Interactive Lecture Demonstration 10 – Lenz's Law

In this activity, you will push and pull a bar magnet in and out of a coil of wire, thereby changing the magnetic flux through the coil. According to Faraday's Law, an emf (and therefore a current) will be induced in the coil. According to Lenz's Law, the magnetic field produced by the induced current will always oppose the change in magnetic flux that caused the induced current. By measuring the directions of the induced currents in the coil with a galvanometer, you will test Lenz's Law. 

Every pair of students will need: galvanometer, coil of wire, bar magnet, compass, clip leads

1.      Orient your coil so that its terminals are facing you. Observe how the wire is coiled. Underline the word that correctly completes the following sentence: If current enters the left terminal of the coil, it will move (clockwise, counterclockwise) around the coil. Sketch the two terminals and several turns of the coil on the cylinder below. 

2.      Check your bar magnet with a compass (check the compass first!) to make sure you know which pole is really N.

3.      Now let's review how to measure current with a galvanometer.  Make a series circuit of a battery,  resistor, and the meter.  Connect the meter so that the current reads positive.  Underline the correct words that complete the sentence below:

A positive reading on the meter means that conventional (+) current enters the meter at the (positive, negative) terminal and exits the meter at the (negative, positive) terminal.

4.      Connect the positive lead of the galvanometer to the right terminal of the coil and the negative lead to the left terminal.  Now you're going to push the magnet into the coil.  You'll push the N pole into the coil as shown in the diagram above. Get ready to read the meter.  When you push the magnet into the coil, you'll get a small change in the reading.  You don't have to record the number but be ready to note whether the reading goes positive or negative.  Now push the magnet in quickly and stop. Record in the table on the next page whether the reading was positive or negative.

 

5.      Now you need to determine the direction of the induced current in the coil.  You'll need to review what you found in step 3.  Show the direction of the induced current on your coil diagram above.  Tell briefly below how you found the direction.

6.      Given the direction of the induced current, determine where the N pole of the coil's induced field is located.  Label this pole on your diagram.  Describe briefly how you determined which pole was N.

7.      Is your result for the direction of the induced current consistent with Lenz's Law ? Explain.

8.      Now do the following tests, each time determining whether the meter reading goes positive or negative and which end of the coil (left or right) is the N pole of the induced field. In each case, pull or push the magnet in or out of the side of the coil that has the terminals. In each case, predict first (using Lenz's Law) the direction you thin the induced current will be. Record your predictions and actual results in the table below.

a.       With the magnet inside the coil, pull the magnet out quickly with the N pole coming out last.

b.      Now push the S pole in quickly.

c.       Finally, pull the magnet out (quickly, of course) with the S pole coming out last.