5370705 9 1 cathode ray oscilloscope doc

Sek Men Sains Muzaffar Syah

170

9.1

Explainthermionicemission

Thermionic emission is the process of emission of electrons from a heatedmetal’s surface.

Metal consists of a large number of electrons which are free to move. At room temperature, the electrons are free to move but remain inside

the metal. The electrons cannot escape at the surface because they are held back

by the attractive forces of the atomic nucleus. If the metal is heated at a high temperature, some of the free electrons

may gain sufficient energy to escape from the metal.

What iscathoderays?

Thermionic emissions can be used to produce a continuous flow ofelectrons in a cathode ray tube. These electrons can be made tomove at a high speed in a vacuum tube.

The vacuum tube is an evacuated glass tube with a positiveelectrode called anode and a negative electrode called cathode.

When the cathode is connected to the anode by a high voltagesupply, the electrons are accelerated at a high speed from thecathode to the anode.

The beam of electrons moving from the cathode to the anode iscalled cathode rays.

Cathode rays can be used in picture tube of a television, a cathode rayoscilloscope and the visual display on a radar screen.

The properties of cathode rays can be summarized:1. Negatively charged particles called electrons.2. Travel in straight lines and cast sharp shadows.3. Travel at very high speed and have kinetic energy.4. Can cause fluorescence. (A process where the kinetic energy of

the electrons is converted into light energy)5. Deflected by electric and magnetic fields.

Describethepropertiesofcathoderays


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Activity 1: To Investigate the properties of electron streams in a Maltese cross cathode ray tube.

Maltase cross tube

Observation

Procedure Observation Explanation Conclusion

6V heatersupply isconnected

The shadow is formedby the ray from the________ __________

Light rays travel in a___________ line

6V heatersupply and 3kV powersupply areconnected

The shadow isformed by the _____________

Cathode rays travel in a _______line.Cathode rays cause __________.Cathode rays carry ________energy and converts to_________ energy when they hitthe screen.

A bar magnetis broughtclose to thecathode rays

The _________ forceis produced becausethe cathode rayscarry a _________

Cathode rays can be deflected by________ fields.The Fleming’s ______-hand ruleis used to determine the directionof motion.

1. In diagram (a) the shadow of the Maltese cross is not deflected by any magnetic field. Indiagrams (b) and (c), two magnets are placed by the side of the bulb. Draw the shadow of thecross in (b) and (c) to show how the shadow of the Maltese cross is deflected.

(a)(b)

(c)


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Activity 2: Investigate the properties of cathode rays in an electric field by using a deflectingcathode ray tube

1. Connect the deflecting cathode ray tube tothe 6.0 V ac power supply

2. Connect the anode and cathode to EHTpower supply.

3. Connect the deflecting plates using aseparate EHT.

(a) Top plate is connected to the positive EHTand lower plate is connected to thenegative terminal EHT.

(b) Top plate is connected to the negative EHTand lower plate is connected to the positive terminal EHT.

Procedure Observation Explanation Conclusion6V heater supply and3 kV power supplyare connected

6V heater supply and3 kV power supplyare connectedand also 1000 Vpower supply isconnected to themetal plates

6V heater supply and3 kV power supplyare connectedand also 1000 Vpower supply isconnected to themetal plates inreverse

Types of motion of the cathode rays in a cathode rays tube

Region Types of energyPQ : Cathode toanodeQR: Anode to screen

Region Types of MotionP : Anode

QR: Anode to cathodeand screen


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DescribetheCathodeRayOscillo-scope

A cathode ray oscilloscope (C.R.O) is an instrument that convertselectronic and electrical signals to a visual display. The graph produced consist of a horizontal axis which is normally a

function of time, and a vertical axis which is a function of the inputvoltage.

The components in a cathode ray tube consists of a vacuum glass tubewith an electron gun, a deflection system for deflecting the electronbeam and a fluorescent coated screen.

Describe the function of the cathode ray oscilloscope, CRO

The electron gun is used to produce a narrow beam of electrons.Is heated when current flows through it. It is used to heat upthe cathode.Heated cathode emits electrons through the process ofthermionic emissions.

Control the number of electrons in the electron beams.The more negative the grid, the fewer the electrons areemitted from the electron gun and the less the brightness ofthe bright spot on the screen.To focus the electrons into a beam and to attract electronsfrom the area of the control grid.To accelerate the electron beam towards the screen.

Electrongun


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DeflectingSystem

The deflection system allows the electron beam to be deflected from itsstraight-line path when it leaves the electron gun.

