Question

Identify the correct statements among the following.
A. The torque acting on a planar current loop in a magnetic field changes when its shape is changed without changing its area.
B. Two parallel wires carrying currents in opposite directions attract each other.
C. An ideal ammeter has zero resistance and an ideal voltmeter has infinite resistance.
D. A galvanometer can be converted into a voltmeter by connecting a high resistance in series.

Choose the correct answer from the options given below:

• A. A, B and C only
• B. B, C and D only
• C. C and D only
• D. A and C only

Hint:

Torque depends on area, not shape. Opposite currents repel.

Answer 1

The Correct Option is C

Step 1: Understand the question.
We have to judge four statements about current loops, parallel wires and meters. Then we pick the option that lists only the true ones.

Step 2: Check statement (A).
The torque on a planar loop is \(\tau = NIAB\sin\theta\). It depends only on the current, the area and the field, not on the shape. If the area stays the same, the torque stays the same. So (A) is FALSE.

Step 3: Check statement (B).
Parallel wires with currents in the same direction attract. Wires with currents in opposite directions repel. So (B) is FALSE.

Step 4: Check statement (C).
An ammeter is joined in series, so it should add no extra resistance. An ideal ammeter has zero resistance. A voltmeter is joined in parallel, so it should draw no current. An ideal voltmeter has infinite resistance. So (C) is TRUE.

Step 5: Check statement (D).
A galvanometer becomes a voltmeter when a large resistance \(R = \frac{V}{I_g} - G\) is put in series with it. This keeps the current through the coil small. So (D) is TRUE.

Step 6: Match with the options.
Only C and D are true, which is option 3. Options 1 and 2 include B, and options 1 and 4 include A, so they are wrong.

Final Answer:
Statements C and D are correct, so option 3 is right. \[ \boxed{\text{Option 3: C and D only}} \]