Upcoming Schedule

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Upcoming Schedule
Nov. 3 20.8, 21.1-21.2 Nov. 5 boardwork Nov. 7 21.3-21.5 Quiz 7
Nov. 10 boardwork Nov. 12 21.4-21.5 Nov. 14 boardwork Quiz 8
Nov. 17 review Nov. 19 Exam 3 Chap. 20-21 Nov. 21 21.6-21.7 18.8
Check announce-ments!
Physics-Speak when we say this It is clear
that much additional work will be required
before a complete understanding is reached.
Physics-Speak when we say this
we really mean It is clear that much additional
work I dont understand any will be required
before a complete of
this. understanding is reached.
2
Homework Problem 21.7 (a) If the resistance is
slowly increased, what is the direction of the
current induced in the small circular loop?
The current flowing as shown gives rise to a
magnetic field pointing out of the screen (but
only for the area inside the larger loop).
?
?
?
?
If the current remains constant, the magnetic
field, and therefore the magnetic flux, through
the inner loop remains constant.
?
No current would flow in the inner loop.
3
If the resistance is increased, the current in
the outer loop decreases, and the flux through
the inner loop decreases.
Less flux through the inner loop (indicated by
the smaller symbols), so it wants to restore
the flux to its before status.
?
?
?
?
?
?
?
?
Current flows in the inner loop to make the
magnetic field inside the loop point out of the
plane of the screen with its original magnitude.
?
?
increase R
Counterclockwise current, just like the book says!
4
(b) What if the small loop were to the left,
outside the larger one?
Outside the larger loop, the magnetic field
points into the screen.
?
Increasing resistance ? less current ? less flux
pointing into the screen ? small loop makes
current flow to make more flux point into the
screen (inside itself) ? clockwise current.
?
?
?
?
5
21.4 Changing Magnetic Flux Produces an Electric
Field
From chapter 16, section 6
OSE E F / q
From chapter 20, section 4
OSE F q v B sin?
For v ? B, and in magnitude only,
F q E q v B
E v B.
We conclude that a changing magnetic flux
produces an electric field. This is true not
just in conductors, but any-where in space where
there is a changing magnetic field.
6
Example 21-5 Blood contains charged ions, so
blood flow can be measured by applying a magnetic
field and measuring the induced emf. If a blood
vessel is 2 mm in diameter and a 0.08 T magnetic
field causes an induced emf of 0.1 mv, what is
the flow velocity of the blood?
OSE ? B? l v?
v ? / (B? l)
In Figure 21-11 (the figure for this example), B
is applied ? to the blood vessel, so B is ? to v.
The ions flow along the blood vessel, but the
emf is induced across the blood vessel, so l is
the diameter of the blood vessel.
v (0.1x10-3 V) / (0.08 T 0.2x10-3 m)
v 0.63 m/s
7
21.5 Electric Generators
Lets begin by looking at a simple animation of a
generator. http//www.wvic.com/how-gen-works.htm
Heres a freeze-frame.
Normally, many coils of wire are wrapped around
an armature. The picture shows only one.
Brushes pressed against a slip ring make
continual contact.
The shaft on which the armature is mounted is
turned by some mechanical means.
8
Lets look at the current direction in this
particular freeze-frame.
B is down. Coil rotates counter-clockwise.
B is down.
Put your fingers along the direction of movement.
Stick out your thumb.
Bend your fingers 90. Rotate your hand until
the fingers point in the direction of B. Your
thumb points in the direction of conventional
current.
I can see it for this part of the loop, but have
great difficulty for this part of the loop.
9
Alternative right-hand rule for current direction.
B is down. Coil rotates counter-clockwise.
Make an xyz axes out of your thumb and first two
fingers.
Thumb along component of wire velocity ? to B.
1st finger along B.
2nd finger then points in direction of
conventional current.
Hey! The picture got it right!
10
I know we need to work on that more. Lets zoom
in on the armature.
v
v??B
v?B
I
B
11
Forces on the charges in these parts of the wire
are perpendicular to the length of the wire, so
they dont contribute to the net current.
For future use, call the length of wire shown in
green h and the other lengths (where the two
red arrows are) l.
12
One more thing
This wire
connects to this ring
so the current flows this way.
13
Later in the cycle, the current still flows
clockwise in the loop
but now this wire
connects to this ring
so the current flows this way.
Alternating current! ac!
Again http//www.wvic.com/how-gen-works.htm
14
Dang! That was complicated. Are you going to
ask me to do that on the exam?
No. Not anything that complicated. But you
still need to understand each step, because each
step is test material.
Click here and scroll down to electrodynamics
to see some visualizations that might help you!
Understanding how a generator works is good,
but we need to quantify our knowledge.
We begin with our OSE ? B? l v?. (l was
defined on slide 17.) In our sample generator on
the last 7 slides, we had only one loop, but two
sides of the loop in the magnetic field. If the
generator has N loops, then ? 2 N B? l v?.
15
Back to this picture
This picture is oriented differently than Figure
21-13 in your text. In your text, ? is the angle
between the perpendicular to the magnetic field
and the plane of the loop.
16
?
The angle ? in the text is the same as the angle
between v??B and the vector v.
Thus, v? v sin ?.
17
B is ? to the wire, so
But the coil is rotating, so ? ? t, and v ? r
? (h/2). The diameter of the circle of
rotation, h, was defined on slide 17.
where A is the area of the loop, f is the
frequency of rotation of the loop, and ? 2 ? f.

18
Example 21-6 The armature of a 60 Hz ac
generator rotates in a 0.15 T magnetic field. If
the area of the coil is 2x10-2 m2, how many loops
must the coil contain if the peak output is to be
?0 170 V?
You should read about dc motors and alternators
in section 21.5, but I wont test you over that
material.