles the circuit shown in Figure 7. The capacitance of the capacitor C is not known.

The output pd of the power supply is set to zero. The student connects the flying lead to socket X and adjusts the output pd until the voltmeter reading is full scale (15 V). She disconnects the flying lead from socket X so that C discharges through the voltmeter.

She measures the time T ½ for the voltmeter reading V to fall from 10 V to 5 V.

She repeats this process several times.

Table 1 shows the student’s results, none of which is anomalous.

Determine the percentage uncertainty in T ½.

Show that the time constant for the discharge circuit is about 17 s.

The student thinks that the time constant of the circuit in Figure 7 is directly proportional to the range of the meter. To test her theory, she repeats the experiment with the voltmeter set to the 3 V range. She expects T ½ to be about 2.5 s.

Explain:

• what the student should do, before connecting capacitor C to the 0 V and 3 V sockets, to avoid exceeding the full-scale reading on the voltmeter

• how she should develop her procedure to get an accurate result for the time constant

• how she should use her result to check whether her theory is correct.

The student wants to find the resistance of the voltmeter when it is set to the 15 V range. She replaces C with an 820 µF capacitor and charges it to 15 V. She discharges the capacitor through the voltmeter, starting a stopwatch when V is 14 V. She records the stopwatch reading t at other values of V as the capacitor discharges.

Table 2 shows her results.

Suggest two reasons why the student selected the values of V shown in Table 2. Explain each of your answers.

Figure 8 shows a graph of the experimental data.

Show, using Figure 8, that the resistance of the voltmeter is about 16 kΩ.

Determine the current in the voltmeter at t = 10 s.