Simulation · Physics · Class 12
From galvanometer to ammeter and voltmeter
From the lesson The moving coil galvanometer in Moving Charges and Magnetism. Change the values and watch what happens.
The idea behind it
NCERT §4.10
- A moving coil galvanometer (MCG) has a many-turn coil free to turn about a fixed axis in a radial magnetic field. A soft iron core makes the field radial and stronger.
- Because the field is radial, the plane of the coil is always along B and the torque is τ = NIAB. A spring gives a restoring torque kφ, where k is the torsional constant (restoring torque per unit twist).
- At equilibrium kφ = NIAB, so the deflection φ = (NAB/k)I is proportional to the current. A pointer on the spring reads φ off a scale.
- As a null detector (as in a Wheatstone bridge) the zero is at the centre of the scale and the pointer swings either way with the current's direction.
- A bare galvanometer is a poor ammeter: it reaches full scale at currents of the order of μA, and its large resistance would change the current it is meant to measure.
- Ammeter: add a small shunt rₛ in parallel. The combination has resistance R_G rₛ/(R_G + rₛ) ≈ rₛ, and most of the current goes through the shunt.
- Voltmeter: add a large resistance R in series, so the meter's resistance is R_G + R ≈ R, large, and it draws very little current from the circuit it is connected across.
- Current sensitivity φ/I = NAB/k; voltage sensitivity φ/V = NAB/(kR). Doubling N doubles the current sensitivity, but it also roughly doubles the coil's resistance, so the voltage sensitivity may not change.
- Example 4.12: with a 3 Ω circuit on 3 V, a 60 Ω galvanometer as the meter gives I = 3/63 = 0.048 A; with a 0.02 Ω shunt the total is about 3.02 Ω and I = 0.99 A; an ideal ammeter would read 1.00 A.
More simulations in Moving Charges and Magnetism
1 more