Physics Alternating Current NUMS 2015
PMDC Verified Question 101 of 127

An A.C. emf of \( E = 100 \sin(100 \pi t) \) volt is connected across a choke of negligible resistance. In order to produce a current of amplitude \( 1\text{ A} \), the inductance of the choke should be:
A
200 H
B
\(2\pi\text{ H}\)
C
\(1 / \pi\text{ H}\)
D
\(2 / \pi\text{ H}\)
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option C: \(1 / \pi\text{ H}\)
Concept:

A choke is an inductor designed to limit AC current. For a purely inductive circuit, the opposition to the alternating current is called inductive reactance.

Formula:

The inductive reactance is defined as:

$$X_L = \omega L$$

and the current amplitude (peak current) is:

$$I_0 = \frac{E_0}{X_L}$$

Solution:

Compare the given voltage equation \( E = 100 \sin(100 \pi t) \) to the standard equation \( E = E_0 \sin(\omega t) \):

  • Peak EMF, \( E_0 = 100\text{ V} \)
  • Angular frequency, \( \omega = 100 \pi\text{ rad/s} \)
  • Peak current, \( I_0 = 1\text{ A} \)


Substitute these values to solve for inductance \( L \):

$$I_0 = \frac{E_0}{\omega L} \implies 1 = \frac{100}{100 \pi \times L}$$

$$L = \frac{100}{100 \pi} = \frac{1}{\pi}\text{ H}$$

Why other options are incorrect:

  • Other options like \( 200\text{ H} \), \( 2\pi\text{ H} \), or \( 2/\pi\text{ H} \) are incorrect due to standard errors in algebraic rearrangement, such as multiplying by \( 2 \) instead of dividing or incorrect cancellation of \( 100 \).

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