AP Physics

The Laws Of Faraday And Lenz


The purpose of this experiment is to observe the current caused by an Emf induced by a changing magnetic field, and verify Faraday's Law and Lenz' law. Faraday's Law relates the induced Emf of a changing magnetic field.  Lenz's Law is apparent in the negative sign of Faraday's equation and can be stated as follows: an induced current produces a magnetic field that opposes the change in magnetic flux that originally produced it.

Materials: voltage source, 100 ohm resistor, wires, coil, magnet, compass, galvanometer, switch


I. Correlate direction of current through the galvanometer with motion of the needle.

1. Connect a 100 ohm resistor in series with the galvanometer to allow a small current to pass through the galvanometer. Create a sketch that shows which terminal the current enters into the meter when the needle points in each direction (the + red and - black signs on the meter do not indicate direction). Try reversing the direction of the current through the meter to confirm your observation.

II. Testing the compass and magnet polarity.

1. It is not atypical for a magnet or compass purchased for laboratory work to be unmarked or incorrectly marked for its polarity. It is a good idea to check the polarity of your magnet and compass. Connect the solenoid to the voltage source (use 10-15 V) and examine its coiling direction. Use the right hand rule to predict the north and south poles of the solenoid, then confirm the poles on your compass and magnet using the solenoid's N/S polarity.

III. The induced current due to a permanent magnet.

1. Eliminate the power supply and connect the solenoid to the galvanometer as shown at the right. Move the magnet as indicated (i.e., north pole towards the solenoid) and observe the galvanometer deflection.

Sketch the direction of the galvanometer and the direction of the 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 four sketches. Your observations should agree with predictions using the right-hand rule!

IV. The induced current due to an electromagnet.

1. Working with another group, connect one of the coils and a switch to the dc power source and lay it sideways on the table. This will become the source coil. Connect the other 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. Close the switch, and verify with the compass that your prediction is correct.

3. Place the source coil upright on the table, then place the detector coil on top of the source coil. Observe the direction of the induced current in the detecting coil when:

i) The current is turned on;

ii) The current is steady;

iii) The current is turned 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. Reverse the direction of the detecting coil and repeat the experiment.

Be sure to think about (and answer) this question: Why are your observations the same when the current is steady and when it is completely off? What causes north/south polarity in a coil?

Summarize the Laws of Faraday and Lenz as it relates to your observations in this lab. Discuss the terms in Equation 1 as part of your conclusion.

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