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Q8. A capacitor of 100 μF is charged from a 120 V d.c supply via a 100 kΩ resistor for 4 second,

(a) Calculate EACH of the following:

(i) The time constant of the circuit;

(ii) The voltage to which the capacitor has charged after 4 seconds.

(b) A second capacitor of 80 μF is now charged for 4 seconds from the same supply via the same 100 kΩ resistor. Both charged capacitors are now connected in parallel.

Calculate EACH of the following:

(i) The final steady state voltage across the pair of capacitors;

(ii) The total energy stored in the two capacitors.

Q7. A silicon heavy power transistor with the characteristics given in Worksheet Q3 is operated from a 12 V supply and has a maximum power rating of 18 W.

(a) Plot the 18 W power dissipation curve on the characteristics.

(b) Determine the minimum safe collector load resistance for the transistor.

(c) If the transistor is biased at the base with 80 mA and a sinusoidal signal of +/- 40 Ma is applied to the base, calculate EACH of the following:

(i) the variation in collector current;

(ii) the corresponding variation in collector voltage;

(iii) the a.c. power output of the transistor.

Q3. A coil of inductance 0.1 H has a power factor of 0.7 and is connected in parallel with a capacitor ‘C’ across 120 V 60 Hz supply. Calculate EACH of the following:

(a) The resistance of the coil;

(b) The value of ‘C’ if the total current is 4 A at p.f. 0.4 leading;

(c) The power dissipated by the combined circuit;

(d) The kVA for the combined circuit.

Q2. A three-phase star connected load has three identical legs each comprising a 40 Ω resistor in series with a 100 μF capacitor. It is supplied at 415 V, 50 Hz, from a three-wire supply.

Calculate EACH of the following:

(a) The current in each phase;

(b) The current in each phase if due to a fault, the red line becomes disconnected;

(c) The current in each line if the red phase becomes short circuited.