Ch. 33 slides

Phys 133 -- Chapter 33 1

Chapter 33

Electromagnetic Induction


Demo Faraday


Some experiments


Motional emf


Motional emf (cont.)Reach equilibrium


A potential difference of 0.050 V is developed across a 10-cm long wire as it moves through a magnetic field at 5.0 m/s. The magnetic field is perpendicular to the wire axis. What are strength and direction of field?

Problem 33.1


Problem 33.1 (ans)At equilibrium

FeFm

Out of page


--moving charges in B field, force --> current

--current-carrying wire in field, force

Induced current


Do workbook 33.1

Answer:A)ClockwiseB)Counter-clockwiseC)No current


Eddy currents


Do workbook 33.2 & 4


Eddy currents (cont.)


Demo eddy currents


Do workbook 33.4


Magnetic flux


There is an induced current in a closed, conducting loop if and only if the magnetic flux through the loop is changing. The induced current creates an induced magnetic field which opposes the change in flux.

--changing flux produces induced current

--induced current creates induced field

--induced field opposes the change

Lenz’s law


Does the loop of wire have a clockwise current, a counterclockwise current, or no current under the following circumstances?A)CWB)CCWC)No current

1)The magnetic field strength is increasing?2)The magnetic field strength is constant?3)The magnetic field strength is decreasing?

Two loops of wire are stacked vertically, one above the other. Does the upper loop have current in the following circumstances.A)CWB)CCWC)No current?

1)Before the switch is closed?2)Immediately after the switch is closed?3)Long after the switch is closed?4)Immediately after the switch is reopened?


1) As the magnet is inserted into the coil, does current flow right to left or left to right through the current meter, or is the current zero?

2) As the magnet is held at rest inside the coil…?

3) As the magnet is withdrawn from the coil…?

4) If the magnet is inserted into the coil more rapidly than in part a, does the size of the current increase, decrease or stay the same? Explain


Do workbook 33.12, 13 & 19

c. Just after the switch on the left coil is reopened, does current flow right to left or left to right through the current meter of the right coil? Or is the current zero?


The current in the solenoid is decreasing. The solenoid is surrounded by a conducting loop. Is there a current in the loop? If so, is the current in the loop cw or ccw?

Problem 33.9


Problem 33.9 (ans)

B1

Initially

later

B2

∆Change in flux to left

• Find change in flux

• Find induced field

Induced field points to right

• Find induced currentInduced current is clockwise from behind


field is there independent of wireInduced fields


--law of nature

--usually helpful

Faraday’s law

-field can change

-area can change

-both can change


Do workbook 33.21

A conducting loop around a magnetic field contains two bulbs, A and B. The wires connecting the bulbs are ideal, with no resistance. The magnetic field is increasing rapidly.

1) Do the bulbs glow? Why or why not?

2) If they glow, which bulb is brighter? Or are they equally bright? Explain.


Problem 33.12bThe loop is being pushed out of the 0.20 T magnetic field at 50 m/s. The resistance of the loop is 0.20 . What are the direction and magnitude of current in the loop?


Problem 33.12b (ans)

L

w

Counter-clockwise


Two 20-turn coils are tightly wrapped on the same 2.0-cm-diameter cylinder with 1.0-mm-diameter wire. The current through coil 1 is shown. A positive current is into the page at the top of the loop. Determine the current in coil 2 as current vs. time t = 0 s to 0.4 s. (Assume field of coil 1 passes completely through coil 2.

0.1 0.3

Problem 33.35


Problem 33.35 (ans)

B1

(+)

2

I1

d2/dt

2

I2


Problem 33.35 (ans)

(+)

I1

I2

Right to left through resistor


Generator


Transformer


Coulomb and non-Coulomb electric fields


-symmetric in E and B

Maxwell and induced magnetic field


E & M equations


Maxwell’s equations


+math

Light is an E&M wave!!!

Maxwell-no charges or current


EM waves


Problem 33.49A U-shaped conducting rail that is oriented

vertically in a horizontal magnetic field. The rail has no electrical resistance and does not move. A slide wire with mass m and resistance R can slide up and down without friction while maintaining electrical contact with the rail. The slide wire is release from rest.

a) Describe the motion of the slide wire.b) Find the terminal velocity of the slide wire

Ch. 33 slides

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