fig 31-co, p.967 p.968 ch 31 faraday’s law 31.1 faraday’s law of induction = -nd b /dt

Histogram for PHYS202 Exam 2 Spring 2008Average = 76 Low = 36 High = 99

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Histogram for PHYS202 Total 2 Spring 2008Average = 73 Low = 42 High = 91

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Fig 31-CO, p.967

Ch 31 Faraday’s Law

31.1 Faraday’s Law of Induction

= -NdB/dt

CT1: In Faraday’s Electromagnetic Lab (PHET), I redo the experiment with the bar magnet and pick-up coil, but with the polarity of the bar magnet reversed. As before, with the bar magnet to the left of the pick-up coil, I first move the bar magnet towards the pick-up coil. Second I pull the bar magnet away from the pick-up coil. What will happen to the signs of the observed voltage across the pick-up coil?

A. Both signs stay the same.B. Both signs reverse.C. The first sign reverses and the

second sign stays the same.D. The second sign reverses and the

first sign stays the same.

CT2: In Faraday’s Electromagnetic Lab (PHET), I redo the experiment with the bar magnet and pick-up coil, but pull the bar magnet away more rapidly. What will happen to the magnitude of the observed voltage across the pick-up coil?

A. The magnitude will stay the same.B. The magnitude will decrease.C. The magnitude will increase.

CT3: In Faraday’s Electromagnetic Lab (PHET), I redo the experiment with the bar magnet and pick-up coil, but with the number of turns in the pick-up coil increased. What will happen to the magnitude of the observed voltage across the pick-up coil?


CT4: In Faraday’s Electromagnetic Lab (PHET), I redo the experiment with the bar magnet and pick-up coil, but with the area of the the pick-up coil increased. What will happen to the magnitude of the observed voltage across the pick-up coil?


CT5: In Faraday’s Electromagnetic Lab (PHET), I put a pick-up coil near the electromagnet. When will the pick-p coil will register a voltage?

A. The current in the electromagnet is DC.B. The current in the electromagnet is AC.C. The current in the electromagnet is either

DC or AC.D. Not in any case.

CT6: In Faraday’s Electromagnetic Lab (PHET), I have an electromagnet and pick-up coil (transformer). If I increase the magnitude of the AC current, the magnitude of the voltage in the pick-up coil will

A. stay the same.B. decrease.C. increase.


31.1 Faraday’s Law of Induction

= -NdB/dt

B = B·dA

B = BA if B is constant and along dA

P31.7 (p.888)

P31.67 (p.896)

x

infinitesimal area element of width dx and length L at x


31.2 Motional emf

V = Blv

CT7: Knowing that the terminal velocity is determined by zero acceleration when FB = ILB = mg, how does vT depend on mg? Recall I = V/R.

A. independent of mgB. mgC. mg2

D. 1/mgE. 1/(mg)2

CT8: Knowing that the terminal velocity is determined by zero acceleration when FB = ILB = mg, how does vT depend on R? Recall I = V/R.

A. independent of RB. RC. R2

D. 1/RE. 1/R2

P31.43 (p.893)

y

= -dB/dt =

-d(BA)/dt =

-d(Bwy)/dt =

-Bwdy/dt =

-Bwv (cw)

I = /R = Bwv/R

I

FB

An induced current creates a magnetic field that opposes the change in magnetic flux that created it. Alternatively, the induced magnetic field tends to maintain the magnetic flux by opposing the change in flux.


31.3 Lenz’s Law

Fig 31-14a, p.978

Fig 31-14c, p.978

CT9: A magnet is released at the top of each copper tube – one without a slit and one with.

A.The magnet in the tube without the slit reaches the bottom first.B. The magnet in the tube with the slit reaches the bottom first.C. The magnets in both tubes reach the bottom at the same time.

CT10: For the jumping ring demo, if I use a ring that is twice as thick and all else is equal, the ring will

A. not moveB. jump approximately half as highC. jump approximately twice as highD. jump approximately the same height

CT11: For the jumping ring demo, if I use a ring that is has a slot cut along its length and all else is equal, the ring will

A. not moveB. jump approximately half as highC. jump approximately twice as highD. jump approximately the same height

x

P31.28 (p.891)

CT12-15

A. in

B. out

C. right

D. left

E. up

F. down


31.4 Induced emf and Electric Fields

E·ds = -dB/dt

P31.33 (p.891)

CT16: In figure (a), a solenoid produces a magnetic field whose strength increases into the plane of the page. An induced emf is established in a conducting loop surrounding the solenoid, and this emf lights bulbs A and B. In figure (b), points P and Q are shorted. After the short is inserted,

A. bulb A goes out; bulb B gets brighter.B. bulb B goes out; bulb A gets brighter.C. bulb A goes out; bulb B gets dimmer.D. bulb B goes out; bulb A gets dimmer.E. both bulbs go out.F. none of the above


31.4 Generators and Motors

P31.54 (p.999)

P31.64 (p.896)

Polling Question: How many hours per week to you spend outside of class on average for PHYS151?

A. 2 or less

B. 3

C. 4

D. 5

E. 6

F. 7

G. 8 or more

fig 31-co, p.967 p.968 ch 31 faraday’s law 31.1 faraday’s law of induction = -nd b /dt

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