In the given figure an ammeter A consists of a 240 Ω coil connected in parallel to a 10 Ω shunt. The reading of the ammeter is ________ mA.

Solution

Physics sir · 4 steps

The ammeter is not ideal, so first find its own resistance and add it to the circuit.

1. Ammeter resistance. The 240 Ω coil and the 10 Ω shunt are in parallel. Together they act as one resistance.

   R_A = (240 × 10)/(240 + 10) = 2400/250 = 9.6 Ω

2. Total resistance. The ammeter is in series with the 140.4 Ω resistor, so R = 140.4 + 9.6 = 150 Ω.
3. Current. By Ohm's law, I = 24/150 = 0.16 A. The ammeter reads the full current entering it, which is the sum of the coil and shunt currents.
4. So. I = 0.16 A = 160 mA, so the reading is 160.

The common slip: 171. Treating the ammeter as ideal and using only 140.4 Ω gives 24/140.4 ≈ 0.171 A, about 171 mA, not 160.

The idea to keep. A real ammeter has resistance: coil parallel to shunt. Put that in series with the rest of the circuit, and remember the reading is the total current, not just the coil current.

Answer from the NTA final answer key. Solution by PrepWiser.