1 Orange Coast College Physics 280 Experiment #7 Electromagnetic Induction The purpose of this experiment is to help you become familiar with some of the principles of Faraday’s law of electromagnetic induction. Apparatus: Two flat wound coils Two voltmeters One ammeter One DC Power supply One AC power supply One Galvanometer One soft-iron U-shaped bar that will hold the two coils Theory Electromagnetic induction theory predicts that a voltage will be induced (also called an “emf”) every time the magnetic flux ! B through a conducting loop changes, as given by the relation: emf = ! d" B dt The magnetic flux ! B through a coil can be expressed as: ! B = ! B " d ! A # = BdA cos ! () # A change in the magnetic flux ! B through a coil can be generated by: (i) changing the magnetic field through the coil, or (ii) changing the area of the coil (either by stretching or compressing the coil), or (iii) by rotating the coil relative to the direction of the magnetic field. The negative sign in the equation above means that the direction of the induced current in the conducting loop is such as to oppose whatever caused it. This is known as Lenz’s law. In this activity you will become familiar with the qualitative response of coils when subjected to a changing magnetic flux through them. You will also build a simple transformer.
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1
Orange Coast College Physics 280
Experiment #7
Electromagnetic Induction The purpose of this experiment is to help you become familiar with some of the principles of Faraday’s law of electromagnetic induction. Apparatus: Two flat wound coils Two voltmeters One ammeter One DC Power supply One AC power supply One Galvanometer One soft-iron U-shaped bar that will hold the two coils Theory Electromagnetic induction theory predicts that a voltage will be induced (also called an “emf”) every time the magnetic flux !B through a conducting loop changes, as given by the relation:
emf = ! d"Bdt
The magnetic flux !B through a coil can be expressed as:
!B =!B "d!A# = BdAcos !( )#
A change in the magnetic flux !B through a coil can be generated by: (i) changing the magnetic field through the coil, or (ii) changing the area of the coil (either by stretching or compressing the coil), or (iii) by rotating the coil relative to the direction of the magnetic field. The negative sign in the equation above means that the direction of the induced current in the conducting loop is such as to oppose whatever caused it. This is known as Lenz’s law.
In this activity you will become familiar with the qualitative response of coils when subjected to a changing magnetic flux through them. You will also build a simple transformer.
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Procedure Part I.
Connect the galvanometer, as shown in figure 1. Watch the galvanometer needle while you
thrust the north pole of a bar magnet through the stationary coil. Describe what you observe?
Use the following symbols in this report to answer the pertinent questions:
L = Left, R = Right, N = North, S = South.
Questions: (Circle the best choice, if given)
(1a) What is the direction of the galvanometer needle when you thrust the north pole of the bar
magnet (at constant speed) toward the stationary coil? _______L, R_______
when you move the coil toward the stationary bar magnet? __________L, R_______
move both the coil and bar magnet toward each other? _____________L, R______________
(1b) What general statements can you make concerning the preceding observations?