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A.A.E. (I) (48)
CRANWELL
(1)
4
ROYAL AIR FORCE — AIRCRAFT APPRENTICES
No. 1 RADIO SCHOOL, CRANWELL
INTERMEDIATE EXAMINATION IN EDUCATIONAL
SUBJECTS
JANUARY 1947 ENTRY
JANUARY, 1948
BASIC RADIO PRINCIPLES
Time allowed — Three hours
Attempt SIX questions only
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1. |
(a) |
What do you understand by the following:— (i) power, (ii) the specific
resistance of a material, (iii) a 240-volt 60-watt lamp ? |
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(b) |
Make a diagram of the following circuit :—
Four 12-volt 36-watt lamps, joined in parallel, are connected in series with
a variable resistor and a fixed resistor of 0.34 ohm to a battery of E.M.F. 24
volts and total internal resistance of 1/6 ohm.
Two pilot lamps each of resistance 8.5 ohms are connected in series and
supplied by the potential difference across the 0.34 ohm resistance.
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(c) |
For the circuit at (b), if the variable resistor is adjusted so that the
36-watt lamps operate at their rated current, find:—
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(i) |
the value of the rated current and the resistance of each 36-watt
lamp, |
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(ii) |
the potential difference across the 0.34 ohm resistance, |
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(iii) |
the power of one of the pilot lamps, |
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(iv) |
the value to which the variable resistor is adjusted, |
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(v) |
the terminal potential difference of the battery, |
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(vi) |
the energy dissipated by one of the 36-watt lamps in 5
minutes. | |
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2. |
(a) |
With the aid of a clear, well-labelled diagram describe the construction and
explain the action of an attraction-type moving-iron ammeter. |
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(b) |
The coil of a moving coil ammeter has a resistance of 0.498 ohm and gives
full scale deflection with a current of .005 ampere. How could this movement be
used to measure currents up to 25 amperes ? Calculate the value of any resistor
used. |
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(c) |
The above instrument is to be used as a voltmeter reading from 0-50 volts.
State how this could be arranged and calculate the value of any resistor
needed. |
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3. |
(a) |
Explain briefly the following terms :—
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(i) |
Self induction. |
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(ii) |
Mutual inductance. | |
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(b) |
In the circuit of Fig. 1 the switch S is first moved to position 1 for two
seconds and then to position 2 for two seconds. With the aid of carefully drawn
diagrams of current and voltage, describe and explain what happens in the above
case. Assume that the time of change over from position 1 to 2 is
negligible. |
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(c) |
What was Faraday's discovery in connection with electro-magnetic induction ?
Show how it applies in the case of a moving-coil
microphone. |
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4. |
(a) |
Sketch the theoretical diagram of a shunt-wound direct current motor, and
explain the basic principles of its action. |
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(b) |
Show in detail how the direction of rotation of the armature may be
determined, and also explain what would happen if the supply voltage were
reversed. |
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(c) |
Explain what would be the effect of increasing the value of the resistance in
series with the field winding. |
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(d) |
What do you understand by the term armature reaction in the case of an
electric motor ? |
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5. |
(a) |
State clearly what you understand by the terms :—
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(i) |
Phase difference. |
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(ii) |
Reactance. |
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(iii) |
Impedance. | |
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(b) |
An alternating voltage of constant value but variable frequency is applied in
turn to each of the elements in Fig. 2. |
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Explain briefly with the aid of graphs how the resistance or reactance of
each of these varies with change of frequency. |
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(c) |
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(i) |
Calculate the R.M.S. value of the voltage across the inductance in the
circuit shown in Fig. 3. |
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(ii) |
Supposing that a condenser of capacity 1/3600 µF. were added in series with
the circuit shown, calculate the new value of the voltage across the
inductance. | |
6. Assuming that the circuits shown in Fig. 4 are both tuned to the same
frequency :—
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(a) |
State whether the circuits are behaving as series or parallel tuned circuits.
Show by vectors the relation between current and voltage in each circuit when
the frequency of the applied voltage is below the resonant
frequency. |
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(b) |
If L1 = 125µH, C1 = 500µµF and
L2 = 500µH, find the value to which C2
must be adjusted. |
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(c) |
If the effective resistances of the circuits R1 and
R2 are 10 ohms and 62.5 ohms respectively, calculate the
"Q" of each circuit at resonance and compare their selectivity. |
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(d) |
Define "coupling factor, K," and show by a set of curves the effect on the
overall response of the circuit of increasing progressively the value of
K. |
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7. |
(a) |
Describe carefully the construction and action of a hard diode valve,
indicating what is meant by (i) thermionic emission, (ii) space charge, (iii)
saturation current, (iv) A.C. resistance. |
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(b) |
Compare the characteristic curves of .(i) a hard diode, (ii) a mercury vapour
diode, (iii) a metal rectifier. Mention one advantage of each of the
above. |
8. The following table of results refers to a triode valve:—
Anode current in mA with Vg at:—
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Vg |
-0 |
-1 |
-2 |
-3 |
-4 |
-5 |
-6 |
| Va |
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| 175 |
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14.1 |
12.0 |
10.0 |
8.0 |
5.9 |
3.8 |
2.0 |
| 150 |
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12.0 |
10.0 |
8.0 |
6.0 |
4.0 |
2.0 |
0.6 |
| 125 |
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9.9 |
7.9 |
5.9 |
3.9 |
2.0 |
0.5 |
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| 100 |
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7.8 |
5.8 |
3.8 |
1.8 |
0.4 |
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| 75 |
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5.6 |
4.0 |
1.9 |
0.4 |
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| 50 |
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3.2 |
2.0 |
0.7 |
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(i) |
Plot the Ia — Va curves of the
valve. |
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(ii) |
Calculate the values of Ra, µ and g m of the valve (showing how they are obtained from the curves)
for values of Va = 100 volts and Va =
0 volts. |
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(iii) |
Calculate the voltage amplification factor of the circuit when the anode load
is 25 kilohms and the supply voltage is 175 volts. |
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(iv) |
Plot the relevant load line, and then with Vg = -3 volts
and a signal voltage of 2 volts peak, find from the graph the resulting swings
of anode current and
voltage. | |
