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Lab10

Electric Guitar and Faradays Law

Scott Vari

PHYS208

Section 10

December 5, 2005

Introduction

Throughout this lab, we will be examining the behaviors
of induced current from changing magnetic fields.  This lab will
help us begin to understand the concept of Faradays Law.  This
lab will also help us understand the emf that influences the induced
current in a coil and will teach us how to measure time dependent voltages
with an oscilloscope.

Experiments

The equipment used in this lab is as follows: 
multimeter, magnets, field coil, magnetic field sensor, electric guitar
string/pickup apparatus, and an oscilloscope.

Experiment 1:  In
this experiment we will connect a coil of a certain number of turns
to a multimeter and examine the emf voltage across the coil when the
magnet is in motion near the coil.  We will then measure our B
field over different distances the magnet moves in relation to the coil.

Experiment 2:  In
this experiment we will measure the signal given off by the string through
the pickup by connecting the pickup to the oscilloscope.  We will
then magnetize and demagnetize the string in order to see how our results
change if they change at all.  We will then analyze the magnetic
field produced by measuring the guitar strings width and using the
density of iron.

Results and Analysis

Experiment One:  The sign of the emf changes
with the magnet changes direction while moving.  It does not matter
if you use the south or north end of the magnet because the magnetic
field is always present.  The only thing that will change when
using a different side of the magnet is the sign of the emf.  Another
condition that would change the sign of the emf is if the magnet remained
still and the coil was moved in different directions.  If the magnet
is brought toward the coil and then held still, the emf will go to zero. 
The faster you move the magnet, the higher positive and negative emf
you will have.  The slower you move the magnet, the lower positive
and negative emf you will have.  When the magnet is perpendicular
to the coil direction, the emf is the greatest.  This is because
perpendicular is completely opposite of parallel, which is when there
is no emf.

We can see from the graph
on the following page the emf as the magnet is moved towards the coil
at different speeds. This graph is accurate for the B field and for
the 1/t case.  Speed will be a factor starting at zero and will
be shown by increasing the variable by one to show a greater speed.

Experiment Two: 
The frequencies heard when plucking the guitar string are audible. 
There is no oscillation when the string is plucked, so we dont think
that the string is magnetized.  After placing the magnet on the
string over the coil, there is oscillation, which means the string is
magnetized.  The frequency at which the string is moving is roughly
5 Hz. 

Faradays Law: emf = (dB/dt)

By this law, we can see
that the greater the magnetic field over a smaller amount of time, the
greater the emf.  By magnetizing the string and measuring the emf
shortly after doing so, the string is still very magnetized, yielding
a greater frequency response in the oscilloscope and a greater emf.

Summary

In this lab, we successfully learned about the concept
of Faradays Law.  We examined the behaviors of induced current
from changing magnetic fields.  We also better understand the emf
that influences the induced current in a coil and we measured the time
dependent voltages with an oscilloscope.  This lab was very fun
because it showed us the electric guitar and a real-life application
for Faradays Law and magnetism.