PHYS 1B Lecture Notes - Lecture 20: Magnetic Flux, Surface 2

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13 Mar 2017

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Document Summary

Induced current: experiments show that a changing magnetic field can induce current to flow in a loop of wire. If magnet moved toward the loop, gives negative current. A/the normal vector of a is parallel to the magnetic field: therefore, the magnetic flux is 2 wb. Faraday"s law of i(cid:374)du(cid:272)tio(cid:374: experiments show that emf (potential difference) is induced in a loop of wire when the magnetic flux through the loop is changing in time. If the loop of the wire has resistance r, this emf will cause the current to flow given (cid:271)y oh(cid:373)"s law, of (cid:373)ag(cid:374)itude i = ||/r. Important application: generation of electricity: consider an electric motor setup. Clicker question: what must by true of the induced current if the generator loop rotates at a constant angular speed: ac = alternating current, direction varies in time, dc = direct current, meaning constant.

Related Questions

1a) Faraday discovered that a current is induced in a coil

A) only when the N pole of a magnet is inside the coil
B) only when the S pole of a magnet is inside the coil
C) only when there is a magnetic field inside the coil
D) only when the magnetic field inside the coil is changing

1b). A conductor moves through a uniform magnetic field at speed v and has a motional emf V created across it. If the conductor were to move twice as fast through the magnetic field, the motional emf would be:

A) V
B) 2V
C) 4V
D) V/2
E) V/4

1c) In the conducting rail discussed in section 25.2 (where the rail is moving at constant speed), what can you say about the power input due to the person pulling on the rail and the power dissipated in the resistance?

A) Power input from pull = power dissipated in R
B) Power input from pull > power dissipated in R
C) Power input from pull < power dissipated in R

1d) The electric generator as shown in the reading generates electricity by:

A) rotating a loop inside a magnetic field
B) moving a magnet in and out of a loop
C) sliding a loop over a magnet
D) running current through a loop in a magnetic field

1e) The magnetic flux through a loop:

A) is proportional to the number of magnetic field lines going through it
B) depends just on the B field going through the loop
C) is maximized when the B field lines in the plane of the loop
D) is zero when the B field is perpendicular to the plane of the loop
E) doesn't depend on the direction of the B field

1f) According to Lenz's Law, the induced B field (due to the induced current) will:

A) be in the opposite direction of the existing B field
B) be in the same direction as the existing B field
C) be in the same direction as the change in the existing B field
D) be in the opposite direction of the change in the existing B field

1g) A stationary coil experiences a doubling of its magnetic field (no change in direction of the field) in a time T and has a given induced EMF. If the same change were to have happened in half the time (T/2), the induced EMF would have been:

A) half as large
B) the same
C) twice as large
D) one-fourth as large
E) four times as large

1h) Eddy currents:

A) make it hard to pull a nonmagnetic material through a magnetic field
B) make it hard to pull a magnetic material through a magnetic field
C) speed up a magnetic material moving through a magnetic field
D) speed up a nonmagnetic material moving through a magnetic field

cerisejackal809

1)As shown in the figure, a uniform conducting rod of length 0.40 m and mass 0.075 kg is in rotational equilibrium about the hinge at point O. A uniform magnetic field of 0.23 T is directed out of the plane of the page, perpendicular to the rod. Due to a battery and wires not shown, there is a current of 4.0 A in the rod directed toward point O. Determine the angle θ.

2)As shown in the figure, a conducting rod with a length of 0.79 m is placed on a horizontal frictionless surface and attached to a power supply by two springs, each with a spring constant of 81 N/m. The springs are initially at their equilibrium length and are in a region where there is a uniform magnetic field of 0.17 T that is perpendicular to the frictionless surface when the power supply supplies a current that causes the wire to move in such a direction that the two springs are stretched.