Moving Coil Galvanometer - UNSOLVED PRACTICE SET

Q7. Explain the construction of a moving coil galvanometer with a labelled diagram.

Q8. Why is a radial magnetic field preferred in a moving coil galvanometer? Explain with a diagram.

Q9. A galvanometer has coil resistance G = 50 Ω and full-scale deflection current I_g = 1 mA. Calculate the voltage required for full-scale deflection.

Q10. Define current sensitivity and voltage sensitivity of a galvanometer. How are they related?

Q11. The current sensitivity of a galvanometer is doubled by increasing the number of turns. What happens to the voltage sensitivity? Explain.

Q12. Why is phosphor-bronze used for the suspension wire in a galvanometer?

Q13. Explain the principle, construction, and working of a moving coil galvanometer. Derive the expression for the deflection: θ = (NIBA/C), where C is the torsional constant of the suspension.

Q14. Explain why a moving coil galvanometer should have (a) a radial magnetic field, (b) a large number of turns, (c) a strong magnetic field, and (d) a low torsional constant for high sensitivity. What are the practical limitations of increasing sensitivity?

Q15. A moving coil galvanometer has a coil with N = 100 turns, area A = 2 cm², suspended in a radial magnetic field B = 0.2 T. The suspension wire has torsional constant C = 10⁻⁸ N m/degree and the coil resistance is G = 80 Ω.

(a) Calculate the deflection for a current of 50 μA.  

(b) Calculate the current sensitivity.  

(c) Calculate the voltage sensitivity.  

(d) What is the figure of merit?  

(e) If the number of turns is doubled, how do the sensitivities change?

Q16. In a school physics lab, students are given a moving coil galvanometer with the following specifications: coil resistance G = 100 Ω, full-scale deflection current I_g = 50 μA, number of turns N = 200, coil area A = 1.5 cm², and magnetic field B = 0.15 T.

(a) Calculate the deflection constant (torsional constant C) of the suspension.  

(b) Calculate the current sensitivity and voltage sensitivity.  

(c) The students want to use this galvanometer to measure currents up to 10 mA. Calculate the shunt resistance required.  

(d) They also want to measure voltages up to 5 V. Calculate the series resistance required.  

(e) Explain why the same galvanometer can be converted into both an ammeter and a voltmeter with different external resistances.

Q17. A sensitive moving coil galvanometer has the following parameters: N = 500 turns, A = 4 cm², B = 0.1 T, C = 4 × 10⁻⁹ N m/degree, G = 200 Ω.

(a) Calculate the full-scale deflection current if the maximum deflection is 50°.  

(b) Calculate the current sensitivity and voltage sensitivity.  

(c) The galvanometer is to be converted into an ammeter of range 0–1 A. Calculate the shunt resistance and the effective resistance of the ammeter.  

(d) The same galvanometer is to be converted into a voltmeter of range 0–50 V. Calculate the series resistance and the effective resistance of the voltmeter.  

(e) Compare the resistances of the converted ammeter and voltmeter. Which should have lower resistance and why?

Q18. A galvanometer with resistance G = 50 Ω shows full-scale deflection at I_g = 1 mA. It is to be converted into a multi-range ammeter with ranges 0–1 A, 0–5 A, and 0–10 A using a universal shunt (Ayrton shunt).

(a) Design the Ayrton shunt circuit showing all resistances.  

(b) Calculate the shunt resistances for each range.  

(c) Calculate the effective resistance of the ammeter for each range.  

(d) Explain why the Ayrton shunt is preferred over individual shunts for each range.  

(e) If the galvanometer is accidentally connected directly across a 12 V battery without any shunt, what happens? Calculate the current and explain the consequences.