University of Virginia
Physics Department

The Magnetic Field

A Physical Science Activity


Adapted from Experiment 27: Magnetic Field Explorations in Physical Science with CBL published by Venier Software

2003 Virginia SOLs

 Objectives

Students will

Motivation for Learning

Discrepant Event: Magnetic Bar World

Materials:

Procedure
  1. The goal of this motivation is to show that the Earth is a big magnet, with the geographic North Pole and the geographic South Pole being its magnetic ends. You can have the students gather around you as this is hard to see.
  2. Show the students the compass with the magnet removed so that it does not affect the compass. Keep the compass away from metal (including jewelry, belt buckles, etc) and note that the needle points in the north direction. Tell the class about the Earth being a large magnet.
  3. Tell the students you will prove it to them by pretending the smaller bar magnet is the Earth. When you place the magnet underneath the compass (just as the earth's magnetic core is underneath us), the needle will point in the direction of the north end of the magnet.


  1. Rotate the magnet around the bar magnet to show that the needle always points in the direction of the North Pole of the magnet.

Demonstration 2: Creating a Compass

Materials:


 

Procedure

  1. Turn the needle into a magnet by stroking it along the surface of the magnet (it doesn't matter which direction). Always stroke in the same direction and do it 60 times. You will need to do this every time you do the experiment, because the needle will not retain its magnetism very long.
  2. Carefully push the needle into the end of the cork or sponge.
  3. Place the cork into the water and try to make it be at rest. Wait until it stops moving, and look at the way it points
  4. Turn the glass around and show that it points in the same direction. This does not happen quickly.
  5. Show that the "real" compass points in the same direction (or in the opposite; you may have produced a south pole).
  6. Show that by placing the magnet close to the needle, the needle will spin towards the magnet, just like the "real" compass did.


Background Information

Magnets are abundant in nature. Lodestone was apparently known to the Chinese over a thousand years ago as a magnetic device, but it is not clear they used them for navigation like the Europeans did later. The Earth is mostly made up of metal. We live on a thin shell that has cooled off. But in the center there is iron and other metallic elements moving slowly around. The center of the Earth is very hot, and the heat allows the metals to stay molten and to flow. This flowing of the charged particles in the metals and rock creates a magnetic field, turning the whole Earth into a big magnet.

The Earth's magnetic field magnetizes certain rocks on the surface as well. If a rock that contains iron is located in a strong part of the Earth's magnetic field, it often becomes magnetized, just like we did when creating the compass by stroking the needle.

A compass is just a magnet that can move easily. It lines up with the Earth's magnetic field, which means that one side points in the direction of the geographic North Pole of the Earth. Note that if the north pole of a magnet is attracted to the geographic North Pole of the Earth, then the geographic North Pole must be the south pole of a magnet. Confusing huh? A compass is not special at all; we can create one by placing a magnetized piece of metal on a cork that is free to rotate, as we did in the Motivation.

One question that might be asked is, since the iron inside the Earth is so big and massive, why doesn't it overpower the magnetic field created by the magnet? In this activity the students will be exploring the size of magnetic fields using the CBL Magnetic Field Detector and will learn that the magnitude of magnetic field depends on distance from its source.

One useful fact is that bar magnets lose their strength over time. Many bar magnets that are purchased commercially may not be as effective as you might expect. We have found that ceramic magnets are inexpensive and strong. A good source of them is:

The Magnet Source Miami Magnet Company 6073 NW 167 St, Suite C - 26 Miami, FL 33015 800-222-7846 www.magnetsource.com

The magnets we use are Part No. CB41-IP and can be purchased in quantities of 25 for $0.27 each, 50 for $0.20 each, and 100 for $0.18 each. There is a minimum order, so you probably want to buy in quantity. Note: these magnets are polarized from top to bottom, so the best readings are given if you stand the magnet on its end and orient to give positive readings. Also the best data is given if you use 2 or 3 magnets stacked together.

Note about this student activity: the magnetic field sensor may need to be calibrated, which the teacher should do before the activity. Directions for calibration come in the box with the Vernier sensor. There are several situations that may contribute to a magnetic field requiring zeroing or calibration of the sensor each time. If something nearby contains iron, there will be a magnetic field associated with it. For example, you may need to be careful in this activity with the stand holding the sensor; this field can be avoided by using a longer clamp to hold the sensor. If a table has a metal frame underneath, it may be necessary to do the experiment on top of books on the table to avoid that field.

