Ac Generator 2

Question 1

Q. 8.
A light bulb and a solenoid are connected in series across an ac source of voltage. Explain, how the glow of the light bulb will be affected when an iron rod is inserted in the solenoid.

Answer 1

When iron rod is inserted in the coil, the inductance of coil increases; so impedance of circuit
V
increases and hence, current in circuit decreases. There fore brightness of bulb decreases.

Question 2

Why is the use of ac voltage preferred over de voltage? Give two reasons.

Answer 2

i)The generation of ac is more economical than dc.

(ii) Alternating voltage can be stepped up or stepped down as per requirement during
transmission from power generating station to the consumer.

iii) Alternating current in a circuit can be controlled by using wattless devices like the choke coil.

(iv) Alternating voltages can be transmitted from one place to another, with much lower energy
loss in the transmission line.

Question 3

Sharpness of resonance is also known as

Answer 3

Quality factor

Question 4

Both alternating current and direct current are measured in amperes. But how is the ampere
defined for an alternating current?

Answer 4

An ac current changes direction with the source frequency and the attractive force would average to zero. of current. Joule’s heming elotis suin property same prop that is independen volte direction

Question 5

Explain why the reactance provided by a capacitor to an alternating current decreases with increasing frequency.

Answer 5

A capacitor does not allow flow of direct current through it as the resistance across the gap is infinite. When an alternating voltage is applied across the capacitor plates, the plates are alternately charged and discharged. The current through the capacitor is a result of this changing voltage (or charge). Thus, a capacitor will pass more current through it if the voltage is changing at a faster rate, i.e., if the frequency of supply is higher. This implies that the reactance offered by a capacitor is less with increasing frequency; it is given by 1/wC.

Question 6

10.
Explain why the reactance offered by an inductor increases with increasing frequency of an alternating voltage.

Answer 6

An inductor opposes flow of current through it by developing an induced emf according to Lenz’s law. The induced voltage has a polarity so as to maintain the current at its present value. If the current is decreasing, the polarity of the induced emf will be so as to increase the current and vice versa. Since the induced emf is proportional to the rate of change of current, it will provide greater reactance to the flow of current if the rate of change is faster, i.e., if the frequency is higher. The reactance of an inductor, therefore, is proportional to the frequency, being given by wL

Question 7

State the underlying principle of a transformer. How is the large scale transmission of electric
Ans.
energy over long distances done with the use of transformers?

Answer 7

• The principle of transformer is based upon the principle of mutual induction which states that due to continuous change in the current in the primary coil an emf gets induced across the secondary coil.
• At the power generating station, the step up transformers step up the output voltage which reduces the current through the cables and hence reduce resistive power loss.
• Then, at the consumer end, a step down transformer steps down the voltage.
• Hence, the large scale transmission of electric energy over long distances is done by stepping up the voltage at the generating station to minimise the power loss in the transmission cables.