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4 months ago

Alternating Current Physics NCERT Exemplar Solutions Class 12th Chapter Seven

As the frequency of an AC circuit increases, the current first increases and then decreases. What combination of circuit elements is most likely to comprise the circuit?

(a) Inductor and capacitor

(b) Resistor and inductor

(c) Resistor and capacitor

(d) Resistor, inductor and capacitor

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Payal Gupta

Contributor-Level 10

This is a multiple choice answer as classified in NCERT Exemplar

(a, d) inductive reactance XL= 2 $π ϑ$ L

For an LCR circuit Z=

\sqrt[]{R^{2} + (X_{L} - X_{C})}

As frequency (ν) increases, Z decreases and at certain value of frequency know as resonant frequency (ν₀), impedance Z is minimum that is Z_min= R current varies inversely with impedance and at Z_min current is maximum.

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4 months ago

Alternating Current Physics NCERT Exemplar Solutions Class 12th Chapter Seven

The output of a step-down transformer is measured to be 24 V when connected to a 12 W light bulb. The value of the peak current is

(a) 1/ $\sqrt[]{2}$ A

(b) $\sqrt[]{2}$ A

(c) 2A

(d) 2 $\sqrt[]{2}$ A

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Payal Gupta

Contributor-Level 10

This is a multiple choice answer as classified in NCERT Exemplar

Secondary voltage V_s= 24V

Power associated with secondary P_s= 12W

I_s= $\frac{P_{S}}{V_{S}}$ = 12/24= 0.5A

I₀= Is $\sqrt[]{2}$ = 1/ $\sqrt[]{2}$ A

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4 months ago

Alternating Current Physics NCERT Exemplar Solutions Class 12th Chapter Seven

An inductor of reactance 1 ? and a resistor of 2 ? are connected in series to the terminals of a 6 V (rms) AC source. The power dissipated in the circuit is

(a) 8 W

(b) 12 W

(c) 14.4 W

(d) 18 W

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Payal Gupta

Contributor-Level 10

This is a multiple choice answer as classified in NCERT Exemplar

Average power dissipated is E_rmsI_rmscos $\emptyset$

Z= $\sqrt[]{R^{2} + X_{L}^{2}}$

= $\sqrt[]{1 + 4}$ = $\sqrt[]{5}$

I_rms= 6/ $\sqrt[]{5}$ A

cos $\emptyset$ =R/Z=2/ $\sqrt[]{5}$

p_av= 6 $* \frac{6}{\sqrt[]{5}} * \frac{2}{\sqrt[]{5}}$ = 14.4W

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4 months ago

Alternating Current Physics NCERT Exemplar Solutions Class 12th Chapter Seven

Which of the following combinations should be selected for better tuning of an L-C-R circuit used for communication?

(a) R = 20 ?, L = 1.5 H, C = 35μF

(b) R = 25 ?, L = 2.5 H, C = 45 μF

(c) R=15?, L = 3.5H, C = 30 μF

(d) R = 25 ?, L = 1.5 H, C = 45 μF

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Payal Gupta

Contributor-Level 10

This is a multiple choice answer as classified in NCERT Exemplar

(c) As we know that Q= $\frac{1}{R} \sqrt \frac{L}{C}$ to make Q high R should be low, L should be high and C should be low.

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4 months ago

Alternating Current Physics NCERT Exemplar Solutions Class 12th Chapter Seven

To reduce the resonant frequency in an L-C-R series circuit with a generator

(a) The generator frequency should be reduced

(b) Another capacitor should be added in parallel to the first

(c) The iron core of the inductor should be removed

(d) Dielectric in the capacitor should be removed

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Payal Gupta

Contributor-Level 10

This is a multiple choice answer as classified in NCERT Exemplar

As we know resonant frequency is v₀= $\frac{1}{2 π \sqrt[]{L c}}$

To increase capacitance we must connect it to another capacitor parallel to the first.

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4 months ago

Alternating Current Physics NCERT Exemplar Solutions Class 12th Chapter Seven

When a voltage measuring device is connected to AC mains, the meter shows the steady input voltage of 220 V. This means

(a) Input voltage cannot be AC voltage, but a DC voltage

(b) Maximum input voltage is 220 V

(c) The meter reads not v but < v²> and is calibrated to read v²

(d) The pointer of the meter is stuck by some mechanical defect

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Payal Gupta

Contributor-Level 10

This is a multiple choice answer as classified in NCERT Exemplar

(c) The voltmeter connected to AC Mains reads mean value (2>) and is calibrated in such a way that it gives value of, which is multiplied by form factor to give rms value.

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4 months ago

Alternating Current Physics NCERT Exemplar Solutions Class 12th Chapter Seven

An alternating current generator has an internal resistance R_gand an internal reactance X_gIt is used to supply power to a passive load consisting of a resistance R_g and a reactance X_L. For maximum power to be delivered from the generator to the load, the value of X_L is equal to

(a) Zero

(b) X_g

(c) -X_g

(d) R_g

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Payal Gupta

Contributor-Level 10

This is a multiple choice answer as classified in NCERT Exemplar

(c) For delivering maximum power from the generator to the load, total internal reactance must be equal to conjugate of total external reactance.

X_L=-X_g

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4 months ago

Alternating Current Physics NCERT Exemplar Solutions Class 12th Chapter Seven

If the rms current in a 50 Hz AC circuit is 5 A, the value of the current 1/300 s after its value becomes zero is

(a) 5 $\sqrt[]{2} A$

(b) 5 $\frac{\sqrt[]{3}}{2}$ A

(c) 5/6A

(d) 5/ $\sqrt[]{2}$ A

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Payal Gupta

Contributor-Level 10

This is a multiple choice answer as classified in NCERT Exemplar

(b) I₀= $\sqrt[]{2}$ Irms= $\sqrt[]{2}$ (5)A

I=I₀sinwt = 5 $\sqrt[]{2}$ sin $π / 3$ = 5 $\frac{\sqrt[]{3}}{2}$ A

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4 months ago

Alternating Current Physics NCERT Exemplar Solutions Class 12th Chapter Seven

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

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Payal Gupta

Contributor-Level 10

This is a short answer type question as classified in NCERT Exemplar

An inductor opposes flow of current through it by developing a back 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. Mat

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4 months ago

Alternating Current Physics NCERT Exemplar Solutions Class 12th Chapter Seven

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

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Payal Gupta

Contributor-Level 10

This is a short answer type question as classified in NCERT Exemplar

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.

Mathematically, the reactance can be

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