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Learning objectives

By the end of this section, you will be able to:

  • Calculate the flux of a uniform magnetic field through a loop of arbitrary orientation.
  • Describe methods to produce an electromotive force (emf) with a magnetic field or a magnet and a loop of wire.

The information presented in this section supports the following AP® learning objectives and science practices:

  • 4.E.2.1 The student is able to construct an explanation of the function of a simple electromagnetic device in which an induced emf is produced by a changing magnetic flux through an area defined by a current loop (i.e., a simple microphone or generator) or of the effect on behavior of a device in which an induced emf is produced by a constant magnetic field through a changing area. (S.P. 6.4)

The apparatus used by Faraday to demonstrate that magnetic fields can create currents is illustrated in [link] . When the switch is closed, a magnetic field is produced in the coil on the top part of the iron ring and transmitted to the coil on the bottom part of the ring. The galvanometer is used to detect any current induced in the coil on the bottom. It was found that each time the switch is closed, the galvanometer detects a current in one direction in the coil on the bottom. (You can also observe this in a physics lab.) Each time the switch is opened, the galvanometer detects a current in the opposite direction. Interestingly, if the switch remains closed or open for any length of time, there is no current through the galvanometer. Closing and opening the switch induces the current. It is the change in magnetic field that creates the current. More basic than the current that flows is the emf that causes it. The current is a result of an emf induced by a changing magnetic field , whether or not there is a path for current to flow.

The picture shows Faraday’s apparatus for demonstrating that a magnetic field can produce a current. It consists of a cylinder shaped battery. The positive end of the battery is connected to an open switch. There is a ring shaped iron core consisting of a set of coils one on the top and another at the bottom. The other end of the switch is connected to one end of the top coil. The other end of the top coil is connected back to the battery. Both the ends of the bottom coil are shown connected across a galvanometer box which shows a null deflection.
Faraday’s apparatus for demonstrating that a magnetic field can produce a current. A change in the field produced by the top coil induces an emf and, hence, a current in the bottom coil. When the switch is opened and closed, the galvanometer registers currents in opposite directions. No current flows through the galvanometer when the switch remains closed or open.

An experiment easily performed and often done in physics labs is illustrated in [link] . An emf is induced in the coil when a bar magnet is pushed in and out of it. Emfs of opposite signs are produced by motion in opposite directions, and the emfs are also reversed by reversing poles. The same results are produced if the coil is moved rather than the magnet—it is the relative motion that is important. The faster the motion, the greater the emf, and there is no emf when the magnet is stationary relative to the coil.

The diagram shows five stages of an experiment done by moving a magnet relative to a coil and measuring the e m f produced. The first stage of the experiment shows a wire coil with two loops connected across a galvanometer. The loop is in horizontal plane. A cylindrical rod shaped magnet is moved upward through the loop with the north pole of the magnet facing the loop and the South Pole away from the loop. The magnetic lines of force of the magnet are shown to emerge out from the North Pole and intersect the coil. A current is shown to be induced in the coil in clockwise direction. The galvanometer needle is shown to deflect toward right. The second stage of the experiment shows the next state of the first stage of the experiment. The cylindrical rod shaped magnet is now moved downward away from the loop with the north pole of the magnet facing the loop and South Pole away from the loop. The magnetic lines of force of the magnet are shown to emerge out from the North Pole and intersect the coil. A current is shown to be induced in the coil in anti clockwise direction. The galvanometer needle is shown to deflect toward left. The third stage of the experiment shows a wire coil with two loops connected across a galvanometer. The loop is in horizontal plane. A cylindrical rod shaped magnet is moved upward through the loop with the south pole of the magnet facing the loop and the North Pole away from the loop. The magnetic lines of force of the magnet are shown to merge into the South Pole and intersect the coil. A current is shown to be induced in the coil in anti clockwise direction. The galvanometer needle is shown to deflect toward left. The fourth stage of the experiment shows the next state of the third stage of the experiment. The cylindrical rod shaped magnet is now moved downward away from the loop with the south pole of the magnet facing the loop and the North Pole away from the loop. The magnetic lines of force of the magnet are shown to merge into the South Pole and intersect the coil. A current is shown to be induced in the coil in clockwise direction. The galvanometer needle is shown to deflect toward right. The fifth stage of the experiment shows a wire coil with two loops connected across a galvanometer. The loop is in horizontal plane. A cylindrical rod shaped magnet is held stationary near the loop with the north pole of the magnet facing the loop and south away from the loop. The magnetic lines of force of the magnet are shown to emerge out from the North Pole and intersect the coil. No current is induced in the coil. The galvanometer needle does not deflect.
Movement of a magnet relative to a coil produces emfs as shown. The same emfs are produced if the coil is moved relative to the magnet. The greater the speed, the greater the magnitude of the emf, and the emf is zero when there is no motion.

The method of inducing an emf used in most electric generators is shown in [link] . A coil is rotated in a magnetic field, producing an alternating current emf, which depends on rotation rate and other factors that will be explored in later sections. Note that the generator is remarkably similar in construction to a motor (another symmetry).

Questions & Answers

What is meant by dielectric charge?
It's Reply
what happens to the size of charge if the dielectric is changed?
Brhanu Reply
omega= omega not +alpha t derivation
Provakar Reply
u have to derivate it respected to time ...and as w is the angular velocity uu will relace it with "thita × time""
Abrar
do to be peaceful with any body
Brhanu Reply
the angle subtended at the center of sphere of radius r in steradian is equal to 4 pi how?
Saeed Reply
if for diatonic gas Cv =5R/2 then gamma is equal to 7/5 how?
Saeed
define variable velocity
Ali Reply
displacement in easy way.
Mubashir Reply
binding energy per nucleon
Poonam Reply
why God created humanity
Manuel Reply
Because HE needs someone to dominate the earth (Gen. 1:26)
Olorunfemi
why god made humenity
Ali
Is the object in a conductor or an insulator? Justify your answer. whats the answer to this question? pls need help figure is given above
Jun Reply
ok we can say body is electrically neutral ...conductor this quality is given to most metalls who have free electron in orbital d ...but human doesn't have ...so we re made from insulator or dielectric material ... furthermore, the menirals in our body like k, Fe , cu , zn
Abrar
when we face electric shock these elements work as a conductor that's why we got this shock
Abrar
how do i calculate the pressure on the base of a deposit if the deposit is moving with a linear aceleration
ximena Reply
why electromagnetic induction is not used in room heater ?
Gopi Reply
room?
Abrar
What is position?
Amoah Reply
What is law of gravition
sushil Reply
what is magnetism
Sandeep Reply
Practice Key Terms 2

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Source:  OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
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