Moving coil galvanometer and its conversion — practice questions

Question 1 · difficulty L1 · recall

A moving-coil galvanometer detects small currents by using

• A. torque on a current-carrying coil in a magnetic field
• B. electrostatic attraction only
• C. heating of a resistor
• D. electromagnetic induction without current

Question 2 · difficulty L1 · understanding

Current sensitivity of a galvanometer is defined as

• A. current per unit deflection
• B. deflection per unit current
• C. voltage per unit resistance
• D. torque per unit magnetic field only

Question 3 · difficulty L2 · understanding

A moving coil galvanometer has 100 turns and each turn has an area of $2.0 \mathrm{~cm}^2$. The magnetic field produced by the magnet is $0.01 \mathrm{~T}$ and the deflection in the coil is 0.05 radian when a current of $10 \mathrm{~mA}$ is passed through it. The torsional constant of the suspension wire is $x \times 10^{-5} \mathrm{~N}-\mathrm{m} / \mathrm{rad}$. The value of $x$ is _______ .

Question 4 · difficulty L2 · understanding

The current sensitivity of a galvanometer can be increased by : (A) decreasing the number of turns (B) increasing the magnetic field (C) decreasing the area of the coil (D) decreasing the torsional constant of the spring Choose the most appropriate answer from the options given below :

• A. (B) and (C) only
• B. (C) and (D) only
• C. (A) and (C) only
• D. (B) and (D) only

Question 5 · difficulty L2 · understanding

For a radial magnetic field, the galvanometer torque on an N-turn coil is

• A. NIAB sinθ with a changing θ factor
• B. NIB/A
• C. NIAB
• D. NI/B

Question 6 · difficulty L2 · application

A galvanometer is converted to an ammeter by connecting

• A. a high resistance in series
• B. a capacitor in parallel
• C. an inductor in series
• D. a low resistance shunt in parallel

Question 7 · difficulty L3 · understanding

A moving coil of galvanometer when shunted with $2 \Omega$ resistance gives a full scale deflection for a current of 500 mA . When a resistance of $470 \Omega$ is connected in series it gives a full scale deflection for 10 V potential applied on it. The value of resistance of galvanometer coil is $\_\_\_\_$ $\Omega$.

Question 8 · difficulty L3 · understanding

In a moving coil galvanometer, two moving coils $\mathrm{M}_1$ and $\mathrm{M}_2$ have the following particulars : $$\begin{aligned} & \mathrm{R}_1=5 \Omega, \mathrm{~N}_1=15, \mathrm{~A}_1=3.6 \times 10^{-3} \mathrm{~m}^2, \mathrm{~B}_1=0.25 \mathrm{~T} \\ & \mathrm{R}_2=7 \Omega, \mathrm{~N}_2=21, \mathrm{~A}_2=1.8 \times 10^{-3} \mathrm{~m}^2, \mathrm{~B}_2=0.50 \mathrm{~T} \end{aligned}$$ Assuming that torsional constant of the springs are same for both coils, what will be the ratio of voltage sensitivity of $M_1$ and $M_2$ ?

• A. $1: 1$
• B. $1: 3$
• C. $1: 2$
• D. $1: 4$

Question 9 · difficulty L3 · understanding

STATEMENT 1 : The sensitivity of a moving coil galvanometer is increased by placing a suitable magnetic material as a core inside the coil. and STATEMENT 2 : Soft iron has a high magnetic permeability and cannot be easily magnetized or demagnetized.

• A. Statement 1 is True, Statement 2 is True; Statement 2 is a CORRECT explanation for Statement 1
• B. Statement 1 is True, Statement 2 is True; Statement 2 is a NOT CORRECT explanation for Statement 1
• C. Statement 1 is True, Statement 2 is False
• D. Statement 1 is False, Statement 2 is True

Question 10 · difficulty L3 · understanding

In a moving coil galvanometer, torque on the coil can be expressed as $$\tau=k i$$, where $$i$$ is current through the wire and $$k$$ is constant. The rectangular coil of the galvanometer having numbers of turns $$\mathrm{N}$$, area $$\mathrm{A}$$ and moment of inertia I is placed in magnetic field B. Find (A) $$k$$ in terms of given parameters $$\mathrm{N}, \mathrm{I}, \mathrm{A}$$ and B; (B) The torsional constant of the spring, if a current $$i_{0}$$ produces a deflection of $$\frac{\pi}{2}$$ in the coil; (C) The maximum angle through which coil is deflected, if charge $$\mathrm{Q}$$ is passed through the coil almost instantaneously. (Ignore the damping in mechanical oscillations.)

• A. $$\left[\right.$$ (A) $$\mathrm{NAB}$$, (B) $$\mathrm{C}=\frac{\mathrm{~N} i_{0} \mathrm{AB}}{\pi}$$ (C) $$\left.Q_{\max }=Q \sqrt{\frac{\mathrm{NAB} \pi}{I i_{0}}}\right]$$.
• B. $$\left[\right.$$ (A) $$\mathrm{NAB}$$, (B) $$\mathrm{C}=\frac{3 \mathrm{~N} i_{0} \mathrm{AB}}{\pi}$$ (C) $$\left.Q_{\max }=Q \sqrt{\frac{\mathrm{NAB} \pi}{3 I i_{0}}}\right]$$.
• C. $$\left[\right.$$ (A) $$\mathrm{NAB}$$, (B) $$\mathrm{C}=\frac{2 \mathrm{~N} i_{0} \mathrm{AB}}{\pi}$$ (C) $$\left.Q_{\max }=Q \sqrt{\frac{\mathrm{NAB} \pi}{2 I i_{0}}}\right]$$.
• D. $$\left[\right.$$ (A) $$\mathrm{3NAB}$$, (B) $$\mathrm{C}=\frac{2 \mathrm{~N} i_{0} \mathrm{AB}}{\pi}$$ (C) $$\left.Q_{\max }=Q \sqrt{\frac{\mathrm{NAB} \pi}{2 I i_{0}}}\right]$$.