# Lenz' Law Lab

Created by

#### Julie Bresowar

PROJECT PHYSLab '99 Palo Alto

#### Equipment

1 air track with air supply

1 air track glider with four masses attached

1 set of neodynium magnets

1 regular pulley, 1 smart pulley

Vernier ULI interface with LoggerPro program

1 50 g mass hanger

string

extra masses (for extension)

#### Background Information

A changing magnetic field produces an electric current, called an induced current, which is a phenomenon known as electromagnetic induction. Passing a magnet over a conductor induces a current in the conductor, and that current sets up a magnetic field that opposes the motion of the magnet. This is known as Lenz's law, and the drag force that is created is Lenz's Force. The magnitude of Lenz's force depends on the strength of the magnet used and the speed of the magnet over the conductor. This lab will study the relationship between the size of Lenz's force and the speed of the magnet over the conductor.

To study this relationship, neodynium magnets attached to an airtrack glider will be passed over an aluminum airtrack. A constant force will be supplied to pull the glider along the airtrack. Using a smart pulley to collect data, the decrease in the acceleration will be studied as a function of the increase in velocity of the glider.

#### Experimental Setup

Setup the equipement as shown below. The string should be long enough so that the glider can be pulled back at least 120 cm on the airtrack and so that the mass hanger does not hit the ground. You should place a stopper on the airtrack close to the pulley end so that the glider stops before it slams into the pulley.

The smart pulley should be connected to port 2 of the ULI Interface.

#### Procedure

1. After setting up the equipment, check to make sure that the glider can be run at least 1 meter on the airtrack and that the glider will be stopped before the mass hanger hits the ground. Make a mark on the airtrack at the starting position of the glider, so that it will be released from the same place for each run.

2. Pull up the program called Lenz's Law using Logger Pro. You should see graphs of distance, velocity and acceleration vs. time.

3. Turn on the air supply, pull back the glider to the starting position and collect one run. When you press the collect button, wait to release the glider until you see the data being collected.

4. After collecting one run, select from the data the portion of the graph that corresponds to the forward motion of the glider. You will only want to select the data from the velocity column and the accleration column. Copy this data using the copy command.

5. Open up a spreadsheet on the computer and transfer the copied data into the spreadsheet. If you are using excel, highlight the copied data and then click on the graph icon in the toolbar. Give the graph an appropriate title and make sure that velocity is plotted on the x-axis and acceleration is plotted on the y-axis.

#### Analysis

1. Looking at the accleration vs. time graph in the Loggerpro program, what happens to the acceleration of the glider as the glider moves forward?

2. How does this acceleration trend compare to the accleration of the glider without any magnets attached (you will have to figure out what the acceleration would look like without the Lenz Force effect)? Why are the two different?

Make a sketch below of the acceleration with magnets and the acceleration without magnets.

3. Looking that the acceleration vs. velocity graph, what relationship do you see?

Using the spreadsheet capabilites, add a trendline that best fits the data. What is the formula for your data?

4. Using what you know about forces and accleration vs. velocity for Lenz's law, describe the relationship between Lenz's law Force and velocity of the magnets?

#### Extensions

1. Using the Lenz's law program, make several runs using different masses on the mass hanger. Plot acceleration vs. velocity for each run and compare the slope of each run. Is this number constant or does it vary? Is there a pattern to the varience, and if so, explain why?

2. Using what you have learned about Lenz's law, explain how this phenomenon can be used in amusement parks rides? Go to the library and look up other uses for Lenz's law and present one use to the class.

Sample Data for Lenz's Law Lab

After one run of data using the 50 g mass hanger, the graphs for distance, velocity and acceleration vs. time are below. Note that the only portion of the graph that is of interest for this lab is when the glider is moving forward.

A plot of acceleration vs. velocity for the same run is shown below. A linear trendline has been added and the regression equation is also shown.

#### Notes For The Teacher

Just a couple of notes to help the lab run smoother.

In order to run this lab as I have set up, I am assuming that the teacher has a working knowledge of the ULI and the LoggerPro program.

1. To create the Lenz Law program, open Logger Pro and have the ULI scan with the smart pulley in port 2. If the ULI recognizes a smart pulley, then the variable called position (in units of revolutions) will appear on the graph and in the data table. Create three new columns in the data table called Distance, Velocity, and Acceleration. Their definitions are as follows:

Distance = position*2*pi*(radius of pulley wheel in meters)

Velocity = derivative of distance

Acceleration = derivative of velocity

After creating those three columns, go to graph options and select the three pane graph layout. The top graph should be distance vs. time. The second graph should be velocity vs. time and the third graph should be accleration vs. time. You will probably have to change some of the graphs so that they are displaying the proper variables. The time for a single run should be about 2.5 seconds. The program should autoscale the y-axis for each of the graphs (if not, each y-axis range can be changed manually).

At this point the program is ready to run. After taking a data run, have the students select the data values that correspond to the forward motion of the glider and then copy these values (velocity and acceleration only) into a spreadsheet so that they can be graphed. It is possible to graph accleration vs. velocity using the LoggerPro program, but their are extra data points (corresponding to the time before the glider was released and the time after it reaches the stopper) on the graph and it can be confusing if you don't know what you are looking for.

2. A couple of layers of masking tape works as an adequate stopper when taped onto the air track at the place where you want the glider to stop.

3. In order to get better data, I found that you should add some mass to the glider so that it does not move so quickly along the track.

### Other Lenz Law Demonstrations

http://fermi.bgsu.edu/~ptak/resource/demos/electricity.html

http://www.ph.utexas.edu/~tai/weeklies/2-28.html

http://www.physics.odu.edu/htmlstuf/physicspage/lenz.htm