Alternatingcurrent 1

For the same rms value of A.C. and D.C., the peak value of current of D.C. is

while the peak value of current of A.C. is

. Hence, the peak value of current of A.C. is

times greater than that of the D.C. So, A.C. is more dangerous than D.C. of the same voltage.

2. The average power consumed by an inductor is zero. Why?

As there are two cycles of current running periodically in A.C. cycle. During the first half cycle, the magnetic energy is stored in the coil. this energy is supplied by emf source supplying the A.C. current. When the source current rises, current increases in the inductance also. A back emf is produced in it which opposes the rise in source current.

During the another cycle, the magnetic energy stored in the coil returns to the source. When the source current decreases, the back emf induced in the coil opposes the fall in the source current. the coil has to do work for which it spends its stored energy. As a result, during the total cycle the power consumed by an inductor is zero.

3. An A.C. circuit containing capacitor only does not consume power. Why?

During the first cycle of A.C., the source current rises due to which the capacitor is charged. When the capacitor is charged, the potential developed across it opposes the rise in the source current. For the source to rise its current in order to charge capacitor during the first cycle, it has to do work which is stored in the capacitor as a electrical energy.

During another half cycle, the source current starts falling. The voltage across the capacitor becomes greater than the source voltage. As a result, the capacitor discharges through the source and stored energy is returned back to the source. Hence, an A.C. circuit containing capacitor only does not consume power.

4. Why is a series LCR circuit called an acceptor circuit?

The overall impedance of a series LCR circuit is given by The current becomes maximum during the resonant frequency in a series LCR circuit. When alternating voltage of various frequencies are applied to it, the circuit oscillates for each frequencies. The amplitude of current varies with frequency but the current becomes maximum for the frequency which is natural or resonant. For all frequency rather than resonance frequency, the current in LCR circuit is much small. During resonance, it accept the information as the supply frequency matches with the natural frequency due to getting of maximum current. Hence, a series LCR circuit is called an acceptor circuit.

5. A parallel LCR circuit is called rejector circuit?

The current becomes minimum at resonance in a parallel LCR circuit and the value can be zero also if the value of resistance is zero. When alternating voltages of various frequencies are applied in the parallel LCR circuit, the circuit oscillates with all frequencies varying amplitude. The amplitude of the current for the resonant frequency is minimum or zero. So, during resonance the voltage is rejected due to which a parallel LCR circuit is called rejector circuit.

6. What is choke coil and why it is considered superior to a rheostat?

A choke coil is an electrical device in which there are a number of turns of insulated coil made to change the electrical energy to magnetic effect. The choke coil has high inductance and low resistance.

If the rheostat is used in the electric circuit then the energy is lost in the form of heat due to the resistance offered by it. But, in case of choke coil, the current is wattless so, the choke reduces current without any loss of power. Hence, choke coil is considered superior to a rheostat.

7. Why is A.C. not used for electrolysis?

In A.C. supply, there are two cycles (positive half cycle and negative half cycle) which changes periodically and in cyclic manner. During the process of electrolysis by using A.C., if there is deposition of ions in negative half cycle then that will again be removed during another cycle i.e. positive cycles. As a result of which there is no electrolysis occur by A.C. Hence, A.C. is not used for electolysis.

8. Why can't A.C. be used to magnetize a substance?

In we use A.C. to magnetize a substance then the molecular arrangement of that substance changes at different cycle of A.C. which disturbs to be in order of the arrangement of molecules vastly. So, A.C. is not used to magnetize a substance.

9. Allowing A.C. current around a magnet destroys its magnetism. Why?

As the A.C. current is passed through a magnet, the molecular arrangement arranged in a proper way is disturbed and later the molecules of a magnet are arranged in disorder or in the haphazard way resulting in the destruction of its magnetism. Hence, allowing A.C. current around a magnet destroys its magnetism.

10. What is the role of A.C. in the vibration of a sonometer wire?

The vibration of a sonometer wire follows the Fleming's left hand rule which states that if the thumb and first two fingers of left hand mutually perpendicular to each other are hold in such a way that if the fore finger points along the direction of the current and mid finger point in the direction of magnetic field, then the thumb will point in the direction of motion of the conductor (of force). The experimental setup of the sonometer is done according to the Fleming's left hand rule.

During each cycle of A.C., the direction of the current reversed which result in the vibration of the sonometer wire due to the force developed on the wire results the to and fro motion of that wire. The force on a current carrying conductor in a magnetic field is due to the interaction of the magnetic field produced by the current with the external magnetic field.

11. It is said that once a person is electrocuted by D.C. it is difficult to separate the person from the wire but is easy in case of A.C. Why?

As there is a constant distribution of voltage in D.C., if a person is electrocuted by D.C. the charge continuously flows via person to the ground which disturbs in sending the message of the removal of his hands from the wire from the spinal cord or brain to the effected areas i.e. it disturbs the nervous system so the person is unable to separate his hands from that wire.

But, in case of A.C., the current and voltage changes instantly which is given by the relation

So, at some instant, the value of current or voltage becomes zero. The value of current or voltage becomes zero two times in each cycle. So, when the value of current or voltage becomes zero, the spinal cord or brain is capable of sending the information to the electrocuted area for the removal from the wire. Hence, the person is able to remove his electrocuted area at that instant quickly. So, it is said that once a person is electrocuted by D.C. it is difficult to separate the person from the wire but easy in case of A.C.

12. A heater coil when operated by D.C. gets fused at only one end when used for a long time but when operated by A.C. there is a chance of getting fused at both the ends. Why?

In D.C., the direction of current flow is always constant (i.e. the polarity of D.C. is always constant) but in case of A.C., the direction of current changes periodically (i.e. the polarity of A.C. reverses periodically). Hence, a heater coil when operated by D.C. gets fused at only one end when used for a long time as the direction of current flow in D.C. is only in one direction but when operated by A.C. there is a chance of getting fused at both ends because the polarity of A.C. reverses periodically so the direction of current changes in two cycles.

13. A lamp is connected in series with a capacitor. Why does this connection works with A.C. but not with D.C.?

The reactance offered by a capacitor of capacitance C in an A.C. circuit is given as

where, f is the frequency of A.C. source which in case of D.C. becomes zero i.e.

So,

Hence a capacitor offers infinite reactance to D.C. and for D.C., capacitor is an open circuit. A lamp does not work with D.C. as a result of which the lamp does not glow.

In case of A.C., frequency is not equal to zero so, the lamp will glow because capacitor can flow A.C. Reducing capacitance will increase impedance of capacitor and the lamp will shine less brightly than before. Hence, this connection works with A.C. but not with D.C.

14. Why does a capacitor cannot be used with D.C.?

The reactance offered by a capacitor of capacitance C in an A.C. circuit is given as

where, f is the frequency of A.C. source which in case of D.C. becomes zero i.e.

So,

Hence a capacitor offers infinite reactance to D.C. and Hence a capacitor cannot be used with D.C.

15. How is it possible to rectify an AC?

As the AC changes its polarity with certain frequency. AC can be rectified using a diode like P-N junction diode as it conducts current only in one direction because it has a low resistance for one polarity whereas high resistance for the opposite polarity.