Bihar Board - Class 12 Physics - Chapter 7: Alternating Current Short Answer Question
Short Answer Type Questions
1. What do you mean by the sinusoidal nature of an alternating current?
Answer: An alternating current is said to be sinusoidal in nature if it varies either as a sine curve or as a cosine curve with time.
2. What is the average value of an AC over one complete cycle, and why?
Answer: The average value of an AC over one complete cycle is zero because in magnitude with time such that for the other half cycle, the current is negative. So, the average value is zero for one complete cycle.
3. Define inductive reactance. On what factors, does it depend?
Answer:The inductive reactance of an inductor is a measure of the opposition offered by it for flow of an AC through it. The inductive reactance depends on the product of inductance (L) and the angular frequency (ω) of AC.
4. How does resistance of an ohmic resistor change with frequency of a source?
Answer: The resistance R of an ohmic resistor does not depend on frequency of AC. An ohmic resistor offers the same resistance R for AC as well as DC at a given temperature.
5. What is the difference between reactance and resistance?
Answer:The main difference between reactance and resistance are as follows:
(a) Resistance is a measure of the opposition to the flow of current by an ohmic resistor but reactance is the opposition to the flow of current either by an inductor or by a capacitor.
(b) Value of resistance does not depend on frequency of AC supply. However, inductive reactance decreases with increase in frequency.
6. What is a phasor and a phasor diagram?
Answer:A phasor is a vector which rotates about the origin with angular speed and is represented by phasors. The analysis of the use of a phasor diagram.
7. What is the phase difference, for an RLC series AC circuit, between the voltage across inductor (VL) and voltage across capacitor (VC)?
Answer:The phase difference between VL and VC is π radian. So, the voltages across the inductor and that across the capacitor are in mutually opposite phases.
8. If the frequency of the AC source in a RLC series AC circuit is increased, how does the current in the circuit change?
Answer: As frequency of an AC source applied across a RLC series circuit is gradually increased, the current initially increases with increase in frequency, then it acquires a maximum value for a frequency of AC equal to the resonant frequency. As the frequency is further increased, the current again begins to fall again.
9. What is the power consumed during one complete cycle in:
(a) purely inductive, and
(b) purely capacitive AC circuit?
Answer: The net power consumed during one complete cycle is zero for:
(a) purely inductive circuit as well as, and
(b) purely capacitive AC circuit.
10. Define the term' wattless current'.
Answer: A wattless current is that which does not dissipate power although there is a flow of current in the circuit. Current flowing in a purely inductive or purely capacitive or a LC circuit is a wattless current.
11. What are the limiting values of power factor in an AC circuit?
Answer: The minimum value of power factor can be 0 while the maximum value can be 1. Thus, effectively, 0 ≤ cosΦ ≤ 1.
12. Under which condition, is the power factor of an AC circuit equal to one?
Answer: Power factor of an AC circuit is having its maximum value, equal to one, either if the circuit is a purely resistive circuit or LCR circuit is in its resonance condition .
13. What is the resulting effect on oscillations of a LC circuit, if some resistance is also present in the circuit?
Answer: Due to the presence of small resistance in L-C oscillatory circuit, there is dissipation of electromagnetic energy at a uniform rate. It results in a damping effect on the oscillations amplitude and the oscillations finally die away.
14. Mention the two characteristic properties of the material suitable for making the core of a transformer.
Answer: The material used for making the core of a transformer should have high value of magnetic permeability but a low value of magnetic hysteresis so that magnetic flux linked with each turn of primary or secondary coil is large and energy loss due to hysteresis phenomenon is the least possible.
15. State the underlying principle of a transformer.
Answer: A transformer works on the principle of mutual inductance. If an AC is flown through the primary coil of the transformer, magnetic field and hence magnetic flux per turn of primary as well as secondary coil changes and consequently, an induced emf is set up in the secondary coil too.
16. When a value is given for AC voltage or current,to which quantity does it correspond to:
(a) peak value,
(b) rms value, or
(c) average value?
