AQA A Level Physics Paper 3 Section B: Turning points in physics June 2019 question paper 7408/3BD

Figure 1 shows an experiment to measure the charge of the electron.

Negatively charged oil droplets are sprayed from the atomiser into the gap between the two horizontal metal plates. A potential difference is applied between the metal plates.

One of the droplets remains stationary.

Identify the forces acting on the stationary droplet. In your answer you should state the relationship between the forces.

The upthrust on the droplet due to the air it displaces is negligible.

The potential difference between the plates is changed to zero and the droplet falls at

Show that the radius of the droplet is about 1 × 10⁻⁶ m.

Assume that the droplet is spherical.

The potential difference between the plates is restored to its initial value and the droplet becomes stationary.

A student suggests that, if the droplet splits into two spheres of equal size, both spheres would remain stationary.

Deduce whether this suggestion is correct.

Newton used a corpuscular theory of light to explain reflection.

Figure 2 shows how corpuscles would reflect from a horizontal surface.

What happens to the horizontal and vertical components of the velocity of the corpuscles, according to the theory, when they are reflected?

Tick (✓) one box.

Newton used the corpuscular theory to explain the refraction of light at an interface between air and water.

Huygens used the wave theory to explain the refraction of light at the interface.

Discuss the evidence that led to the rejection of Newton’s corpuscular theory.

In your answer you should include

• how each theory explains refraction

• how experimental evidence led to the acceptance of the wave theory.

Light is now known to behave as an electromagnetic wave.

Describe a plane-polarised electromagnetic wave travelling through a vacuum. You may wish to draw a labelled diagram.

Figure 3 shows part of the apparatus used to investigate electron diffraction.

Electrons were accelerated through a potential difference to form a beam which was then incident on a thin metal foil.

Regions of maximum and minimum intensity formed on a photographic film behind the

State de Broglie’s hypothesis.

The voltmeter in Figure 3 shows a reading of 3.5 kV.

Determine whether this voltmeter reading is consistent with a de Broglie wavelength for the electrons in the beam of about 0.02 nm.

The experiment is repeated using a similar arrangement to that shown in Figure 3. Figure 4 shows the diffraction patterns from the two experiments.

State and explain two independent changes that could be made to the arrangement in Figure 3 to produce the result shown for the second experiment in Figure 4.

Table 1 shows data of speed v and kinetic energy E_k for electrons from a modern version of the Bertozzi experiment.

Classical mechanics predicts that E_k v².

Deduce whether the data in Table 1 are consistent with this prediction.

Discuss how Einstein’s theory of special relativity explains the data in Table 1.

Calculate, in J, the kinetic energy of one electron travelling at a speed of 0.95 c.