Electromagnetic Induction Lab. Magnet through a coil

[Serendipitydo]

I apologize in advance if some of my answers don't quite make sense. I'm generally articulate, but not when it comes to physics I'm barely grasping.

I did a lab and part of it was dropping a magnet through a coil, which produced a Voltage vs Time graph using LabPro. For trial 1 we had the N end facing down. For trial 2 we had the S end facing down. The solenoid remained unchanged. Trial 1 has a graph going from 0 to ~-1 up to ~+1 and back to 0. Trial 2 has the opposite graph( 0-> +1 -> -1 -> 0)

I'm not sure how to correctly answer the questions:
"Why does the voltage switch from negative to positive, or positive to negative? " and "Explain your results in terms of V=BLv"

I have written down:
"The voltage switches because the top of the coil is N. As the N end of the magnet enters, the current flows in one direction. As the s end of the magnet leaves, the current flows in the opposite direction, and vice versa. The change of the current direction results in a change of the polarity of the voltage."

and

"Voltage depends on the magnetic field strength and direction, the length of the coil and velocity. The results are consistent, but opposite because the coil length and velocity remained constant. The bar magnet was always dropped from rest, straight down. The pole of the magnet, and thus magnetic field direction in the coil were the only things changed"

The things I know/understand/have learned
- the magnetic field of a magnet goes from N to S
- the magnet moving in the solenoid/coil produces a current
- the induced current inside the solenoid creates a field that is opposite that of the magnetic field of the magnet(a bit fuzzy on the induced current/coil magnetic field relationship)
- the pole of the solenoid switches as the magnet passes through(the solenoid end will be N, at least from what I've gathered from the questions we've done)
- V=BLv
- Fm=BIL
- V=IR

Thanks to anyone who can help. This is due in ~12 hours, so any help before then will be appreciated. Anything after that will definitely help me for my test, Monday.

 

[Nate810]

The key points to understand here are the relationship between the magnetic field of the magnet, the induced current in the coil, and the voltage created by the change in the direction of the magnetic field. When a magnet is dropped through a coil, the magnetic flux passing through the coil changes, inducing a current. This current then produces its own magnetic field which opposes the magnetic field of the magnet. When the North pole of the magnet is entering the coil, the magnetic field of the magnet and the coil are in the same direction, resulting in an increase in voltage. When the South pole of the magnet is entering the coil, the magnetic fields are in opposite directions, resulting in a decrease in voltage. In terms of V=BLv, your results can be explained by the fact that the length of the coil and velocity remain the same. The only thing that changes is the direction of the magnetic field. The change in the magnetic field causes a change in the voltage, as described above.