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Cathode Rays and Cathode Ray Tube

Background.

The electron was the first of the sub-atomic particles to be identified. It was discovered by Sir Joseph John Thomson in 1897. His experiments were concerned with the passage of electricity through gases at low pressure. With development of vacuum pumps and the techniques of sealing metal electrodes in glass tubes made it easier to carry out experiments with gas discharge tubes.

Objectives

By the end of the lesson you should be able to:

-Describe production of cathode rays

-State the properties of cathode rays

-Explain the functioning of a C.R.O. and T.V tube.

-Explain uses of the C.R.O.

-Solve problems involving the C.R.O.

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Introduction.

In this topic we look at how gases at low pressure start to conduct electricity steadily in enclosed tubes and movement of electrons in evacuated tubes.

Production of cathode rays

By heating a metal to a high temperature, electrons can be emitted from its surface. This process is known as thermionic emission and the electrons so produced are called thermions. A stream of fast moving electrons is called cathode rays. Play the animation by clicking on its play button and observe what happens.


Observation

When air is pumped out, initially there is no effect but as the pressure decreases to a few mm of mercury thin streams of luminous gas appear between the cathode and the anode. Further reduction of pressure causes the tube to fluoresce.
Explanation

Some kind of radiation proceeds in straight lines from the cathode, C towards the anode, A and consists of fast moving electrons 'shot' out from the cathode with high velocity. The released electrons are repelled by the negative cathode and attracted by the anode hence moving at high velocity.

Cathode ray tube

The cathode ray tube is used for generating cathode rays. The animation below shows how the cathode ray tube is used to produce cathode rays. Click on the play button and observe.

Observations

The millimeter pointer deflects slightly indicating that current flows from the anode to the cathode.

Explanation

When the cathode is heated from the low tension source of about 6V, its temperature rises and thermal electrons are emitted from its surface. The emitted electrons are repelled by the cathode and further accelerated by the anode at positive potential. The cathode is coated with Barium or Strontium oxides to increase supply of electrons. Electrons can reach the anode without hindrance or loss of energy as there is a vacuum in the tube.

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Properties of the cathode rays

The properties of the cathode rays can be displayed using a cathode ray tube. Some of them are outlined using some illustrations and others are animated. Play the animation below and observe.

Observations

1. The image of the cross is projected onto the fluorescent screen.

2. The paddle wheel starts rotating.

3. The cathode rays are seen to deflect in a curve towards the North Pole.

4. The rays are deflected towards the positive plate in a curve.

5. The particles of gas become ionized / charged and move towards the positive plate.

6. The photographic plate is fogged.

Explanation

1. The cathode rays move in straight lines hence the cross blocks some of them as the rest continue in their path. This forms a shadow on the screen.

2. The cathode rays move with high speed hence possesses kinetic energy which pushes the plates of the paddle wheel making it to rotate.

3. The deflection shows the particles have small mass and are negatively charged.

4. The deflection towards the positive plate indicates they are negative.

5. The ionization confirms that the cathode rays possess mass and energy and are charged.

6. The fogging of the photographic plate shows the rays possess high energy and hence move at high speed.

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The Cathode Ray Oscilloscope

It is a device that uses cathode rays to display waveforms and is used to study their properties on a fluorescent screen. Play the video below to see a working CRO.

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Parts of the cathode ray oscilloscope

The cathode ray oscilloscope has the following parts:

(i) The electron gun.

(ii) A system of deflecting plates.

(iii) An evacuated strong glass tube.

(iv) A fluorescent screen at one end of the glass envelope.

Place the mouse over the different labeled parts to get an explanation of what they are and their functions.

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Working of the CRO

The animation below shows the working of the CRO, click on its play button and make observations.

Explanation

When the cathode- grid potential is made more negative, it repels electrons and only those with very high kinetic energy will manage to proceed. Therefore the grid determines the intensity of cathode rays leaving the cathode. The anode is used to attract the cathode rays from the cathode hence giving them more kinetic energy. The more positive the anode is made the greater the kinetic energy of the cathode rays. Hence the spot will appear brighter. The X-plates are placed vertical (X1 and X2) such that one plate is positive while the other is negative. The potential difference across the plates causes the cathode rays to drift horizontally. These plates are connected to the time-base which varies and determine the frequency of the horizontal drift of the cathode rays. The Y-plates are placed horizontally facing each other such that one plate is positive and the other negative (Y1 and Y2). The potential difference between them causes the cathode rays to drift vertically. When the X-plates is switched on while the time base is off, the spot will move slowly from left horizontally, before starting again. On turning on the time- base, the spot moves faster and when increased a horizontal line is produced. When both the X and Y plates and time-base are on the spot will drift both vertically and horizontally producing a wave form of the voltage input. As the time-base is increased, the wave form is saw-tooth shaped to sinusoidal shape.

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Television tube

A T.V consists of a short electron gun, two pairs of coils and a fluorescent screen.


 

Discussion

The black and white television works in a similar way as the CRO. except that the deflection is by magnetic fields produced by currents in coils. It has two time-bases one connected to the Y-plates and the other to the X-plates such that they sweep the dot vertically and horizontally respectively. The deflections set the electron beam to move repeatedly across and down the screen forming a complete picture consisting of adjacent lines in about 1/625 seconds. The scan produces slightly different pictures enabling movement to be portrayed in the same way as in a film with rapid succession of still images giving an impression of continuous movement. A colour television works on the same basic principle but uses three electron guns one for each primary colour; red, blue and green. Though the electron beams are not coloured themselves, each affects only one set of red, blue and green phosphors on the screen and the amalgamation of the three-coloured images produces a full colour picture. In colour televisions, the incoming signal is processed by a decoder. The beams pass through a metal shadow mask with thousands of holes arranged symmetrically so that the beam from each of the three guns only strikes those phosphor dots of appropriate colour.

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Uses of the Cathode Ray Oscilloscope

The Cathode Ray Oscilloscope can be used to display any electrical signal whose variations can be put in form of a voltage. Some of its uses include measurement of voltage, measurement of frequency, Measurement of small intervals of time among others. The uses stated here are captured in the animations which follow.

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Measurement of a.c voltage

Click on the play button and observe how this measurement is done.

Observations

When connected a straight vertical line is displayed on the screen

Explanation

The length of the displayed line shows the peak to peak voltage of a.c voltage

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Measurement of d.c voltage

Direct current can be measured using the CRO as shown in the following animation. Click on the play button and observe how this is done.

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Measurement of frequency

A signal of unknown frequency like the heart beat of a person or some musical sound can be fed to the input terminals of the oscilloscope. Using the time-base, the settings are adjusted to obtain waveforms that appear stationary. Frequency of the signal is equal to that of the electron beam oscillating between X - plates. The time-base is read off from the scale division and the mean is determined by counting the number of full waves in a certain distance on the X - axis. Mean wavelength, is equal to the distance measured on the screen divided by the number of waves displayed. Knowing the wavelength and the time-base, the mean period T for a full wave is found. Frequency, f = 1/T.Click on the play button on the animation below to see how frequency can measured using a CRO.

Observation

A sinusoidal-wave is displayed on the screen

Measurement of small intervals of time

If a drum is struck twice consecutively and sound picked by a microphone and connected to a C.R.O. The microphone will convert sound energy into electrical pulses. When these pulses are fed to the input terminals of the oscilloscope and the time-base put on, a signal will be displayed on the screen. If time base of the C.R.O and the distance between the signals of the pulse is known, the time interval between them can be calculated.

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