This diagram shows an electron tube. Electrons emitted from the cathode accelerate towards the anode and then pass into a uniform electric field created by two oppositely charged horizontal metal plates.

a i. Explain why the beam curves upwards between the plates.
ii. Explain how the pattern formed on the fluorescent screen shows that all the electrons have the same speed as they leave the anode.
b. Write down an equation relating the speed of the electrons v to the potential difference V _ac between the anode and the cathode.
c. The deflection of the beam upwards can be cancelled by applying a suitable uniform magnetic field in the space between the parallel plates.
i. State the direction of the magnetic field for this to happen.
ii. Write down an equation relating the speed of the electrons v, the electric field strength E that exists between the plates and the magnetic flux density B needed to make the electrons pass undeflected between the plates.
iii. Determine the value of B required, using the apparatus shown in the diagram, given that for an electron the ratio e/m_e = 1.76 x 10¹¹C Kg^-1.

Transcribed Image Text:

electron gun beam |fluorescent +1,0 kV +500 V screen cathode anode 10 cm 5.0 cm ov Figure 25.19