___________
ANSWERS
__________________
PHYS1200
PHYSICS II
SPRING 2006
Class 7 Activity: Faraday’s Law
The relation between an induced electromotive force (emf) and a changing magnetic field is
given by Faraday’s Law. Today, you will use Faraday’s Law, a coil of wire, and a computer to
measure the strength of a magnetic field.
Let us examine Faraday’s Law to see how it can be used to measure a magnetic field. If a coil of
wire, consisting of
N
turns, experiences a changing magnetic field, the magnetic flux through it
will change. That will induce an electromotive force,
E
, as described by Faraday’s Law:
E
E
= 
= 
N
d
dt
dt
N d
B
B
Φ
Φ
,
so
If both sides of the equation on the right are integrated, the result is,
(
29
E
dt
N
d
N
t
B
B f
Bi
Bi
B f
0
∫
= 
∫
= 

Φ
Φ
Φ
Φ
Φ
That is, the integral of
E
over time is equal to the change in flux through the coil times the
number of turns in the coil. Now imagine that the coil is initially in a strong magnetic field, and
ends up in a magnetic field of zero. Then the equation becomes,
(
29
i
B
i
B
t
N
N
dt
Φ
=
Φ


=
∫
0
0
E
.
However, the flux through the coil, when it is in the magnetic field, is equal to the average value
of the magnetic field in the coil,
B
av
, times the area of the coil,
A
. Since the coil is circular, this
can be written as,
2
r
B
A
B
av
av
i
B
π
=
=
Φ
.
Then the equation for the integral becomes,
2
0
r
NB
dt
av
t
π
=
∫
E
.
Finally, the equation can be solved for
B
av
to give,
B
dt
N
r
av
t
=
∫
E
0
2
π
With the help of LoggerPro,
∫
t
dt
0
E
can be measured, and
B
av
can be calculated.
1
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
___________
ANSWERS
__________________
You will need LoggerPro in today’s activity.
Make sure that one person in each group has a
computer hooked up to a LabPro and running LoggerPro. Connect a LabPro to your
computer through a USB port.
There are links to the LoggerPro file that you will need for this experiment at the Physics II web
site under “Activities”
If you are running LoggerPro 2.2 do the following:
Select the link “Faraday for 2.2”.
Download the file “Faraday2.MBL” to your desktop.
Double click on the icon to start the program.
If everything is working correctly, your screen will show a “Collect” button above a graph of
potential vs. time.
If you are running LoggerPro 3.2 do the following:
Select the link “Faraday for 3.2”.
Download the file “Faraday3.xmbl” to your desktop.
This is the end of the preview.
Sign up
to
access the rest of the document.
 Spring '06
 Stoler
 Physics, Force, Magnetic Field, Faraday

Click to edit the document details