Model Question Paper with Answer - 3 (PART A)

MODEL QUESTION PAPER - III.

1. Determine the power delivered by each source for the given circuit in the figure.

Ans :

Nett voltage = 50 - 20

= 30 V

Total resistance = 1 + 2 = 3Ω

Current = 30 / 3 = 10 A

I flows away from positive polarity of 50 V.

Power delivered by 50V source = 50 × 10 = 500 W

Power received by 20V source = 20 ×  10 = 200 W

2. State ohm's law

The ratio between the potential difference across two terminals of a conductor and the current through it remains constant, when the physical conditions of the conductor remain unchanged. V = IR

3. State Thevenin's theorem.

Any linear active network with output terminals A and B as shown in fig. (a) can be replaced by a single voltage source (V_Th = V_0c) in series with a single impedance (Z_Th = Z_i)

V_Th is the Thevenin's voltage. It is the voltage between the terminals A and B on open circuit condition. Hence it is called open circuit voltage denoted by V_0c

4. In the given circuit of the figure, R_g varies between 2 and 55 ohms. What value of Rg results in maximum power transfer across terminals AB? Calculate the maximum power.

Ans: Expressions in Page 9.74 are useful.

Let the source voltage by E_g

Power delivered to R_L = P_L.

From the above expression for P_L, it is seen that R_g must be minimum for maximum P_L. As R_g varies between 2 and 55 Ω,

P_L is maximum when R_g = 2Ω

 Maximum P_L = 100000 / (2 + 10)² + 25

= 592

= 592 Watts

5. Differentiate between transient and steady state.

Ans: A circuit having constant sources is said to be in steady state if the currents and the voltages do not vary with time. In a circuit containing energy storage elements, with change in excitation, the voltage and currents change from one state to other. The behaviour of the voltage or current when it is changed from one state to other is called the transient state.

6. Sketch the transients for i, V_R, V_L in a series RL circuit.

7. List out the methods of power measurements in the three phase balanced circuits.

(i) Three wattmeter method

(ii) Two wattmeter method

(iii) Single wattmeter method

8. A 3ϕ 400 volts supply is given to a balanced star connected load of impedance 8+j6 ohms in each branch. Find the line current.

Star connection:

I_L = I_ph

V_L = V_p /√3 = 400/√3= 230.9

I = V/Z = 230 / 8 + j6 =  23 ∠ -36.9°

9. Derive the formula for the resonant frequency of series resonant circuit.

As Z   = R + j (X_L - X_C)  (i)

10. Define coupling coefficient

K = M / √L₁ L₂