One of the great advantages of ac over dc for electric power distribution is that it is much easier to step the voltage levels up and down with ac than with dc.

1. Name the device used for the above purpose and write its working principle.

2. Write any three major energy loses in this device and state how these lose are minimized in a practical situation.

Power developed in an ac circuit can be expressed as P=VI cos ∅. In certain circuits no power is developed even though current flows through it.

a) Identify such a circuit from the following:

(i) purely inductive circuit

(ii) purely resistive circuit

(iii) inductive and resistive circuits

(iv) resistive and capacitive circuits

b) Which of the following circuit can be used to produce oscillations?

(i) L-R Circuit (ii) LCR Circuit

(iii) LC Circuit (iv) RC Circuit

c) Explain how oscillations are produced in the chosen circuit.

(a) The core of a transformer has the following properties:

(i) core is laminated.

(ii) hysteresis loop is narrow.

Explain the significance of each property.

(b) what is meant by resonance in an LCR circuit?

1. (a) The S.I unit of inductive reactance is

(i) Henry (ii) Ohms (iii) Volt (iv) No unit.

(b) Figure given below shows a series LCR

circuit to a variable frequency source.

Determine the source frequency at resonance.

The circuit shown in figure is called tank circuit in which a charged capacitor (C) is connected to an inductor (L).

1. Complete the statement given below.

Capacitor and inductor can store ………………………… and ……………………………….. energy respectively. The charge on the capacitor and the current in the circuit exhibit a phenomenon called ………………………………………..

2. Figure below shows three different tank circuits. If are the angular frequencies corresponding to circuits shown in fig(a), fig (b) and fig (c) respectively, choose the correct alternative.(i)                                (ii)                                 (iii)                                (iv)

110 V variable frequency ac source is applied to a circuit as shown in the figure. At a particular frequency the voltage across the capacitor and inductor are found to be equal. (Given L = 0.38 H, C = 27 μF and R = 48 Ω).

1. Write one practical application of the circuit shown.

2. Find the current through the circuit.

3. What is the frequency of the applied ac source?

4. How does the circuit current change if the plates of the capacitor are (i) brought closer (ii) moved away?

Phasor diagram of a series LCR circuit is shown. V_R, V_L and V_C represent the respective potential difference across resistor, inductor and capacitor and I represents the circuit current.

Answer the following questions based on the phasor diagram shown.

1. In the phasor diagram two quantities have the same phase. Identify them. (1)

2. Redraw the given phasor diagram and complete it to show the applied voltage. (2)

3. Is the frequency applied to the circuit above or below the resonant frequency? (1)

The current through an AC circuit depends on the magnitude of the applied voltage and impedance of the circuit.

1. Write any two factors on which the impedance of a series LCR circuit depends.

2. Draw the impedance diagram of a series LCR circuit and write the expression for the power factor from the diagram.

3. When 110 V, 50 Hz AC is applied to an electric component labeled X, a current of 0.25 A flows through it and it is in phase with the applied voltage. When X is replaced with another component Y the magnitude of current remains the same but it leads the voltage by Identify the components X and Y. Now if X and Y are connected in series with the same source find the current and phase difference between current and voltage.

The current through fluorescent lights are usually limited using an inductor.

1. Obtain the relation for an inductor across which an alternating emf is applied.

2. Why it is better to use an inductor rather than a resistor to limit the current through the fluorescent lamp?

3. When 100 V DC source is connected across a coil a current of 1 A flows through it. When 100 V, 50 Hz AC source is applied across the same coil only 0.5 A flows. Calculate the resistance and inductance of the coil.

A voltage source is connected to an electrical component X as shown in figure

1. Identify the device X.

2. Which of the following equations can represent the current through the circuit?

             (i)         (ii)     (iii)

       3.Draw the phasor diagram for the circuit.

Qn.

An alternating voltage is shown in figure. Answer the questions based on the voltage shown.

1. What is the peak value of voltage?

2. What is the rms value of the voltage?

3. What is the frequency of the voltage?

4. Write the general equation of the voltage.

A capacitor of capacitance 2 μF is in an a.c. circuit of frequency 1000 Hz. If the rms value of the applied emf is 10 V, find the effective current flowing in the circuit.

A capacitor blocks d.c but allows a.c. Explain.

In LCR circuit if resistance increases quality factor

(a) increases finitely                               (b) decreases finitely

(c) remains constant                               (d) None of these

Ifan LCR series circuit is connected to an ac source, at the resonance, the voltage across

(a)R is zero

(b)R equals the applied voltage

(c)C is zero

(d)L equals the applied voltage

In LCR circuit if resistance increases quality factor

(a) increases finitely                            (b) decreases finitely

(c) remains constant                           (d) None of these

A capacitor has capacitance C and reactanceX . capacitance and frequency become double, then reacance will be

(a) 4X                                 (b) X/2

(c) X/4                                (d) 2X

Phase difference between voltage and current capacitor in an ac circuit is

(a) π                               (b) π/2

(c) 0                               (d) π/3

A capacitor acts as an infinite resistance for

(a)DC                                              (b) AC

(c)DC as well as AC                         (d) neither AC nor DC

The average power dissipated in a pure inductance is

(a) LI² /2                                           (b) LI²

(c) LI²/4                                          (d) zero

With increase in frequency of an A.C. supply, the inductive reactance

(a)decrease   directly with frequency                (b)increases directly with frequency

(c)increases as square of frequenc                   (d)decreases inversely with frequency

An alternating voltage is connected to a box with some unknown circuital arrangement as shown. The applied voltage and current through the circuit are

measured as  and 

1. Does the current lead or lag the voltage?

2. Is the circuit in the box largely capacitive or inductive?

3. Is the circuit in the box at resonance?

4. What is the average power delivered by the box?

For many purposes, it is necessary to change an alternating voltage from one value to another. This is done with a transformer.

a] The basic principle behind a transformer is ..............................

b] Give an expression for the voltage and current in a transformer

A friend from abroad presents you a coffeemaker when she visited you. Unfortunately it was designed to operate at 110 V line to obtain 960 W power that it needs.

1. Which type of transformer you use to operate the coffeemaker at 220 V? 

2. Assuming the transformer you use as ideal, calculate the primary and secondary currents. 

3. What is the resistance of the coffee maker? 

One of the great advantages of ac over dc for electric power distribution is that it is much easier to step the voltage levels up and down with ac than with dc.

1. Name the device used for the above purpose and write its working principle.

2. Write any three major energy loses in this device and state how these lose are minimized in a practical situation.

The circuit diagram and the phase relationship between current and voltage for three different circuits are shown in the table below. Match the second and third columns of the table with first column.