PHYS1220 Lecture Notes - Lecture 10: Counter-Electromotive Force, Magnetic Flux, Electric Motor

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aquamarinebadger817

10 Oct 2018

School

University of Newcastle

Department

Physics

Course

PHYS1220

Professor

Michael Gladys

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Document Summary

For an arbitrary surface or non-uniform field the equation for magnetic flux is the following: Equal to the time rate of change of magnetic flux through a circuit. Negative as the emf is trying to oppose the direction of the original change in flux to. An induced emf is always in the direction that opposes the original change in flux conform with the conservation of energy. The coil is rotated within a magnetic. Identical to a motor running in reverse that caused it field to generate a current. In a motor, current is supplied to the coil and the magnetic force on the loop causes rotation generates emf. As the coil rotates, the flux through it changes and. From lenz"s law, the induced emf must oppose the: called back emf or counter emf original emf. Back emf reduces the overall voltage through the circuit. One of the most widely used electronic devices.

Related Questions

A loop of wire is lying flat on a tabletop. A uniform magneticfield is directed vertically UPWARDS such that it is perpendicularto the tabletop (and the loop) and pointing toward the ceiling.Beyond the edges of the table, the magnetic field is zero. Imaginethat you are looking DOWN on the loop from above. Which of thefollowing statements are true about the induced EMF in the wireloop?

1)If the magnetic field suddenly decreases, the induced currentwill be in a counter-clockwise direction.
2)If the magnetic field suddenly decreases, the induced currentwill be in a clockwise direction.
3)If the loop suddenly shrinks in size, the induced current will bein a clockwise direction.
4)If you pull the loop out of the field, by sliding it off thetable, the induced current will be in a clockwise direction.
5)Lenz's Law states that the induced current always leads to amagnetic field that OPPOSES the change in magnetic flux.
6)If you keep the loop on the table, but pull it to the right,there will be no induced current.

A coil of wire is connected to a power supply, and a currentruns in the coil. A single loop of wire is located near the coil,with its axis on the same line as the axis of the coil. The radiusof the loop is 2 cm.

At time t1 the magnetic field at the center of the loop, due to thecoil, is 0.4 T, in the direction shown in the diagram; the currentin the coil is constant.

(a) What is the absolute value of the magnetic flux through theloop at time t1?
Î¦ mag = T m2
(b) What approximations or assumptions did you make in calculatingyour answer to part (a)?

Check all that apply.

1.The magnitude of the magnetic field due to the coil is uniformover the area of the loop.
2.The magnetic field outside the loop is zero.
3. The magnetic field due to the coil is uniform in direction overthe area of the loop.

(c) What is the direction of the "curly" electric field inside thewire of the loop at time t1? (Remember that at this time thecurrent in the coil is constant.)
E = 0

At a later time t2, the current in the coil begins todecrease.

(d) Now what is the direction of the "curly" electric field in theloop?
counter-clockwise
At time t2 the rate of change of the magnetic field at the centerof the loop, due to the coil, is -0.36 T/s.
(e) At this time, what is the absolute value of the rate of changeof the magnetic flux through the loop?
|dÎ¦mag/dt| = T m2/s
(f) At this time, what is the absolute value of the emf in theloop?
|emf| = V
(g) What is the magnitude of the electric field at location P,which is inside the wire?
|E| = V/m
(h) Now the wire loop is removed. Everything else remains as it wasat time t2; the magnetic field is still changing at the same rate.What is the magnitude of the electric field at location P?
|E| = V/m

Faraday's law describes how electric fields and electromotive forces are generated from changing magnetic fields. This problem is a prototypical example in which an increasing magnetic flux generates a finite line integral of the electric field around a closed loop that surrounds the changing magnetic flux through a surface bounded by that loop. A cylindrical iron rod with cross-sectional area is oriented with its symmetry axis coincident with the z axis of a cylindrical coordinate system as shown. It has a uniform magnetic field inside that varies according to .

1. Find , the electromotive force (EMF) around a loop that is at distance R from the z axis, where R is restricted to the region outside the iron rod as shown. Take the direction shown in the figure as positive

2. Due to the cylindrical symmetry of this problem, the induced electric field can depend only on the distance R from the z axis, where R is restricted to the region outside the iron rod. Find this field

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PHYS1220 Lecture Notes - Lecture 10: Counter-Electromotive Force, Magnetic Flux, Electric Motor

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