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INTRODUCTION

    The purpose of this experiment is to observe the current caused by an emf induced by a changing magnetic field: Faraday's Law and Lenz's law.

THEORY

    Michael Faraday found that an emf is induced in a coil of wire that is proportional to the rate of change of the magnetic flux through the coil:

where is the emf induced in the loop, N is the number of loops of wire, Δ&phi:_B is the change in magnetic flux, and Δt is the time interval over which &phi:_B changes.

    The negative sign in Faraday's Law comes from Heinrich Lenz, who discovered that the direction of the induced current in a coil of wire is such that the coils own B field opposes the original change in the flux that induced the current. Therefore, the coil will always resist a change in magnetic flux.

EXPERIMENT

1. Correlate the direction of current through the galvanometer with the motion of the needle.
   • Connect a 100 kΩ resistor in series with the galvanometer to allow a small current to pass through the meter, as shown in the circuit diagram (If you connect the resistor incorrectly, you'll fry the meter!). Draw a circuit diagram that shows which side the current enters the meter when the needle points in each direction (the + and – signs on the meter do not indicate direction). Reverse the direction of the current through the meter to confirm your observation.

   
   

2. The induced current due to a permanent magnet.
   1. Use a known compass to determine the polarity of your bar magnet (recall that field lines leave the north pole, and enter the south).

   2. Tape the large red or yellow coil vertically on the table, and connect it to the galvanometer so that the wire connected to the left side of the galvanometer passes over the top of the coil, as shown below:

      

   3. Sketch the directions of the galvanometer needle, the induced current, and the induced B field through the coil due to i) inserting and ii) withdrawing a north and a south pole, from each end of the coil (you will have a total of eight sketches similar to the figure above). Draw your coils carefully; your observations should agree with predictions using the right-hand rule! (If they don't agree, you've misidentified the polarity of your magnet, or your coil is not connected properly!)

3. The induced current due to an electromagnet.

   

   1. Following the diagram at right, connect one of the red/yellow coils and a knife switch to the DC power source built into the lab bench, and lay it sideways on the table. This will become the source coil. Connect the other red/yellow coil (the detector) to the galvanometer, and set it aside.

   2. Use the right hand rule to determine which electromagnetic pole will be at the "top" of the source coil when a current passes through it (check with your instructor to make sure your coil is wired correctly). Close the knife switch, and verify with a known compass that your prediction is correct.

   3. Now place the detector coil on top of the source coil (as shown above). Create sketches of the coils to show the observed direction of the induced current and induced B in the detecting coil: i) the instant the switch is closed (current turned on); ii) the switch stays closed (steady current); iii) the instant the switch is opened (current turned off); and iv) the switch stays open (current off).

      One way to think about what is occurring is that turning the current on is like inserting a magnet into the detector coil. Turning the current off is like withdrawing a magnet from the detector coil.

   4. Now, answer this question: Why are your observations the same when the current is steady and when it is completely off? If you can answer this, then you truly understand the theory!

4. Did I Understand This? At the front table, you will find a coil and several magnets with tape covering the ends. Your instructor will ask you to determine the polarity for one of these unknown magnets without the aid of a compass. You will have to explain your answer; the instructors' initials will signify that you have mastered the Laws of Faraday and Lenz.

©	St. Lawrence University	Department of Physics

Revised: 25 Jun 2003

Canton, NY 13617