# Induced emf as a magnet passes through a solenoid

#### fromthepast

I'm doing a lab write-up for physics 2. The experiment is about the title, a bar magnet being dropped through a solenoid.

I have to explain four graphs that plot the change in emf (y axis) vs. time (x) axis. There are incoming and outgoing peaks on these graphs. I have to tie these results with the equation emf = -N(ΔΦ/Δt).

For example, if a bar magnet is dropped with north facing downward through the center of the coil, why is the incoming peak positive? How do the magnetic field, induced magnetic field, and emf all tie together to produce a positive peak?

Thanks!

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#### thepatient

The formula emf = -N ΔΦ/Δt, gives you most of the information you need. The changing magnetic flux induces an emf. The emf is induced opposite of the direction at which the magnetic field "tries" to induce into the solenoid. The direction at which the current would have to go in order to induce a magnetic field in the opposite direction (thus, the negative in the formula). The N simply means the number of loops within the solenoid.

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