Electron Deflectiom

ROOP KANWALJIT KAUR

PHYSICS 04

12TH MARCH, 2008

ELECTRON DEFLECTION

Introduction:

A beam of moving electrons can be deflected in horizontal or vertical direction by the application of electric or magnetic field. In this experiment, first the deflection of the electron beam was measured systematically as a function of the voltage differences across the x-direction plates and then the y-directions plates by using a cathode ray tube. In the second part the deflection produced by applied magnetic field was measured and in the third part the Lorentz force magnetic and electric voltage was measured in order to prove the validity of Lorentz force. This experiment is based on the fact that a beam of particles with charge e and mass m, moving at speed u through a region with uniform electric field E and uniform magnetic field B experience a force and this force is given by:

F = eE + qvÐ"--B

The force in turn is responsible for deflecting the electron from its original path.

Test and procedure:

This experiment uses a large vacuum tube which contains an "electron gun". The gun is a metal filament that is heated so hot that some of the electrons of the metal have enough kinetic energy to leave the filament. They are then accelerated by a potential difference which gives them a kinetic energy, equal to,KE= (mv^2)/2. (e is charge of an electron, -1.6x10-19 coulombs, and m is its mass, 9.1 x 10-31 kg.) The electron beam becomes visible when the electrons strike a fluorescent screen.

As the electrons move in the horizontal (x) direction, an electric force may be applied in the vertical (y) direction by applying a potential difference Vp between two deflecting plates. In the first part of the experiment we will study just the electric deflection; in the second part only the magnetic deflection; and, in the third part a combination of both forces applied in opposite directions so as to cancel each other's effect.

Part a) Electron deflection:

In the beginning the power supply was switched on in order to energise the filament,until seen on the CRT.At this point the voltage labeled Vc In the beginning voltage Vb was set to about 250 V, and Vc was so adjusted that a sharp spot of the electron beam can be possible on the face of the tube.

was adjusted to provide the proper focus, while Vb and Vc voltages together determine the total potential difference (в?†V) between the plates of the CRT.

After leaving the accelerating and focusing electrodes, the beam passes between two sets of deflecting plates which were arranged to deflect the beam in orthogonal direction. The beam travels some distance L to the end of the tube, where it hits the screen and we were able to see it as bright spot. At this point the deflection was set to zero and the

With the tube focused at the highest B+, increase the deflection voltage Vd. The spot on the tube face should move vertically in response to the electric field. Measure the deflection DE for a few values of V, and plot DE as a function of V. Reset the voltage B+ to the lower value and repeat the measurement, plotting the new data on the same graph.

PART B MAGNETIC DEFLECTION:

The magnetic deflection of the electric beam will be studied using two solenoids. The solenoids should be connected in series to the current power supply and placed on either side of the

CRT tube. Make sure the magnetic fields are pointing in the same direction. Connect the DMM to the output of the supply; this supply produces a voltage Vm. The Vm and Vp are adjusted to 0 volts and the position is recorded. This is the zero position for subsequent measurements of the deflection. Measure the deflection Db for a few values of current I, and plot Db as a function of Vm.

PART C LORENTZ FORCE:

The last part is to prove that there is net force acting on the electron called Lorentz force. In order to prove this the electron is deflected using the magnetic voltage and electric voltage is applied in order to bring it back to its original position. This is based on the fact that when the deflection is zero the force is zero.

Results and observations:

Vb and Vc :

The largest value for Vb and Vc for which you can get a sharp spot, and the lowest value for which the spot is bright enough to easily see are 110V and 100V respectively. Largest and smallest values of B+ that your tube allows corresponds to the range of speeds u that is available, and will be used in the deflection measurement.

D vs Vp:

The second measurement taken was the deflection in horizontal and vertical direction when the electric field was applied. With the tube focused at the highest Vb, the deflection voltage Vp is increased. The spot on the tube face should move vertically in response to the electric field.

Deflection DE is measured for a few values of Vp, and plot DE as a function of Vp. The measurements were such taken that voltage was adjusted to be zero volts and the corresponding position was set as zero deflection. After that the position was increases by 1/10th of an inch and the corresponding voltages were measured. Two sets of measurements were taken one in vertical and one in horizontal direction.

VERTICAL DEFLECTION HORIZONTAL DEFLECTION:

Deflection(in) Voltage (V)

0 0

0.1 -1

0.2 -2.2

0.3 -3.3

0.4 -4.4

0.5 -5.9

0 0

0.1 1.1

0.2 2.3

0.3 4.5

0.4 5.7

0.5 6.5