Important Questions
1. Why is the use of a.c. voltage preferred over d.c. voltage? Give two reasons.
2. Define capacitive reactance. Write its S.I. units.
3. When an ideal capacitor is charged by a dc battery, no current flows. However, when an ac source is used the current flows continuously. How does one explain this based on the concept of displacement current?
4. In a series LCR circuit connected to an ac source of variable frequency and voltage v = vm sin ωt,
draw a plot showing the variation of current (I) with angular frequency (ω) for two different values of resistance R1 and R2 (R1> R2). Write the condition under which the phenomenon of resonance occurs. For which value of the resistance out of the two curves, a sharper resonance is produced? Define Q-factor of the circuit and give its significance.
5. An inductor L of inductance XL is connected in series with a bulb B and an ac source. How would
brightness of the bulb change when (i) number of turns in the inductor is reduced, (ii) an iron rod
is inserted in the inductor and (iii) a capacitor of reactance XC = XL is inserted in series in the circuit. Justify your answer in each case.
Sample Questions
1. The teachers of Geeta’s school took the students on a study trip to a power generating station, located nearly 200km away from the city. The teacher explained that electrical energy is transmitted over such a long distance to the city, in the form of alternating current (ac) raised to a high voltage. At the receiving end in the city, the voltage is reduced to operate the devices. As a result, the power loss is reduced. Geeta listened to the teacher and asked questions about how the ac is converted to a higher or lower voltage.
(a) Name the device used to change the alternating voltage to a higher or lower value. State one cause for power dissipation in this device.
(b) Explain with an example, how power loss is reduced if the energy is transmitted over long distances as an alternating current rather than a direct current.
(c) Write two values each shown by the teachers and Geeta.
2. (a) Derive an expression for the average power consumed in a series LCR circuit connected to2a.c. source in which the phase difference between the voltage and the current in the circuit is 0.
(b) Define the quality factor in an a.c. circuit. Why should the quality factor have high value in receiving circuits? Name the factors on which it depends.
Or
(a) Derive the relationship between the peak and the rms value of current in an a.c. circuit.
(b) Describe briefly, with the help of labeled diagram, working of a step-up transformer.
3. A device X is connected across an ac source of voltage V = V0 sin ωt. The current through X is given as I = I0cosωt
(a) Identify the device X and write the expression for its reactance.
(b) Draw graphs showing variation of voltage and current with time over one cycle ac for X.
(c) How does the reactance of the device X vary with frequency of the ac? Show this variation graphically.
(d) Draw the phasor diagram for the device X.
4. (i) With the help of a labelled diagram, describe briefly the underlying principle and working of a step up transformer.
(ii) Write any two sources of energy loss in a transformer.
(iii) A step up transformer converts a low input voltage into a high voltage.
Does it violate law of conservation of energy? Explain.
5. (i) Write the function of a transformer. State its principle of working with the help of a diagram. Mention various energy losses in this device.
(ii) The primary coil of an ideal step up transformer has 100 turns and transformation ratio is also 100. The input voltage and power are respectively 220V and 1100 W Calculate:
(a) Number of turns in secondary.
(b) Current in primary.
(c) Voltage across secondary.
(d) Current in secondary.
(e) Power in secondary.
