electron_deflection.doc  -­‐-­‐  froehlich  

AP Physics Electron Deflection Lab In a cathode ray tube (CRT), electrons travel in a narrow beam from an electron source, called the electron gun, through focusing and deflecting plates, to a luminescent screen. The screen is coated with a monochromatic, short persistence phosphor that emits light when excited by energetic electrons. The beam is initially accelerated by a potential difference between the cathode and anode. Partway along the tube, the electron beam passes between two pairs of deflecting plates, one after the other, called the X and Y plates. When a potential difference is applied across the X plates, the beam is deflected horizontally. Likewise, the Y plates deflect the beam vertically. The purpose of this lab is to study these deflections as a function of the voltages applied to the cathode, anode, and the X and Y plates.

CAUTION: We are going to be using some high potentials in this lab. When making connections to the CRT, make sure the power supply is OFF. Do not attach wires to any part of your body or the body of your lab partner(s) or instructor. Make the initial connections as shown in the illustration to the right. Your power supply may look different than the one shown here, so here are some things to consider:

• The heater is being powered by approximately 6 VAC. What is the function of the heater?

• What is the potential difference on the anode with respect to the cathode? If your power supply is different from the one pictured, connect your supply in such a way that the anode is positive with respect to the cathode. Measure the voltage.

 

Key elements of the cathode ray tube. The focusing device is not shown.  

 

electron_deflection.doc  -­‐-­‐  froehlich  

After you have checked the connections, plug in and turn on the power supply. You should see a spot of light on the face of the CRT. Adjust the focus control on the base of the apparatus to make the spot as small as possible. When you have a well-focused beam, you can connect the X plates to a low voltage source ( < 20 volts) and observe the deflection of the beam as the voltage is changed. The grid on the face of the tube is marked off in millimeters, and may be rotated to coincide with the x-deflection of the beam and provide an x-axis for measurement.

• Measure the x-deflection and note the

voltage present on the X plates.

• Now, reverse the potential by swapping the wires going to the X plates. Observe the new deflection. Are the deflections of equal magnitude?

• Create a plot of the deflection of the

beam as a function of the voltage applied to the X plates.

• From your graph, is the deflection per

volt the same in all cases?

In applications in which a CRT is used in an oscilloscope, the deflections are used to determine an applied voltage. The voltage required to obtain a beam deflection of 1 cm is called the sensitivity of the CRT.

• Is the sensitivity of the CRT the same for horizontal and vertical deflections?

Substantiate your claim with graphical evidence. • The X and Y plates are identical in construction. Which plates are closer to the

face of the tube? How do you know?

• If you change the potential difference between the cathode and anode, how does that affect the sensitivity of the CRT? Substantiate your claim.

Connections to the heater, cathode and anode  

Connections to a pair of deflection plates  

electron_deflection_supplement.doc  -­‐-­‐  froehlich  

AP Physics Electron Deflection Lab Supplement In order to have an intelligent discussion, we need to have uniformity in the way we describe the elements of the cathode ray tube, and their associated measurements. The illustration that follows should suffice.

This illustration depicts, from left to right, the accelerating plates (cathode, anode), the y deflecting plates, and the phosphorescent face of the tube. The dotted line represents the path an electron will follow.

• Va = accelerating potential • Vd = deflecting potential • w is the width of the deflecting plates. This is the x distance under which the

electron experiences a force in the y direction. • L is the distance from the end of the deflecting plates to the screen. • Δy is the y distance the electron is deflected while under the influence of the

electric field between the deflecting plates. • D is the total displacement on the y-axis from the point where the electron strikes

the screen when the deflecting potential is zero. Your challenge is to take the data you have gathered from varying the accelerating and deflecting potentials and devise a simple equation for D, such that D is a function of a constant, Va and Vd.