# Solenoid and Iron Rod

1. Sep 1, 2013

1. The problem statement, all variables and given/known data

A solenoid with a large number of turns of wire is connected in series with an ammeter and a battery.

An iron rod is moved towards the solenoid at constant speed.

What will happen to the reading on the ammeter

(A) as the rod moves closer to the solenoid, before entering the solenoid;
(B) as the rod is in the solenoid;
(C) as the rod moves away from the solenoid, after emerging from the solenoid.

2. The attempt at a solution

I think that magnetic induction will occur. Thus, the rod will be a temporary magnet attracting the solenoid. This will result in more current being drawn as the rod enters and exits from the solenoid. Is that right?

Last edited: Sep 1, 2013
2. Sep 2, 2013

### rude man

V = d/dt(Li). Usually it's V = L di/dt but in this case ...

BTW the problem is badly stated. An ideal solenoid has no resistance so, in series with an ideal ammeter, the current would just build up to infinity before the rod is even inserted.

Assume a resistor in series with the solenoid, and an ideal ammeter (zero internal resistance).