Conceptual Questions Based on Alternating Current
Q.1. Why a d.c. voltmeter and d.c. ammeter cannot read a.c.?
Solution. The average value of a.c. over a cycle is zero. So, a d.c. voltmeter/ammeter will show zero reading with alternating voltage/current.
Q.2. What is meant by the statement that the current through an inductor lags behind the emf across it by π / 2 ?
Solution. This means that in an inductive a.c. circuit whichever value emf attains, current attains a similar value a quarter of cycle later. For example, if emf attains its maximum value at t = 0, then current attains its maximum value at t = T / 4, and so on.
Q.3. What is the average value of a.c. over a cycle and why?
Solution. Zero, because a.c is positive during one half cycle and equally negative during other half cycle.
Q.4. Which value of current do you read with an a.c ammeter?
Solution. Root mean square value of the current.
Q.5. When an alternating current is passed through a moving coil galvanometer, it shows no deflection. Why?
Solution. A moving coil galvanometer measures average value of current, which is zero for a.c. over every cycle. So galvanometer shows no deflection.
Q.6. Can we define rms value of a.c. in terms of the chemical effect of current?
Solution. No, because the chemical effects are reversed when the direction of current is reversed.
Q.7. Voltages across L and C in series are 180° out of phase. Comment.
Solution. Given a current in series LC, voltage in L leads current by 90° phase and voltage in C lags behind current by 90° phase. So voltages in Land C differ by a phase of 180°.
Q.8. Can we use a.c. of frequency 15 cps for lighting purpose?
Solution. Yes, because the fluctuations in current will be so rapid (30 times per second) that the bulb will appear glowing continuously due to persistence of vision.
Q.9. On which effect of current, a.c. ammeters are based? Give reason.
Solution. A.C. ammeters are based on heating effect of current because this effect does not depend on the direction of current.
Q.10. A 110 V d.c. heater is used on an a.c. source, such that the heat produced is the same. What would be the rms value of the alternating voltage?
Solution. By definition, rms value of the alternating voltage = 110 V.
Q.11. 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.
Solution. When the iron core is inserted in the solenoid, the self-inductance L increases. Consequently, the inductive reactance, XL = ωL increases. This decreases the current in the circuit and the bulb glows dimmer.
Q.12.. When L and C are connected in parallel, currents in L and C are 180° out of phase. Comment.
Solution. Given an applied voltage across parallel LC, current in Llags behind voltage by 90° phase, current in Cleads voltage by 90° phase. So currents in Land Care 180° out of phase.
Q.13. Which is the best method of reducing current in an a.c. circuit and why?
Solution. The current in an a.c. circuit can be best reduced by using a choke coil or capacitor. There is no dissipation of energy in these devices.
Q.14. When a series LCR-circuit is brought into resonance, the current in the circuit increases to a large value. Why?
Solution. In resonance condition, the impedance of the LCR-circuit becomes minimum and so current in the circuit rises to a maximum value.
Q.15. When is the current in a.c. circuit wattless?
Solution. The current in an a.c. circuit is wattless when the phase difference between current and voltage is π / 2. It can be obtained by using an inductor or a capacitor in the circuit.
Q.16. When are the voltage and current in LCR-series a.c. circuit in phase?
Solution. When XL = XC, voltage and current in a series LCR-circuit are in same phase.
Q.17. Can we use a capacitor instead of a choke coil for reducing current in an a.c. circuit? Give reason.
Solution: Yes, because the average power dissipated per cycle in an ideal capacitor is also zero.
Q.18. What do you mean by charging and discharging of a capacitor?
Solution. The process of storing charge on the plates of a capacitor is called charging, and the process of withdrawing charge from the plates is called discharging of the capacitor.
Q.19. Does a transformer change the frequency of a.c.?
Solution. No. The frequency of alternating voltage obtained across the secondary is same as that of voltage applied across the primary.
Q.20. With reference to alternating currents and voltages, state any one fundamental difference between resistance and reactance.
Solution. The resistance of any component is independent of the frequency of a.c. while the reactance changes with the change in frequency of the a.c. source.