Conceptual Physics

the potential difference of the alternating current varies sinusoidally with time.

There are ten loops in the top coil. You can vary the number of loops in the bottom

coil from 20 to 50, in increments of 5. Pick a number of loops and then use the

oscilloscope to determine the maximum potential difference across the resistor in

the secondary circuit. You can use that value and the ratio of loops to determine the

maximum potential difference in the primary circuit.

Enter this value in the box provided in the control panel and press CHECK to see if

you are right. If not, redo your calculations, enter a new answer, and press CHECK

again.

If you need help, review the section on transformers.

29.18 - Gotchas

Any magnetic field induces an emf in a loop of wire. No, a changing magnetic field induces an emf in such a loop. A uniform, constant

magnetic field alone induces no net emf in a loop of wire. However, a potential difference can be induced across a segment of wire by moving it

through a magnetic field.

Transformers change potential differences. Power is a function of potential difference, so they change power, as well. No, the current changes

in inverse proportion to the potential difference, so the power stays constant.

29.19 - Summary

A changing magnetic field can generate an emf in a loop of wire. This phenomenon

is called electromagnetic induction.

The motion of a coil in a nonuniform magnetic field can also induce an emf. The

same effect is achieved whether you move the coil through the field, or move the

field past the coil.

Magnetic flux, ĭB is the amount of magnetic field passing perpendicularly through a

surface. The amount of flux can be computed as the dot product of the field and

area vectors, which is the product of the field strength, the area, and the cosine of

the angle between them. The unit of magnetic flux is the weber (Wb). 1 Wb = 1

T·m2.

Faraday’s law states that the induced emf in a circuit is proportional to the rate of

change of magnetic flux through the circuit.

When a change in the magnetic flux due to an external magnetic field induces a

current in a circuit, the induced current creates its own magnetic field. Lenz’s law

states that the induced magnetic field always opposes the change in flux that

caused it. This helps you to deduce the direction of the induced current.

A loop of wire rotating in a magnetic field is the basis of an electric generator. A

torque causes the loop to rotate in the field, constantly moving its wires through the

field and inducing an emf and an electric current in it. The induced current reverses

direction every half turn of the loop, and is called an alternating current (AC). Both the induced emf and the current vary sinusoidally with time.

Currents induced in large solid pieces of conducting material are not like those induced in a wire. When a solid conductor moves through a

nonuniform magnetic field, eddy currents flow through it in complex “whirlpools.” Eddy currents oppose the motion of the conductor through the

nonuniform field.

Mutual induction occurs when a changing current in a circuit generates a changing magnetic field, which in turn induces a current in a second

nearby circuit.

A transformer is a device that increases or decreases potential difference from one alternating-current circuit to another. Both circuits contain

coils wrapped around the same iron core, but with a different number of loops in each coil.

Wire segment moving in B field

ǻV = LvB

Magnetic flux

ĭB = BA cos ș

Faraday’s law

Transformer

(^552) Copyright 2007 Kinetic Books Co. Chapter 29