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Alternating Current question

2023 · 29 Jan · Shift 2 · Q54
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  5. /2023 · 29 Jan · Shift 2 · Q54

Alternating Current question

2023 · 29 Jan · Shift 2 · Q54

JEE MainPhysicsAlternating CurrentMCQ+4 / −1
For the given figures, choose the correct options : JEE Main 2023 (Online) 29th January Evening Shift Physics - Alternating Current Question 51 English
  1. A
    The rms current in circuit (b) can be larger than that in (a)
  2. B
    The rms current in figure (a) is always equal to that in figure (b)
  3. C
    The rms current in circuit (b) can never be larger than that in (a)
  4. D
    At resonance, current in (b) is less than that in (a)
View written solutionFree

Correct answer: C

Let the two figures be the standard AC circuits:

  • (a): a series RLCRLCRLC circuit connected to an AC source
  • (b): a parallel RLCRLCRLC circuit connected to an AC source

We compare their rms currents drawn from the source.


1. Current in series RLCRLCRLC circuit (a)

For a series RLCRLCRLC circuit, the impedance is

Zs=R2+(ωL−1ωC)2Z_s = \sqrt{R^2 + \left(\omega L - \frac{1}{\omega C}\right)^2}Zs​=R2+(ωL−ωC1​)2​

Hence the rms current is

Ia=VZsI_a = \frac{V}{Z_s}Ia​=Zs​V​

Since

Zs≥R,Z_s \ge R,Zs​≥R,

we get

Ia≤VRI_a \le \frac{V}{R}Ia​≤RV​

with equality at resonance:

ωL=1ωC\omega L = \frac{1}{\omega C}ωL=ωC1​

So at resonance,

Ia=VRI_a = \frac{V}{R}Ia​=RV​

which is maximum for the series circuit.


2. Current in parallel RLCRLCRLC circuit (b)

For a parallel RLCRLCRLC circuit, source current is found from admittance:

Yp=(1R)2+(ωC−1ωL)2Y_p = \sqrt{\left(\frac{1}{R}\right)^2 + \left(\omega C - \frac{1}{\omega L}\right)^2}Yp​=(R1​)2+(ωC−ωL1​)2​

Thus rms current drawn from source is

Ib=VYp=V(1R)2+(ωC−1ωL)2I_b = V Y_p = V\sqrt{\left(\frac{1}{R}\right)^2 + \left(\omega C - \frac{1}{\omega L}\right)^2}Ib​=VYp​=V(R1​)2+(ωC−ωL1​)2​

Since

Yp≥1R,Y_p \ge \frac{1}{R},Yp​≥R1​,

we get

Ib≥VRI_b \ge \frac{V}{R}Ib​≥RV​

with equality at resonance.

At resonance,

ωC=1ωL\omega C = \frac{1}{\omega L}ωC=ωL1​

so

Ib=VRI_b = \frac{V}{R}Ib​=RV​

which is minimum for the parallel circuit.


3. Comparison of IaI_aIa​ and IbI_bIb​

From above,

Ia≤VR,Ib≥VRI_a \le \frac{V}{R}, \qquad I_b \ge \frac{V}{R}Ia​≤RV​,Ib​≥RV​

Therefore,

Ib≥IaI_b \ge I_aIb​≥Ia​

for all frequencies, and equality holds only at resonance.

So:

  • circuit (b) can be larger than circuit (a) away from resonance
  • they are not always equal
  • circuit (b) can never be larger is false
  • at resonance, they are equal, not less

4. Check each option

Option A

The rms current in circuit (b) can be larger than that in (a)

This is true because away from resonance,

Ib>IaI_b > I_aIb​>Ia​

Option B

The rms current in figure (a) is always equal to that in figure (b)

This is false. They are equal only at resonance.

Option C

The rms current in circuit (b) can never be larger than that in (a)

This is false.

Option D

At resonance, current in (b) is less than that in (a)

This is false because at resonance,

Ia=Ib=VRI_a = I_b = \frac{V}{R}Ia​=Ib​=RV​

5. Final answer

The correct option is:

A\boxed{A}A​

6. Comparison with stored answer

Stored correct answer: CCC

Our derived answer is AAA, so the stored answer does not match the physics of standard series and parallel RLCRLCRLC circuits. In fact, the comparison shows:

Ib≥IaI_b \ge I_aIb​≥Ia​

not the reverse.

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