Answer: The value given for an alternating voltage or current is ordinarily the rms value . When we say that a fuse of 5A rating is to be used, it means that the rms value of current should not exceed 5 A.
17. Can you define one ampere of AC in the same way as for DC? If not, how can we define 1 ampere of AC?
Answer: Although both AC and DC are measured in amperes, we cannot define 1 A AC in terms of mutual attraction force between two parallel straight wires carrying current as in DC. An alternating current changes its direction and the average attractive force would be zero. One ampere of rms value of an alternating current in a circuit is the current which produces the same average heating effect as 1 A of DC would produce under the same condition.
18. Can an AC source be connected to a circuit and yet deliver no power to it? If so, under what circumstances?
Answer: Yes, it is possible that an AC source be connected to a circuit and yet deliver no power to it. It happens when instantaneous voltage and current in the circuit differ in phase by a phase angle ф=π/2 and the power factor cosф is zero. Examples of such AC circuits are either a pure inductive circuit or a pure capacitive circuit or a LC oscillatory circuit.
19. What is a choke coil? What is its function?
Answer: A choke coil is an inductor of large inductance L and the least possible resistance. When connected in an AC circuit, it reduces the circuit current without dissipation of power. The coil offers a reactance. Hence, impedance of circuit increases and circuit current is reduced. However, the choke coil does not dissipate any power because V and I differ in phase by π/2 in an inductor.
20. Can a capacitor be used instead of a choke coil to reduce current in an AC circuit? Give a reason.
Answer: Yes, a capacitor may also be used to reduce current in an AC circuit without any dissipation of electrical energy. A capacitor, when joined in an AC circuit, offers a capacitive reactance XC . Thus, the impedance of circuit increases and circuit current decreases. However, even for a capacitive circuit, V and I differ in phase by π/2 and there is no dissipation of electrical energy.
21. Can the voltage drop across the inductor or the capacitor in a series LCR circuit be greater than the applied voltage of the AC source? Justify your answer.
Answer:Yes, the voltage drop across L or C in a series LCR circuit can be greater than the applied voltage of the AC source. This is due to the fact that these voltages are not in phase and cannot be arithmetically added up. In fact, VL and VC are in mutually opposite phases and try to balance each other.
22. In India, domestic power supply is at 220 V, 50 Hz while in the USA, it is at 110 V, 60 Hz. Give one advantage and one disadvantage of 220 V supply instead of 110 V supply.
Answer: In India, domestic power supply is at 220 V as compared to 110 V in the USA. Its advantage is that for a given power at higher voltage of 220 V, the circuit current I is less, hence may use a thinner line wire. On the other hand, the disadvantage is that the insulation coating of the line wire must be of superior quality so as to withstand a higher voltage.
23. What are the advantages of AC and DC?
Answer: Main advantages of AC and DC are as follows:
(a) Alternating current generators, motors and other such appliances are more robust and durable than similar DC devices.
(b) Transformers can be used with AC only. By the use of transformers, voltage of AC supply may be suitably raised or lowered as per our need.
(c) Large amount of AC power may be transmitted over long distances at high voltage without much loss during transmission.
24. What are the disadvantages of AC as compared to DC?
Answer:Main disadvantages of AC as compared to DC are as follows:
(a) AC is more dangerous than DC having the same effective voltage.
(b) For a given effective voltage, the peak voltage in AC is √2 times the effective voltage, hence better insulation is required for the AC line.
(c) AC supply cannot be used as such for chemical and magnetic effects of current and in electronic devices.
25. Can an ordinary moving coil ammeter or voltmeter used for DC be used to measure an alternating current or voltage? Give reason for your answer.
Answer: An ordinary moving coil ammeter or voltmeter cannot be used to measure an alternating current or voltage because the net torque experienced by the coil of the ammeter or voltmeter will be zero for an alternating current or voltage. Hence, the pointer will give nil deflection.
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