AQA A Level Physics Paper 3 Section B: Turning points in physics June 2024 question paper 7408/3BD

Figure 1 shows apparatus used in an experiment to measure the specific charge of the electron.

Electrons are accelerated by the potential difference V_A.

The electrons then enter the region between two parallel metal plates, shown shaded in Figure 1. The parallel metal plates are separated by a distance d with a potential difference V_P across them. In the same region there is a uniform magnetic field of flux density B into the plane of the diagram.

Explain why the electron beam is undeflected in the shaded region shown in Figure 1

Determine, using the following data, a value for the specific charge of the electron.

B = 1.59 mT

d = 5.0 cm

Figure 2 shows a cross-sectional view of the arrangement that Millikan used to determine the charge on the electron.

Millikan’s initial step was to determine the radius of the oil droplet.

Explain how Millikan used this apparatus to determine the radius of the oil droplet.

In your answer you should:

• describe the procedure used, the measurements taken and any additional data required

• describe how the radius was determined from the measurements

• state the physical principles and assumptions involved in the determination of the radius.

On one occasion, the radius of a droplet was determined to be 1.20 × 10⁻⁶ m. When the droplet was stationary, the voltmeter reading was 467 V.

Show that the charge on the droplet was approximately 8 × 10⁻¹⁹ C.

Table 1 shows the percentage uncertainty in each quantity.

Show that the absolute uncertainty in your answer to Question 02.2 is

Hertz did an experiment to determine the speed of radio waves.

Describe this experiment.

In your answer you should:

• include a labelled diagram

• state the measurements that were taken

• describe how the data were used to determine the speed of radio waves.

Figure 3 shows a modern version of Bertozzi’s experiment to measure the kinetic energy of high-speed electrons. A timer is used to measure the time taken for a pulse of electrons to travel from the detector to the aluminium block.

A potential difference (pd) of 1.30 MV is used to accelerate the electrons.

Show that each electron gains approximately 2 × 10⁻¹³ J of kinetic energy.

These electrons cause the temperature of the aluminium block to increase by 68.0 K The number of electrons that cause this increase in temperature is 4.50 × 10¹⁷

Deduce whether this increase in temperature is consistent with an accelerating pd of 1.30 MV.

specific heat capacity of aluminium = 903 J kg⁻¹ K⁻¹ mass of aluminium block = 1.50 kg

The speed of the electrons between the detector and the block is 2.88 × 10⁸ m s⁻¹.

Student A suggests that the non-relativistic equation for kinetic energy could be used. Student B suggests that the relativistic equation for kinetic energy is required in this situation.

Evaluate the suggestions of student A and student B. Support your answer with calculations.

The timer in Figure 3 records a time of 29.8 ns.

What is the proper time interval for an electron travelling from the detector to the aluminium block?

Tick (✓) one box.

The electrons in Figure 3 were accelerated from rest in 13 stages.

In each stage the electrons were accelerated by a pd of 100 kV.

As a result, an electron increases its speed and kinetic energy during each stage.

Compare, for an electron,

• its increase in speed for the first stage with that for the last stage

• its increase in kinetic energy for the first stage with that for the last stage.

Justify your answer.

No further calculations are required.