AQA A Level Physics Paper 3 Section B: Turning points in physics June 2018 question paper 7408/3BD

Figure 1 shows apparatus which can be used to determine the specific charge of an electron.

Electrons are emitted from the filament and accelerated by a potential difference between the filament and anode to produce a beam. The beam is deflected into a circular path by applying a magnetic field perpendicular to the plane of the diagram.

Describe the process that releases the electrons emitted at the filament.

Table 1 shows the data collected when determining the specific charge of the electron by the method shown in Figure 1.

Show that the specific charge of the electron is given by the expression

Using data from Table 1, calculate a value for the specific charge of the electron. Give your answer to an appropriate number of significant figures.

At the time when Thomson measured the specific charge of the particles in cathode rays, the largest specific charge known was that of the hydrogen ion.

State how Thomson’s result for the specific charge of each particle within a cathode ray compared with that for the hydrogen ion and explain what he concluded about the nature of the particles.

Figure 2 shows a modern version of the apparatus used by Hertz to investigate the properties of electromagnetic waves. Electromagnetic waves are continuously emitted from a dipole transmitter. The electromagnetic waves are detected by a dipole receiver. An oscilloscope is used to display the amplitude of the detected signal at the dipole receiver.

Figure 3 shows the same apparatus when the dipole receiver has been rotated through an angle of 90º

Sketch a graph on Figure 4 to show how the amplitude detected by the dipole receiver varies with angle of rotation as the receiver is turned through 360° Start your graph from the position shown in Figure 2.

Maxwell derived the equation c =c = for the speed c of electromagnetic waves, where is the permeability of free space and ₀ is the permittivity of free space.

Explain, using a suitable calculation, why this equation led to the conclusion that light is an electromagnetic wave.

Figure 5 shows a vacuum photocell in which a metal surface is illuminated by electromagnetic radiation of a single wavelength. Electrons emitted from the metal surface are collected by terminal T in the photocell. This results in a photocurrent, I, which is measured by the microammeter.

The potential divider is adjusted until the photocurrent is zero.

The potential difference shown on the voltmeter is 0.50 V

The work function of the metal surface is 6.2 eV

Calculate the wavelength, in nm, of the electromagnetic radiation incident on the metal surface.

The intensity of the electromagnetic radiation is increased. No adjustment is made to the potential divider.

The classical wave model and the photon model make different predictions about the effect on the photocurrent.

Explain the effect on the photocurrent that each model predicts and how experimental observations confirm the photon model.

The potential divider in Figure 5 is returned to its original position so that a photocurrent is detected by the microammeter. The potential divider is then adjusted to increase the potential difference shown on the voltmeter.

Explain why the photocurrent decreases when this adjustment to the potential divider is made.

The apparatus shown in Figure 5 is used to investigate three different metal surfaces A, B and C. Table 2 shows, for each of the three surfaces, a voltmeter reading V and the corresponding photocurrent I. The same source of electromagnetic radiation is used throughout the investigation.

Which conclusion about the relationship between the work functions of A, B and C is correct?

Tick (✓) the correct box.

Figure 6 shows a diagram of the Michelson-Morley interferometer that was used to try to detect the absolute motion of the Earth through the ether (æther).

Light from the monochromatic source passes through the semi-silvered glass block and takes two different paths to the viewing telescope. The two paths, PM₁ and PM₂ are the same length. Interference fringes are observed through the viewing telescope.

It was predicted that when the interferometer was rotated through 90^o the fringe pattern would shift by 0.4 of the fringe spacing.

Explain how the experiment provided a means of testing the idea that the Earth had an absolute motion relative to the ether.

Your answer should include:

• an explanation of why a shift of the fringe pattern was predicted

• a comparison of the results of the experiment to the prediction

• the conclusion about the Earth’s absolute motion through the ether.

Extra space is available on the next page

The Michelson-Morley experiment provides evidence for one of the postulates of Einstein’s theory of special relativity.

State this postulate.

State the other postulate of Einstein’s theory of special relativity.

One consequence of the special theory of relativity is length contraction.

Experimental evidence for length contraction is provided by the decay of muons produced in the atmosphere by cosmic rays.

Figure 7 shows how the percentage of the number of muons remaining in a sample changes with time as measured by an observer in a frame of reference that is stationary relative to the muons.

In a particular experiment, muons moving with a velocity 0.990 c travel a distance of

1310 m through the atmosphere to a detector.

Determine the percentage of muons that reach the detector.