2 Marks Questions

Question 512 Marks

Answer the following questions:

A choke coil in series with a lamp is connected to a dc line. The lamp is seen to shine brightly. Insertion of an iron core in the choke causes no change in the lamp’s brightness. Predict the corresponding observations if the connection is to an ac line.

Answer

If an iron core is inserted in the choke coil (which is in series with a lamp connected to the ac line), then the lamp will glow dimly. This is because the choke coil and the iron core increase the impedance of the circuit.

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Question 522 Marks

What is the rms value of alternating current shown in figure?

Answer

$(\text{I}^2)_{\text{mean}}=\frac{\int\limits^{\text{T}}_{0}\text{I}^2\text{dt}}{\int\text{dt}}=\frac{\int\limits^{\frac{\text{T}}{2}}_0(2)^2\text{dt}+\int\limits^{\text{T}}_{\frac{\text{T}}{2}}(-2)^2\text{dt}}{\text{T}}=\frac{\int\limits^{\text{T}}_04\text{dt}}{\text{T}}=4$
$\text{I}_{\text{rms}}=\sqrt{4}=2\text{A}$

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Question 532 Marks

The dielectric strength of air is 3.0 × 10⁶V/m. A parallel-plate air-capacitor has area 20cm² and plate separation 0.10mm. Find the maximum rms voltage of an AC source that can be safely connected to this capacitor.

Answer

E = 3 × 10⁶V/m, A = 20cm², d = 0.1mm.
Potential difference across the capacitor = Ed = 3 × 10⁶ × 0.1 × 10^-3 = 300V
Maximum rms Voltage $=\frac{\text{V}}{\sqrt{2}}=\frac{300}{\sqrt{2}}=212\text{V}.$

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Question 542 Marks

What will be the effect on inductive reactance XL and capacitive reactance XC if frequency of ac source is increased?

Answer

The inductive reactance $\text{X}_{\text{L}}=\omega\text{L}=2\pi\text{vL}$ will increase with the increase of frequency v, while capacitive reactance $\text{X}_{\text{C}}=\frac{1}{\omega\text{C}}=\frac{1}{2\pi\text{vC}}$ will decrease with the increase of frequency v.

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Question 552 Marks

In Exercises 7.3 and 7.4, what is the net power absorbed by each circuit over a complete cycle. Explain your answer.

Answer

In the inductive circuit,
Rrns value of current, I = 15.92A
Rms value of voltage, V = 220V
Hence, the net power absorbed can be obtained by the relation,
$\text{P}=\text{VI}\cos\phi$
Where,
$\phi=$ Phase difference between V and I
For a pure inductive circuit, the phase difference between alternattnq voltage and current is 90º i.e., $\phi=90^{\circ}$
Hence, P = 0 i.e., the net power is zero.
In the capacitive circuit,
Rrns value of current, I = 2.49A
Rms value of voltage, V = 110V
Hence, the net power absorbed can ve obtained as:
$\text{P}=\text{VI}\cos\phi$
For a pure capacitive circuit, the phase difference between alternating voltage and
current is 90º i.e., $\phi=90^{\circ}$
Hence, P = 0 i.e., the net power is zero.

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Question 562 Marks

Can the voltage drop across the inductor or the capacitor in a series LCR circuit be greater than the applied voltage of the a.c. source? Justify your answer.

Answer

Yes; because

As V_Cand V_L have opposite faces, V_C or V_L may be greater than V. The situation may be as shown in figure where V_C > V

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Question 572 Marks

Sketch a graph showing variation of reactance of a capacitor with frequency in an AC circuit.

Answer

Capacitive reactance,

The graph of capacitive reactance XC and frequency ν is shown in figure.

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Question 582 Marks

Draw the graphs showing variation of inductive reactance and capacitive reactance with frequency of applied a.c. source.

Answer

$\text{X}_{\text{L}}=\text{w}_{\text{L}}=2\pi\text{vL};$ graph X_L of f and f is a straight line.
$\text{X}_{\text{C}}=\frac{1}{\omega\text{C}}=\frac{1}{2\pi\text{vC}},$ graph of XC and f is a rectangular hyperbola as shown in figs.

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Question 592 Marks

Answer the following questions:
An applied voltage signal consists of a superposition of a dc voltage and an ac voltage of high frequency. The circuit consists of an inductor and a capacitor in series. Show that the dc signal will appear across C and the ac signal across L.

Answer

The dc signal will appear across capacitor C because for de signals, the impedance of an inductor (L) is negligible while the impedance of a capacitor (C) is very high (almost infinite). Hence, a de signal appears across C. For an ac signal of high frequency, the impedance of L is high and that of C is very low. Hence, an ac signal of high frequency appears across L.

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Question 602 Marks

Answer the following questions:
A capacitor is used in the primary circuit of an induction coil.

Answer

High induced voltage is used to charge the capacitor.

A capacitor is used in the primary circuit of an induction coil. This is because when the circuit is broken, a high induced voltage is used to charge the capacitor to avoid sparks.

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Physics 2 Marks Questions