QPage 276: Q11: HC Verma: Concepts of Physics - Volume 2 Permanent Magnets Page 276: Q11 Solution: A tangent galvanometer is connected directly to an ideal…
Students often incorrectly assume that doubling the number of turns ($n$) will increase the deflection because the magnetic field inside the coil is proportional to $n$. However, since the galvanometer is connected directly to an ideal battery, the current $I$ remains constant, and the deflection angle $ heta$ is determined by $ an heta = rac{B_{coil}}{B_H}$. While the sensitivity (change in deflection per unit current) increases, the specific deflection for a fixed current source does not change if the circuit resistance is negligible or if the question implies the current is maintained by the ideal source without considering the coil's resistance change; strictly speaking, for an ideal battery and negligible coil resistance, $I$ is constant, but the standard interpretation in this context is that the deflection is a function of current, not just turns, and if the current is fixed by the source, the deflection remains the same relative to the new sensitivity scale, or more accurately, the question tests the understanding that $ heta$ depends on $I$, and if $I$ is constant, $ heta$ is constant only if we ignore the resistance change or assume the 'deflection' refers to the reading on a scale calibrated for the new $n$. Actually, the standard trap is confusing the formula $ an heta propto nI$ with the idea that $n$ alone drives $ heta$. The correct insight is that for a fixed current $I$, $ heta$ is fixed. If the coil resistance changes, $I$ might change, but 'ideal battery' usually implies constant voltage. If resistance doubles, current halves, then $nI$ remains constant ($2n cdot I/2 = nI$), so deflection remains unchanged.
Multiple Choice Options
- A increase
- B decrease
- C remain unchanged
- D either increase or decrease