Thermionic

PHYSICAL SCIENCE

MODULE

E0801

THERMIONIC EMISSION

NAME: CLASS:

Return by:Theory 45Practical 25Applications 30Research 50TOTAL: 150

© Hillcrest High School, Private Bag 1012, Hillcrest, 3650All rights reserved. This module or portions thereof may not be reproduced

by any means without the school's permission.

THERMIONIC EMISSION Page 2

Objectives

On completion of this module you should be able to:

1. define thermionic emission;2. explain the operation of a simple thermionic diode;3. define and understand the terminology relating to thermionic diodes;4. describe some of the modern applications of thermionic diodes and electron guns.

Tasks

1. Answer the questions in the Theory section of this module before doing the practical work. The questions are intended to develop your knowledge and understanding of the basic concepts in thermionic emission.

2. Complete the Practical section.3. Complete the Applications section.4. You will have to research the answers to the questions in this module.

Resources and Supportive Material

1. Videos: Thermionic EmissionThe Cathode Ray Oscilloscope

2. Computer: The Way Things Work (CD)Encarta 95 / 96 (CD)Infopedia (CD)The New Grolier Multimedia Encyclopedia (CD)Bookshelf 95 (CD)Electricity & Magnetism (CD)Encyclopedia of Science (CD)

Guidelines

1. You have two weeks from the date of receipt to complete this module.2. Half your raw score for this module will be added to your term marks if the module is

completed by the due date. Failure to meet the deadline will result in your score not counting towards your term mark.

3. Since this is an extension module it is important that your core modules have priority over this one. Make sure that you have understood and completed the work in your current core module before attempting this module.


THEORY

1. What is thermionic emission?_____________________________________________

__________________________________________________________________ [3]

2. Give an alternative name for thermionic emission and explain its origin.

_____________________________________________________________________

__________________________________________________________________ [3]

3. What is an electrode? ___________________________________________________

__________________________________________________________________ [3]

4. Draw and label a sketch of a simple thermionic diode.

[5]

5. Explain what is meant by (a) a directly heated cathode and (b) an indirectly heated cathode.

_____________________________________________________________________

__________________________________________________________________ [4]

6. Will an electric current flow through the thermionic diode if the cathode is not heated? Explain your answer.

_____________________________________________________________________

__________________________________________________________________ [3]

7. What is the name of the second, unheated electrode?________________________ [1]


8. Will an electric current flow through the thermionic diode if the cathode is heated but there is no potential difference between the two electrodes? Explain your answer.

_____________________________________________________________________

__________________________________________________________________ [3]

9. Under what two conditions will a current flow through a thermionic diode?

_____________________________________________________________________

__________________________________________________________________ [4]

10. What, other than supply voltage, limits the maximum current flowing through the thermionic diode? Explain your answer.

_____________________________________________________________________

__________________________________________________________________ [3]

11. What is the name for the maximum current mentioned in 10 above?

__________________________________________________________________ [1]

12. Suggest a way of increasing this current without increasing the potential difference between the electrodes.

_____________________________________________________________________

__________________________________________________________________ [3]

13. Why is a thermionic diode often referred to as a thermionic valve?

_____________________________________________________________________

__________________________________________________________________ [3]

14. Suggest why is it necessary to evacuate the space inside a thermionic diode.

_____________________________________________________________________

__________________________________________________________________ [3]

15. What is the most important characteristic of a thermionic diode?

__________________________________________________________________ [3]

[45]


PRACTICAL

AIM: To determine the characteristics of a thermionic diode.

APPARATUS:

One 15 V d.c. supply; one 5 V d.c. supply; one EZ80 vacuum tube and base (with built-in variable resistor); two d.c. voltmeters; one d.c. ammeter; variable resistor; connecting leads.

CIRCUIT DIAGRAM:

PROCEDURE:

1. Construct the above circuit and ask your teacher to check it before switching it on.

2. Use the heater circuit's built-in variable resistor RH to adjust the heater voltage to 2 V d.c.

3. Adjust the potential difference across the diode to 2 V d.c. using the second variable resistor.

4. Record the ammeter and voltmeter readings for the plate circuit.

5. Increase the potential difference across the fixed resistor to 4 V d.c. and repeat step 4.

6. Continue increasing the voltage by about 2 V until a maximum of 15 V is reached. Record your ammeter and voltmeter readings each time.

7. Adjust the heater voltage to 3 V d.c. and repeat steps 3 to 6.

8. Plot your results on the same set of axes, with potential difference on the x-axis and current strength on the y-axis. Link your data points smoothly.

RESULTS:

V

A

V

+

-RH

15 V max.


Table 1: P.d. versus I for two different cathode temperatures (See Graph 1)

LOW CATHODE TEMPERATURE(2 V ACROSS HEATER)

HIGH CATHODE TEMPERATURE(3 V ACROSS HEATER)

RESISTOR VOLTAGE / V

PLATE CURRENT / mA

RESISTOR VOLTAGE / V

PLATE CURRENT / mA

[15]

CONCLUSIONS:

1. Is a thermionic diode an ohmic conductor? Explain your answer.

_____________________________________________________________________

__________________________________________________________________ [3]

2. What are the saturation currents for the two different heater settings?

_____________________________________________________________________

__________________________________________________________________ [4]

3. What is the relationship between the strength of the thermionic current and the cathode temperature?

_____________________________________________________________________

__________________________________________________________________ [3]

[25]


APPLICATIONS

1. Suggest why it takes old valve radios a few seconds to start playing music, etc. after being switched on.

_____________________________________________________________________

__________________________________________________________________ [2]

2. An alternative device to the thermionic diode is the semiconductor diode. Describe the main differences between the two diodes, and explain very briefly how the semiconductor type works.

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

__________________________________________________________________ [5]

3. List some of the main disadvantages of thermionic diodes compared with semiconductor diodes.

_____________________________________________________________________

__________________________________________________________________ [3]

4. What is one of the main applications of thermionic and semiconductor diodes?

_____________________________________________________________________

__________________________________________________________________ [3]

5. Draw the circuit diagram for the diode circuit used to achieve full-wave rectification of alternating current. This may be a bridge rectifier or a triode circuit.

[5]


6. Draw a fully-labelled sketch of a cathode ray tube. Alternatively, you may print a suitable diagram from a multimedia CD and paste it here.

[7]

7. Briefly describe the operation of a cathode ray tube.

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

__________________________________________________________________ [3]

8. Where are cathode ray tubes used?

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

__________________________________________________________________ [2]

[30]


RESEARCH

You are required to research one of the following topics. Make your choice and report it to your teacher for verification. Your research should be summarised in the form of a report of about 1 000 words. Remember to run an English (UK / International) spelling check, title your research and record your word count. Also ensure that you record the details of all your reference materials in a bibliography as follows: Author (Surname and Initial). "Title", Publisher, Year.

1. Research the history of the development of television.

2. Find out about the use of cathode ray tubes (CRTs) in the "glass" cockpits of modern aircraft. What are their advantages and why are they now so popular?

3. Describe the construction and operation of a colour television set.

4. Describe the construction, operation and use of a CRT oscilloscope.

5. Although CRTs are the most common type of monitor used in the computer industry, lap-tops employ liquid crystal displays (LCDs) similar to those in calculators. Find out how these work, and try to establish whether they are likely to supersede CRTs as the standard television display.

[50]

Documents

Thermionic