Transcript Wednesday`s Slides

Magnetism 11.2
From Forces to Induction
Occurrences
• Today
– A Military Application of Magnetism
– Introduction to Inductors
– Begin next unit
• Friday
– Quiz
– Continue unit.
• Monday
– More of the same old thing.
Remember Force on a Wire
F=Bil
Check this out …
Wire
The Real Deal
The figure shows a uniform magnetic field that is normal to the plane
of a conducting loop with resistance R. Which one of the following
changes will cause an induced current to flow through the resistor?
A)
B)
C)
D)
decreasing the area of the loop
decreasing the magnitude of the magnetic field
increasing the magnitude of the magnetic field
rotating the loop through 90° about an axis in the plane of the paper
E) all of the above
A conducting loop of wire is placed in a magnetic field that is
normal to the plane of the loop. Which one of the following actions
will not result in an induced current in the loop?
A) Rotate the loop about an axis that is parallel to the
field and passes through the center of the loop.
B) Increase the strength of the magnetic field.
C) Decrease the area of the loop
D) Decrease the strength of the magnetic field.
E) Rotate the loop about an axis that is perpendicular to
the field and passes through the center of the loop.
A conducting bar moves to the left at a constant speed v on two
conducting rails joined at the left as shown. As a result of the
bar moving through a constant magnetic field, a current I is
induced in the indicated direction. Which one of the following
directions is that of the magnetic field?
A) toward the right
B) toward the left
C) parallel to the long axis
of the bar
D) into the page
E) out of the page
LET’S TALK ABOUT MIKE FARADAY
Important Definition From Last Time –
Magnetic Flux
Magnetic Field
AREA
FLUX
What did LENTZ
say of the FLUX
changes??
The Magnetic Flux Going Through The
Loop:
   Bi cos(i )Ai
i
Add up all of these pieces
that are INSIDE the loop.
WAIT A SECOND …….
• You said that there is a conducting loop.
• You said that there is therefore a VOLTAGE or
emf around the loop if the flux through the
loop changes.
• But the beginning and end point of the loop
are the same so how can there be a voltage
difference around the loop?
• ‘tis a puzzlement!
REMEMBER when I said E Fields start and
end on CHARGES???
DID I LIE??
The truth
• Electric fields that are created by static
charges must start on a (+) charge and end on
a (–) charge as I said previously.
• Electric Fields created by changing magnetic
fields can actually be shaped in loops.
Why do you STILL think I am a liar?
Because you said that an
emf is a voltage so if I
put a voltmeter from
one point on the loop
around to the same
point, I will get ZERO
volts, won’t I
The POTENTIAL between two points
• Is the WORK that an external agent has to do
to move a unit charge from one point to
another.
• But we also have (neglecting the sign):
V   Es
s
So, consider the following:
emf   Es  E  s
x
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x
Conductor
emf  2RE  zero
E
Through
emf 
the loop
t
Michael Faraday
(1791-1867)
Q: WHICH WAY DOES E POINT?
A: The way that you don’t want it to
point! (Lenz’s Law)
Technically:

emf  
t
A rectangular circuit containing a resistor is perpendicular to
a uniform magnetic field that starts out at 2.65 T and steadily
decreases at 0.25 T/s. While this field is changing, what does
the ammeter read?
Start Working on Unit 14
Friday: QUIZ + Do the Experiment