Also instead of drawing the graph of the magnetic field vs. distance, the TI graphing calculators can be linked to a computer through the TI-Graph Link Cable and the TI-Graph Link software. The graph can then be displayed on a computer screen and also printed. Directions for doing this are in the procedure.

 

Student Activity

To print out the Student Copy only, click here.

Materials

Procedure

  1. Tape the meter stick to the top of the table so that it doesn't move easily.
  2. Set up the stand and clamp so that the magnetic field sensor end is at the 0 marker on the ruler. The small white spot on the end of the sensor should be facing the magnet.
  3. Connect the Vernier magnetic field sensor to the adapter cable and then into the CBL unit in CH 1. Link the CBL unit and the TI graphing calculator by connecting the black link cable into the bottom of both the CBL unit and the calculator.
  4. Prepare the CBL / sensor / calculator for data collection by entering into the PHYSICS program and setting up the magnetic field probe. Select SETUP PROBES, then ONE probe. To set the type of probe, select MORE and then under the new menu choose MAGNETIC FIELD. Use the USE STORED in the calibration menu. Select LOW (GAUSS) from the magnetic field setting menu and make sure that the detector's switch is set to LOW.
  5. Select ZERO PROBES from this menu, CHANNEL 1 and then press TRIGGER on the CBL to set the zero of the probe.
  6. From the main menu, select COLLECT DATA and use the TRIGGER/PROMPT feature. This will allow you to move the magnet, record a reading, then enter in how far away it was.
  7. Start by placing the magnet 4-cm away from the magnetic field sensor by placing it on the ruler. Find the orientation of the magnet that gives the highest field reading. This will mean the polarization is along the meter stick. Rotate the magnet along the meter stick so that the pole facing the sensor gives a positive reading. After each measurement select MORE DATA and then slide the magnet back 2 cm and take the new measurement. When prompted for data entry on the calculator, enter in the correct distance for each measurement you take.
  8. Select STOP and GRAPH from the menu to see a plot of magnetic field vs. distance. Draw the plot that you see on the calculator screen after you have finished collecting data.
  9. To print the graph using TI-Graph Link instead:
    1. Disconnect the calculator from the CBL unit while the graph is displayed on the screen of the calculator.
    2. Connect the gray TI-Graph Link Cable to the computer's serial port and then to the calculator's link port at the base of the calculator.
    3. Start the TI-Graph Link Software and select LINK from the menu at the top of the screen.
    4. Choose GET SCREEN from the drop down menu and then when the box comes up choose GET SCREEN again.
    5. Select PRINT.
    6. Reconnect the TI calculator to the CBL unit.
  10. Fill in the data table by using the left and right arrow buttons to trace from point to point along the graph. Read the magnetic field for each point from the bottom of the screen.

Data Sheet

To print out the Data Sheet only, click here.

Distance (cm)
4
6
8
10
12
14
Magnetic Field (Gauss)            

Graph your data here:

Questions:

  1. What happens to the magnetic field strength as distance increases?




  2. What would the magnetic field strength be at 7 cm?




  3. What would the magnetic field strength be at 16 cm?




  4. How does this help explain why the magnetic field of the earth does not overpower the magnetic field of the bar magnet when we hold a compass to it?  




Answers to Questions:

  1. The magnetic field strength decreases rapidly as the distance increases. This means that if you double the distance, the field doesn't just drop by two. It drops by much more than that. As the magnet gets farther from the sensor, the field decreases by less and less.
  2. At 7 cm, the field strength should be much closer to the value at 8 cm than the 6 cm value because the field strength is dropping more quickly from 6cm to 7cm than it is from 7cm to 8cm.
  3. At 16 cm, the field strength should drop exponentially again. A rough estimate would be to see the percent drop from 12 to 14, then use that percent drop from 14 to 16.
  4. This phenomenon can be explained by this rapid decrease in the field. Since the Earth's magnetic material is so far away, it becomes very weak compared to a small magnet up close. You can imagine how much the field strength would be if we were closer to the center of the Earth! The magnetic field of the earth is 1 Gauss at the earth's surface.

The Drawing should resemble this:

Students with Special Needs

All students should be able to participate in this activity.

Click here for further information on laboratories with students with special needs.

 

Assessment

Data sheet and Questions to be completed during the laboratory.