Keep the cover on the cathode ray tube (CRT). The interior of the CRT is at very low pressure. If the tube is broken, it will implode and hurl glass everywhere at very high speeds.
DO NOT TOUCH BARE WIRES! The voltaes involved in this experiment are on theorder of 500V. Anytime that two parts of your body are at differentpotentials, you will get a shock and the larger the potential differencebetween the two points the greater the shock. At 500V, you will get a BIG shock and could possibly DIE.
There are three sets of charged plates inside the CRT. The first set ofplates accelerate the electrons from rest to the speed at which they willtravel the rest of the way to the screen. This accelerating voltage is equalto |Vc| + |Vb|. The other two pairs of plates are aligned so that the electricfield formed by each pair of plates is perpendicular to the motion of theelectrons - one set has a vertical field and the other pair has a horizontalfield. Both pairs of plates are used to change the direction in which theelectrons move. By adjusting the deflecting voltage, Vp, the amount of deflection is changed. Take one set of data for each of the two setsof deflecting plates, horizontal and vertical.
Deflection of the electron beam can be caused by any electric field. Charged glass and bakelite rods are used to show this effect. Any distributionof charge creates anelectric field around itself, therefore there will be electric field around thecharged rods. By placing the charged rods near the dot on the CRT screen, youshould be able to see deflection of the beam spot. More deflection will occurwhere the field is stronger. Note also the difference between deflectioncaused by static fields and those caused by moving electric fields. You shouldbe able to see the difference between deflections caused when holding thecharged bakelite in a stationary position near the screen and when moving thecharged bakelite back and forth in front of the screen. Record yourobservations and be sure to differentiate between the forces caused by stationary charge and the forces caused by moving charge. Also notice whathappens if there are sparks between the charged rod and the CRT case. Rememberthat a spark is a movement of charge.
Make sure to report the velocity of the moving electrons in the CRT. Comment on whether or not changing the magnitude of the velocity would effect the size of the deflection.
Plot graphs for both horizontal and vertical deflections of the electronbeam. Is D proportional to Vp as shown by your graph? Does oursimplified analysis in the discussion predict your result? Why or why not?
Comment on the two graphs - for horizontal and vertical deflections - are they the same or do they differ? Explain why you think this is so.
Comment on your observations of the deflections caused by the charged rods.Be clear and concise in your explanations of what you saw. Where are theregions of highest and lowest charge density? Be very clear in explaining yourlogic for the interpretation of charge distribution for this plastic rod. Also, state which rod had the greater charge density, glass or plastic. Be sureto differentiate between the forces caused by stationary charges andthe forces caused by moving charge.
Tubes similar to the CRT used in this experiment are used to make equipmentsuch as oscilloscopes, EKG monitors, some computer monitors, and sometelevision sets. Use your observations to suggest methods you would use to convert the simple CRT used in this experiment into one of these pieces ofequipment.
| Section | Points |
|---|---|
| Purpose | 1 |
| Results | 3 |
| Graphs | 3 |
| Calculations | 3 |
| Analysis | 10 |