Homework Help: Not really HW Just want to fully understand induced emf

1. Mar 12, 2010

sarahjt1

Ok... so there really isn't a homework problem that goes directly with what I am asking... I just want to understand how/why induction (magnet through a coil) gave certain results in our Science Workshop program...

Here is what we did in our lab:

We dropped a magnet (south end in first) through a coil hooked up to a voltage sensor and photogate to measure the area of our emf vs. time plot. We didn't do any calculations the computer did... but it used the concept of Faraday's law of induction. The resulting graph had a high point and a low point.

The high point's area beneath the curve was 0.021VoltSec

The low point's area above the curve was -0.020VoltSec

I understand that the area's have to be equal because Lenz's law states something to the effect that whatever change we've made we have to give back.

BUT What I don't fully understand is WHY it is a negative value? Is it because the current flow changes direction?

I don't fully understand the right hand rule either and I don't know exactly which direction the current was flowing as the south end of the magnet entered the coil... but I do know that it will reverse direction once the magnet is leaving the coil and that the point on the graph that y=0 (voltage) is the point when the current is reversing.

Basically, I want to conceptually understand the theory and how it is applied to the lab we did... here is the lab guide http://physics.unr.edu/152labs/Magnetic_Induction.pdf

Last edited by a moderator: Apr 24, 2017
2. Mar 13, 2010

Andrew Mason

The direction of the induced electric field changes as the direction of the magnetic field changes, which causes the induced current direction to change.

$${Emf} = -N\frac{d\phi}{dt} = -NA\frac{dB}{dt}$$

Working out dB/dt is complicated. All you have to know is that as B changes direction so does dB/dt and, therefore, so does the emf.

AM

Last edited by a moderator: Apr 24, 2017
3. Mar 13, 2010

ehild

When the magnet is above the coil, and starts to fall down, approaching and entering the coil, the magnetic flux increases inside the coil. Increasing flux induces voltage according to Faraday's law of induction.
When the magnet traverses the coil the flux does not change much and the the induced voltage gradually drops to zero. The situation changes when the top end reaches the coil. As it the magnet leaves the coil, the flux gradually disappears and the induced voltage has opposite charge.

The shape of the curves is related to the speed of the falling bar.
When is this speed higher, when the magnet enters or when it leaves the coil? What do you think how does the induced field depend on the speed of the magnet?

ehild