Moving-Coil Galvanometer Torque and Current Sensitivity
For N turns of area A in radial field B, equilibrium gives NABI = k theta. Current sensitivity is theta/I = NAB/k, assuming linear spring torque k theta.
Why this shows up in the exam
Finding galvanometer current from deflection · Increasing current sensitivity · Understanding radial-field design
Learn the idea
A galvanometer converts current into angular deflection through magnetic torque on a coil. Current in the coil produces a torque while the suspension spring pushes back. In a radial field the magnetic torque stays proportional to current at every deflection.
🧠 Memory hook: Magnetic torque turns; the spring torque returns.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- N A B I = k theta — torque balance in a radial-field moving-coil galvanometer
- theta/I = N A B/k — current sensitivity
- I_g = k theta_g/(N A B) — full-scale current for deflection theta_g
How to approach it
- 1Write magnetic and spring torques
- 2Set them equal at equilibrium
- 3Use the requested deflection or sensitivity ratio
Common slip-ups that cost marks
- •Using sin theta despite the radial-field design
- •Confusing sensitivity with full-scale current
- •Changing N or A without considering resistance consequences in a real instrument
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A charge of 2 microC moves perpendicular to a 3 T magnetic field at 4 x 10^5 m/s. Find the magnetic force.
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