Question
A precision laboratory resistor is made of a coil of wire 1.50 cm in diameter and 4.00 cm long, and it has 500 turns. (a) What is its self-inductance? (b) What average emf is induced if the 12.0 A current through it is turned on in 5.00 ms (one-fourth of a cycle for 50 Hz AC)? (c) What is its inductance if it is shortened to half its length and counter- wound (two layers of 250 turns in opposite directions)?
1. $1.39 \textrm{ mH}$
2. $3.33 \textrm{ V}$
3. $\textrm{L} = 0$. Wires carrying current in opposite directions create magnetic fields that oppose each other. The net magnetic field will always be zero, so with no changing magnetic field there is no self-inductance.
Solution Video