AQA A Level Physics Paper 3 Section B: Turning points in physics June 2023 question paper 7408/3BD

In Figure 1, a beam of electrons travels through the aperture in the anode and hits the screen.

Explain how the electrons that form the beam are emitted.

Show that the maximum speed of the electrons in the beam is about 1.3 × 10⁷ m s⁻¹.

A student suggests that the apparatus can be used to demonstrate the wave properties of electrons in the beam, provided that the aperture in the anode has a suitable diameter.

Discuss whether the student is correct. Support your answer with a calculation.

In 1897, J J Thomson determined a value for the specific charge of an unknown particle. The unknown particle is now known to be the electron.

Describe one method to determine the specific charge of the electron.

Your answer should include:

• a description of the apparatus used and the measurements made

• a description of how the specific charge can be determined using these measurements

• an explanation of the conclusion made by Thomson from the value that he determined.

In an experiment to determine the electronic charge, a charged oil drop falls from rest between two uncharged plates.

The oil drop has a weight of 1.2 × 10⁻¹⁴ N and a radius of 6.8 × 10⁻⁷ m. Ignore the buoyancy force of the air on the oil drop.

Figure 2 shows the variation of the vertical speed of the oil drop with time.

Calculate the viscosity of the air between the plates.

During the experiment, an electric field is produced between the plates and is adjusted until the oil drop is stationary. The electric field strength is 18.8 kV m⁻¹.

Discuss whether the outcome of the experiment is consistent with the accepted value for electronic charge.

An electromagnetic wave is propagating through space.

Figure 3 shows the variation of the magnetic flux density of the wave with distance. The magnetic field is in the xz plane. The y-axis is at right-angles to the xz plane.

Draw and label on Figure 3:

• the corresponding electric field

• the direction of propagation of the wave.

Which scientist proposed the electromagnetic wave model of light?

Tick () one box.

Another theory of the nature of light was proposed by Isaac Newton.

Describe how Newton’s theory was used to explain the refraction of light as it moves from air into glass.

Describe a demonstration using visible light that can be performed in a school laboratory to show that Newton’s theory is not correct.

Einstein developed his theory of special relativity from two postulates. One postulate states that physical laws have the same form in all inertial frames.

State the other postulate and explain how it is consistent with the equation:

A proton leaves a particle accelerator at point X and moves at a constant speed towards a target at point Y. The speed of the proton is 2.5 × 10⁸ m s⁻¹ in the frame of reference of the target.

The distance XY in the frame of reference of the proton is 38 m.

Calculate the distance XY in the frame of reference of the target.

Show that the kinetic energy E_k of the proton is about 1.2 × 10⁻¹⁰ J.

Sketch on Figure 4 the variation of E_k with speed v for a proton.

To help you, the dashed line represents

where m₀ is equal to the mass of a proton at rest.