Download Presentation_W01D1_answers_jwb

Interlude: The Hardest Part of
this Course
Faraday’s Law
We will do this twice, today
and in about a month
(discovered Aug 29,1831)
Today’s Objectives
Introduce key concepts from electricity and
magnetism through discovery activities,
experiments, concept questions, discussion,
and visualizations.
Later in the course, we will return to the same
concepts.
Today we are just going to have some fun.
What we are trying to get a feel for:
You Tube Link: http://youtu.be/YywaJkGKOaY
Concept Question: Loop in Uniform
Field
While a rectangular wire loop is
pulled upward though a uniform
magnetic field B field penetrating its
bottom half, as shown, there is
1. a current in the loop.
2. no current in the loop.
3. I do not understand the concepts of current and
magnetic field.
4. I understand the concepts of current and magnetic field
but am not sure of the answer.
4
Demo: aluminum sleeve moving
past fixed magnet, students do
this at their tables
Demo: we show the demo of
magnet falling through plastic
tube and aluminum tube
5
Seeing the Unseen:
Faraday’s Law Applet
Applet -- Faraday’s law applet (with a magnet and a
coil):
http://web.mit.edu/viz/EM/visualizations/faraday/faradaysLaw/faradayapp/faradayapp.htm
Play with the application until you are familiar with all
the features. In the Actions Menu: try both Manual and
Generator Mode. You can use the buttons at the
bottom to start, pause and reset the simulation. You
can move the magnet and the ring back and forth
using the mouse. Let each person in the group have a
turn.
Seeing the Unseen: First
Concept Flow
Group Discussion Question
What are some examples of flow of “something”
through an area?
Examples of Flow
Electric Current: Flow Of Charge
Electric Current I: Charge Q flowing across area A
in time t
Q
I
t
8
Current and Magnetic Field
Current produces a magnetic field as
shown in figure
9
Magnetic Field of Bar Magnet
(1) A magnet has two poles, North (N) and South (S)
(2) Magnetic field lines leave from N, end at S
10
Seeing the Unseen:
Magnetic Field
Run the Applet on generator mode and stop the
magnet when it is near the ring
Scroll down on the panel on the right and click on
Magnetic Field: Iron Filings
Seeing the Magnetic Field: Iron
Filings
The iron filings represent the
magnetic field present at the
instant you stopped the magnet .
The direction of the magnetic
field is along the direction of the
iron filings. Does the magnetic
field intercept the area of the
circular wire?
Magnetic Flux Thru Wire Loop
Flux is the
Generalization of
Flow
Product of magnetic
field and area
 B  B A
13
Discussion Question: Magnetic
Flux in Ring
The first graph on the right in the Applet shows a plots of the
external magnetic flux and total magnetic flux in the ring versus
time. Briefly describe where the “external flux” (red plot) is
coming from: that is, what kind of flux is this, what creates it,
over what area is the flux being measured.
More Discussion Questions
About Magnetic Flux
1. Describe different ways that you can change the external flux
2. Explain how the total magnetic flux (blue plot)
is related to the external magnetic flux (red plot).
Current in Ring
The second graph on the right in the Applet shows a
plot of the current in the ring versus time.
Proposing a Hypothesis
Propose a qualitative relationship between magnetic
flux (seen in top graph) and current that flows in the
ring (seen in bottom graph).
Testing Hypotheses
Groups utilizing the application came up with the following
hypotheses.
1.Group A conjectured that the current through the ring is
proportional to the total magnetic flux.
1.Group B proposed that the current through the ring is
proportional to the change in the total magnetic flux.
Use the application to test these two hypotheses. Design and
run a virtual experiment that could rule out any of the
hypotheses. Which did you rule out and why?
Faraday’s Law of Induction
Changing magnetic flux induces a
current
d B d( BA) dB
I:


A
dt
dt
dt
19
Electromotive Force
Electromotive force  looks like a
voltage difference. It’s a “driving force”
for induced current
  IR
20
Faraday’s Law of Induction
Changing magnetic flux is proportional
to electromotive force

d B d( BA) dB
:


A
dt
dt
dt
21
Demo: Electromagnetic Induction
22
Demo: Electromagnetic Induction
23
Demonstration:
Induction
At this point, students again
move the coil of wire in their
experiment just to observe the
current
24
Concept Question: Loop in Uniform
Field
While a rectangular wire loop is
pulled upward though a uniform
magnetic field B field penetrating its
bottom half, as shown, there is
1. a current in the loop.
2. no current in the loop.
3. I do not understand the concepts of current and
magnetic field.
4. I understand the concepts of current and magnetic field
but am not sure of the answer.
25
Concept Question Answer: Loop in
Uniform Field
Answer: 1. The motion changes the magnetic flux through the
loop. The magnetic flux is decreasing in time as more of the
loop enters a region of zero magnetic field. According to
Faraday’s Law there is an induced current through the loop.
26
Concept Question: Loop in Uniform
Field
While a rectangular wire loop is
pulled sideways though a uniform
magnetic field B field penetrating its
bottom half, as shown, there is
1. a current in the loop.
2. no current in the loop.
3. I do not understand the concepts of current and
magnetic field.
4. I understand the concepts of current and magnetic field
but am not sure of the answer.
27
Concept Question Answer: Loop in
Uniform Field
Answer: 2. The motion does not change the magnetic flux
through the loop. The magnetic flux is constant in time.
According to Faraday’s Law there is no induced current
through the loop.
28
Concept Test: Induced Current
We define positive current clockwise
as viewed from the top. As the coil
moves from well below the magnet to
well above that magnet, the induced
current through the coil will look like:
(1)
(2)
(3)
(4)
Try to answer this
question using
your experimental
set-up
(5) I don’t know
Concept Question: Induced
Current Answer
Solution (3).
Lenz’s Law
Direction of Induced Current
31
Discussion Question:
Induced Current
Run the Applet and observe the relation between the
sign of current and the slope of the plot of magnetic
flux. What do you observe?
Minus Sign? Lenz’s Law

d B

dt
Induced EMF is in direction that opposes
the change in flux that caused it
33
Conclusion: Faraday’s Law of
Induction
Changing magnetic flux generates
electromotive force that opposes that
change in flux

d B

dt
34
Jumping Ring
An aluminum ring jumps into the air when the
solenoid beneath it is energized
35
What is Going On?
This is a dramatic example of Lenz’s Law:
When the magnetic field created when the
solenoid is energized tries to permeate the
conducting aluminum ring, currents are induced
in the ring to try to keep this from happening!
36