Y-plates is to move the electron beam vertically up and down the screenwhen an input voltage is applied across it.

No input voltageThe electron beam does notdeflect and the bright spotis at the centre

+ve voltage is applied, theelectron beam deflectupward. The bright spotmoves to the top.

-ve voltage is applied, theelectron beam deflectdownward. The bright spotmoves to the bottom

a.c voltage is applied, theelectron beam deflects upand down. The bright spotmoves up and down to forma bright vertical trace on thescreen

The function of the X-plates is to sweep the electron beam across the screenhorizontally from left to right at a steady speed.

The fluorescent screen is coated on the inside surface with some fluorescentmaterial such as phosphor or zinc sulfide.

When electron beam strikes the screen, the material becomes glows. Thisenables a bright spot to appear whenever an electron beam strikes thescreen.

The moving electrons have kinetic energy. When this electrons strikes thescreen, the fluorescent coating on the screen converts the kinetic energy ofthe electrons into light energy.

FluorescentScreen


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Control knob FunctionPower switch Control the power supplyFocus Control the sharpness of the bright spot

Connected to the focusing anode The sharpness of the bright spot is also affected by the brightness

Brightness To control brightness or intensity of the bright spot Connected to the control grid Brightness level should be set as low as possible to obtain a clear and

sharp traceX-shift To adjust the horizontal position of the bright spot on the screen

Connected to the X-platesY-shift To adjust the vertical position of the bright spot or the trace displayed

Connected to the Y-platesY gain(volts / div)

To control the magnitude of the vertical deflection of the bright spot or thetrace displayed on the screen by adjusting amplitude

Connected to the Y-platesTime-base(time/div)

To control the magnitude of the horizontal deflection of the bright spot orthe trace displayed on the screen by adjusting the frequency

Connected to the X-platesX-input A terminal to connect the voltage to the X-platesY-input A terminal to connect the voltage to the Y-platesAC/DC switch To select the type of input received

When the switch is at DC position, the a.c and d.c voltages will bedisplayed

When the switch is at AC position, only the a.c voltage will be displayed.Any signals of d.c voltage will be blocked by a capacitor in the CRO

Earth To disconnect the input voltage at the Y-plates and to earth the inputterminal

Applicationof CRO

1. Measuring potential difference2. Measuring short intervals3. Displaying wave forms

Describe the working principle of the cathode ray oscilloscope, CRO


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Activity 3: Display wave forms and measuring voltage from a DC source using a CRO

Set the vertical gain of the CRO to 0.5 V/div.Turn off thetime base.

Connect a battery to the Y-input Observe the display on the CRO screen and measure

the DC voltage. Reverse the terminals of the battery and repeat step 4. Turn on the time base. Observe the display on the CRO

screen Reverse the terminals of the battery and observe again

ObservationTurn off time baseNo input

Connected to a dry cell Reversed the dry cellconnections

Turn on time baseConnected to a dry cell Reversed the dry cell

connections


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Activity 4: Use a CRO to display and measure the voltage of a AC voltage supply

Set the vertical gain of the CRO to 0.5 V/div. Connect an AC current of 2 V to the Y-input Observe the display on the screen Reverse the terminals of the AC input and

observe again

ObservationsTime-base off Time-base on

DC voltage = [displacement of the bright spot x [ the selected range from the zero position]of the Y-gain control]

DC voltage with the time-base off

(a) (b)

DC voltages with thetime-base on

(c) (d)What is the value of the dc voltage in figure (a) and(b) if the Y-gain control is 1 V/div?

What is the value of the dc voltage infigure (c) and (d) if the Y-gain control is0.5 V/div?

Measure Potential Difference using the C.R.O


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Peak ac = [Height of vertical trace from x [ the selected range voltage the zeroposition] of the Y-gain control]

Y-gain = 2 V/divHeight of vertical trace from zeroposition =

Peak ac voltage =

The time-base is set to 1 ms/div It means I div = 0.001 s The number of div is counted between two

crests of a wave The short time interval between pulses =

Multiplying the number of division by thetime-base

Length between 2 signals = … div

Time base is set = 10 ms/div

Time taken, t =

Solve problems based on the CRO display

Example 1Diagram 1 shows a trace produced by an acpower supply which is connected to Y-inputof an CRO setting at 20 V/div and 5 ms/div.Calculate:(a) Period(b) Frequency(c) Peak voltage

Diagram 1

Measure Short Time Intervals using the C.R.O

Solve Problems based on the C.R.Odisplay


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Example 2Diagram 2 shows a trace produced by an a.cpower supply connected to a CRO with thetime base is switched of. The Y-gain is set to20 V/div. Find the peak voltage.

diagram 2Example 3Diagram 3 shows a wave produced by an audio generatordisplayed the screen of a CRO. The length between the twocrests is 3 cm.(a) If the time-base is set to 5 ms/div, find the frequency.(b) What is the period of the wave?(c) When the frequency of the wave is double, what is the

length between the two crests?

