L15. Lenz's Law:  Induced Current in a Coil

About this lab and your report:  You'll need to obtain information from the  video clip on Lenz's Law.  Download the clip and view it all the way through.  Then answer the numbered questions below.  Write your answers on paper to be faxed.  Simply list them in the order given.

Goal:  To determine whether the predictions of Lenz's Law are consistent with the direction of induced current

Preparation:  You should have already read through section 4 of Chapter 23 and have completed E.23.01.

Equipment (in the video clip):  microammeter, coil of wire (solenoid), cylindrical magnet, compass

Part A. Prediction

  1. Is the taped end of the cylindrical magnet a north pole or a south pole?  Tell how you know.

  2. The diagram below shows the cylindrical magnet and the coil.  Click here to open a page showing two stages, one with the magnet closer to the coil than the other.   Print the page. You'll be drawing on it and submitting it as part of your report.  Write N or S on the taped end of magnet according to your answer to 1.

  1. Draw the magnetic field lines of the cylindrical magnet in Stage 1.  Be sure to put directional arrows on the field lines.  Draw as accurately as you can, and draw field lines far enough out that they intersect the red end of the coil.

  2. Repeat your magnetic field drawing for Stage 2.  Draw the field as nearly identical to Stage 1 as you can (same number and spacing of lines), because you're going to make a comparison next.

  3. Compare the magnetic flux intercepted by the red end of the coil for the two stages.  Hopefully, your diagrams will make the difference in flux clear.  Tell specifically what is different that makes the flux different in the two stages.  Remember that flux depends on more than one thing.  In your answer, use the definition of magnetic flux.

  4. Use Lenz's Law to predict the direction of the induced field of the coil due to the magnet's motion toward the coil.   Explain your answer, making it clear how you use Lenz's Law.  This means that the phrase "oppose the change" must appear in your answer, among other things.

  5. Now draw the induced field of the coil for Stage 2 only.  Review section 22-7 as needed to refresh your memory about how to draw the field of a solenoid.  Be sure to show the directions of the field lines.

  6. What must be the direction of the induced current in the coil in order to produce the induced field that you drew?  Indicate the current direction by putting dots or crosses inside the small circles on the top and bottom of coil.  These represent the wire carrying current into or out of the paper.  See Figure 22-28 on p. 754 (p. 736 of 2nd ed) if you need to see how the dots and crosses are used.

Part B. Checking your prediction

  1. Now it's time to find out if your prediction of the direction of the induced current coincides with the actual current direction.  In the video clip, did the meter indicate positive or negative current when the taped end of the magnet was pushed into the coil?

  2. When a meter indicates positive current, which direction is the current moving?  In order to answer this, consider a circuit that you used in a previous experiment with a battery, a resistor, and an ammeter as shown below.  P and Q represent the terminals of the meter.  Positive current moves from the positive terminal of the battery to the ammeter, resistor, and back to the battery.  So positive current flows from P to Q inside the meter and from Q to P outside the meter.  In order for the meter to give a positive reading, should P be the positive (red) connection or the negative (black) connection to the meter?  (If you need to review how to connect an ammeter, see the multimeter tutorial.)

  1. In the video clip where the taped end of the magnet was pushed into the coil, which way did current initially flow in the meter:  red to black terminal or black to red terminal?

  2. Was the actual current direction in the coil consistent with your prediction from Part A?

Part C.  A follow up problem

  1. As the taped end of the magnet was being pushed in, the current first increased and then fell back to 0.  Faraday's law says that the induced emf is proportional to the rate of change of magnetic flux.  Since the emf appeared across the coil, which acts as a resistor, the induced current was the ratio of emf to the resistance of the coil (I = V/R).  Thus, the induced current, like the emf, was proportional to the rate of change of magnetic flux.  If the induced current was increasing or decreasing, then the rate of change of flux must also have been increasing or decreasing.  It's important to understand here that not only is the flux changing, but the rate at which the flux changes is also changing.  (The concept is analogous to that of acceleration, which is the rate at which velocity, the rate of change of displacement, changes.)   With all this in mind, explain why the current first increased and then decreased as the magnet was being pushed in.  Note that this relates only to when the magnet was being pushed in and not when it was being pulled out.  Watch the end of the clip again, observe the needle carefully, and note how it was moving as the magnet was being pushed in.