W03D1 Presentation Answers (2)

7/29/2019 W03D1 Presentation Answers (2)

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Introduction to Active

Learning:

Faradays Law


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Todays 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 and

get to know each other.


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Introductions


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What we are trying to get a feel for:

You Tube Link: http://youtu.be/YywaJkGKOaY
http://youtu.be/YywaJkGKOaYhttp://youtu.be/YywaJkGKOaY


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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

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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.


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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


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Seeing the Unseen:

Faradays Law Applet

Applet -- Faradays 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 allthe 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 forthusing the mouse. Let each person in the group have a

turn.
http://web.mit.edu/viz/EM/visualizations/faraday/faradaysLaw/faradayapp/faradayapp.htmhttp://web.mit.edu/viz/EM/visualizations/faraday/faradaysLaw/faradayapp/faradayapp.htm


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Seeing the Unseen: First

Concept FlowGroup Discussion Question

What are some examples of flow of something

through an area?


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Examples of Flow

Electric Current: Flow Of Charge

Electric Current I: Charge Q flowing across area A

in time t

I=

Q

t


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Current and Magnetic Field

Current produces a magnetic field as

shown in figure


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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


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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


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Seeing the Magnetic Field: Iron

FilingsThe iron filings represent the

magnetic field present at the

instant you stopped themagnet . 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?


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Magnetic Flux Thru Wire Loop

B=B

A

Flux is the

Generalization of

Flow

Product of magnetic

field and area


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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.


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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).


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Current in Ring

The second graph on the right in the Applet shows a

plot of the current in the ring versus time.


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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).


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Testing Hypotheses

Groups utilizing the application came up with the followinghypotheses.

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?


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Faradays Law of Induction

I

=

()

=

Changing magnetic flux induces acurrent

C


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Electromotive Force, Current and

Resistance

Vary the resistance in the applet and

observe the current.

Electromotive force looks like a

voltage difference. Its a driving forcefor induced current

= IR


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Faradays Law of Induction

=

()

=

Changing magnetic flux is proportionalto electromotive force


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Demo: Electromagnetic Induction


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Demo: Electromagnetic Induction


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Demonstration:

Induction

At this point, students again

move the coil of wire in their

experiment just to observe the

current

C t Q ti L i U if


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Field


pulled upward though a uniform



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magnetic field.



C t Q ti A L i


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Concept Question Answer: Loop in

Uniform Field

Answer: 1. The motion changes the magnetic flux throughthe loop. The magnetic flux is decreasing in time as more of

the loop enters a region of zero magnetic field. According to

Faradays Law there is an induced current through the

loop.

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C t Q ti L i U if


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Field


pulled sideways though a uniform



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magnetic field.



C t Q ti A L i


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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 Faradays Law there is no induced current

through the loop.

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Lenzs Law

Direction of Induced Current


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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? Try flipping the coil andsee what result you get for the current.


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Minus Sign? Lenzs Law

Induced EMF is in direction that opposes

the change in flux that caused it

= d

B

dt


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Concept Test: Induced Current

We define positive current clockwise

as viewed from the top. As the coilmoves from well below the magnet to

well above that magnet, the induced

current through the coil will look like:

(1) (2)

(3) (4) (5) I dont know

Try to answer this

question using

your experimental

set-up


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Concept Question: Induced

Current Answer

Solution (3).

Conclusion: Faradays Law of


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Conclusion: Faradays Law of

Induction

= d

B

dt

Changing magnetic flux generateselectromotive force that opposes that

change in flux


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Jumping Ring

An aluminum ring jumps into the air when the

solenoid beneath it is energized


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What is Going On?

This is a dramatic example of Lenzs Law:

When the magnetic field created when thesolenoid is energized tries to permeate the

conducting aluminum ring, currents are induced

in the ring to try to keep this from happening!

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W03D1 Presentation Answers (2)