Diagram 3

Example 4An aircraft is detected from a radar station. The twotransmitted signal Y and reflected signal, X are displayedon the screen of a CRO.Diagram 4 shows a wave form displayed on the screen ofa CRO. The time-base is set at 50 ms/div and the speed

of the signal in air is 3 x 108 ms-1 .

Calculate the distance of the aircraft detected from theradar station at that time.

Diagram 4


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TUTORIAL 9.1

1. Which diagram shows the correctmovement of an electron beam in anelectric field? (2005)

2. The diagram shows the trace on the of aCRO when an a.c. power supply isconnected to the Y-input of the CRO.

Which of the following tracing is formedon the screen when the a.c. powersupply is replaced by a dry cell?

3. The diagram shows a waveform isdisplayed on the screen of a CRO whenan a.c. power l is connected to Y-input ofthe CRO .The time-base is switched off

What is the peak voltage of thealternating current when the Y-gain of

the CRO is adjusted at 20 V cm-1?A 80.0 VB 56.6 VC 40.0 VD 28.3 VE 15.9 V

4. The diagram shows a waveform from asignal is displayed on the screen of aCRO.

What is the frequency of the signalwhen the time-base switch is adjusted at

20 ms cm-1?A 12.5 HzB 25 HzC 50 HzD 100 HzE 200 Hz

5. The diagram shows a waveform isdisplayed on the screen of a CRO. Thedistance between two successive peaksis 3 cm.

What is the distance between twosuccessive peaks when the frequency ofthe waveform is doubled?A 0.75 cm B 1.50 cmC 3.00 cm D 4.50 cmE 6.00 cm


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6. The diagram shows a waveform isdisplayed on the screen of a CRO when amoving an a.c. power supply isconnected to Y-input of the CRO. Thefrequency of the alternating current is200 Hz.

What is the time taken from A to B?

A 5 x 10-3

s

B 7.5 x10-3

s

C 1.0 x 10-2

s

D 5.0 x 10-2

s

E 7.5 x 10-2

s

7. The figure shows a waveform obtainedon the screen of CRO at an airport radarstation. The point M and N indicate thetime transmission to an aero plane andtime of receiving the reflected signals bythe radar station .[ Time-base control setting of the CRO =

5 ms cm-1

, the speed of light = 3 x 10 8

m s -1 ]

What is the distance between the radarstation and the aero plane.

A 600 kmB 1500 kmC 300 kmD 100 kmE 50 km

STRUCTURE QUESTIONS

8. The diagram shows a cathode-ray tube.

(a) What is the thermionic emissionmeant?

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(b) What is the function of anode A?

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..............................................................(c) Explain why the tube must be a

vacuum tube?

..............................................................

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(d) What will happen to the electronswhen(i) the current flows through the

filament is increased.

......................................................

......................................................(ii) the voltage of 5 KV power

supply is increases.

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(iii) the switch is closed.

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(e) What happen to the energypossesses by the electrons whenthey hit the screen of the cathode-ray tube?

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9. The diagram shows the construction of asimple cathode-ray oscilloscope.

(a) State the name and the functionof component P .

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..............................................................

(b) The filament can be heated by d.c.or a.c. power supply. Explain why?

..............................................................

..............................................................

(c) Explain how is the control gridcontrol the brightness the screen?

................................................................

..............................................................

(d) The time-base switch of the CRO ison.Draw the waveform in the spacebelow when the Y-input of the CROis connected to(a) a dry cell

(ii) a transformer

(e) Give two advantages use the CRO todetermine the potential differencecompare to the voltmeter.

..............................................................

..............................................................

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10. The diagram shows a waveform obtainedfrom an a.c. power supply connected toY-input of a CRO.

(a) What is

(i) the frequency the current?

(i) the peak voltage of the current?

(b) Draw the waveform display on thescreen of the CRO in figure belowwhen

(i) the time-base switch is off.

(ii) the time-base switch is on andthe a.c. power supply isreplaced by a d.c. generator.

5370705 9 1 cathode ray oscilloscope doc

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