Specific charge

This question is about Thomson’s determination of the electron’s specific charge.

You should not use the values of the electron charge, mass or specific charge from the data and formulae booklet in this question.

Figure 1 shows part of the apparatus that Thomson used to measure the specific charge of the electron.

A modern version of Thomson’s apparatus uses thermionic emission in the production of the electron beam.

Suggest how the electron beam is produced in this modern version. You should include a labelled diagram and suitable values for the power supplies used.

A potential difference (pd) of 750 V is applied between the horizontal parallel plates. Figure 2 shows the plates and the path of the electrons.

The electrons enter the electric field between the plates at S and leave it at T with speed v. Beyond T the electrons travel in a straight line at an angle θ. θ is read from a scale on a fluorescent screen.

The plates are of length 10.0 cm and are separated by a distance of 5.00 cm.

A uniform magnetic field of flux density 0.640 mT is now applied between the plates. This returns the electron beam to its undeflected position.

Show that the horizontal component of the velocity of the electrons is approximately

The magnetic field is switched off and θ is measured to be 25°.

Show that the vertical component of the velocity of the electrons at T is approximately

Show that the vertical acceleration of the electrons between the plates is

Determine a value for the specific charge of the electron.

Explain the significance of the result of Thomson’s determination.

Some students are given a version of the apparatus to repeat Thomson’s experiment.

Figure 3 shows a section of the measuring scale on the fluorescent screen and the spot from the electron beam.

The students consider two methods for minimising the percentage uncertainty in the measurement of the angle.

Method 1:

• choose a magnetic flux density B

• then set the magnetic flux density to B with no pd

• then adjust only the pd until the beam is horizontal

• then switch off the magnetic field and measure the angle θ.

Method 2:

• choose an angle θ

• then set the pd so that the spot from the electron beam is at θ with no magnetic field

• then adjust only the magnetic field until the beam is horizontal.

Discuss both of these methods with reference to the percentage uncertainty in θ that they will produce. In your answer, suggest which method will minimise the percentage uncertainty. No calculations are required.

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

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.

Figure 1 shows a diagram of a discharge tube used by JJ Thomson to investigate cathode rays.

The direction XY is horizontal and at right angles to the axis of the tube.

When correct connections are made to a high-voltage power supply, a cathode ray is produced. The cathode ray hits the centre of the fluorescent screen.

Describe how a cathode ray is produced in the discharge tube in Figure 1.

P and Q are metal plates that can be attached to a second power supply.

In an experiment, a potential difference (pd) is applied across P and Q so that P is positively charged and Q is negatively charged. This deflects the cathode ray.

Then a magnetic field is applied between the plates so that the cathode ray follows its original path to the centre of the screen.

What is the direction of the magnetic field? Tick () one box.

Changes are made to the apparatus so that the particles in the cathode ray travel with a greater speed as they pass between plates P and Q.

Explain how the cathode ray is restored to its original path by adjusting:

• only the electric field strength between P and Q

• only the magnetic flux density.

electric field strength only

magnetic flux density only

Using the apparatus in Figure 1, Thomson determined the specific charge of the particles in the cathode rays. Thomson compared this result with the specific charge of the hydrogen ion.

Discuss the significance of Thomson’s results for the particles in cathode rays, when compared with the specific charge of the hydrogen ion.

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.