Quick Question about the mag field of a solenoid

[JamesL]

A light bulb and a solenoid are connected in series to a battery. An iron rod is thrust rapidly into the solenoid and later rapidly removed. The bulb:

1)No change in or out
2)No change in, brightens out
3)Dims in, Dims out
4)Brightens in, dims out
5)Brightens in, brightens out
6)remains brighter the entire time
7)Dims in, Brightens out
8)Brightens in, no change out
9)Dims in, no change out
10)no change in, dims out

-----------

Im leanin towards #7, dims as the rod is thrust into the solenoid, and brightens as it is removed. since the iron rod is a conductor, it would draw some of the mag force away from the solenoid (and therefore the battery) thus making it dimmer... and then when it is removed, the bulb would brighten back to its original state...

is this reasoning correct? any help?

 

[JamesL]

solved. nevermind

 

[wais]

Yes, your reasoning is correct. As the iron rod is rapidly thrust into the solenoid, it will create a temporary increase in the magnetic field within the solenoid. This will cause the bulb to dim slightly as some of the current is diverted to the rod. When the rod is rapidly removed, the magnetic field will decrease and the current will return to its original path, causing the bulb to brighten again. So, the correct answer is #7, dims in and brightens out.