Magnetic Fields
Materials
2 each bar magnets
iron filings (contained on paper plates)
1 each small solenoid with iron rod
1 each DC power supply
2 each wire leads
large sheet of plain white paper
1 each track post (spacer)
2 each clear magnet boxes (raisers)
1 each metric ruler
1 each computer
Goals
1) To visualize the magnetic fields produced by several different configurations of simple bar magnets using iron filings.
2) To understand magnetic field properties (direction, relative strength, null points)
3) To graph and understand the relationship between field strength and location with bar magnets and solenoids with variable number of coils
Introduction
A magnetic field exerts forces on a compass needle such that the needle tends to align itself with the direction of the field. If the magnetic field is strong enough and additional non-magnetic forces (gravity, etc.) are negligible, then the compass needle points for all practical purposes in the direction of the field.
In this lab the magnetic fields surrounding bar magnets are mapped out using a compass and iron filings.
The end of your compass needle that points toward the magnetic pole of the Earth in the northern hemisphere (when it is far away from any magnet or other magnetic material) is by definition an N pole.
Then we must conclude that the earth’s magnetic pole in northern Canada is actually an S pole, since the N pole of the compass points to it and unlike poles attract. The N pole of the compass needle points toward the S pole of your magnet.
The magnetic poles of all magnets can thus be labeled by means of a compass and the definition of an N pole as just stated.
Exercise 1:
Observing Magnetic Fields with Iron Filings
In the presence of a magnetic field, iron filings act like many small compass needles. By spreading them out on the paper above the magnet a “picture” of the magnetic field is produced. At your lab station you have paper plates with iron filings.
The bar magnets can be placed under the paper plates to demonstrate magnetic field lines.
Warning – Please do not pick up iron filings with the magnet. The filings are very difficult to remove from the magnet. Leave the iron filings on the paper plates. Your lab technician is responsible for collection of the iron filings after the lab.
(1) Draw a full-scale outline of the bar magnet on fresh piece of paper and label the N and S poles. Place the bar magnet on the lab counter and cover it with a paper plate which contains iron filings.
Gently tap the paper plate, and you’ll see the iron filings lining up along the magnetic field lines. Now on the first sheet of paper with the outline of the bar magnet already drawn make a careful free hand sketch of the magnetic field lines as shown by the iron filings.
Each line should start at a point on the edge of the magnet and smoothly flow to another point on the edge of the magnet. On your sketch include the direction of the field lines by means of arrows. By convention the field lines go from the N pole to the S pole outside the magnet itself. Each person in your lab group is expected to draw her/his own sketch.
(2) Repeat this process with the paper plate raised above the magnet by several millimeters (using e.g. pencils, chalk, notebook, etc.).
(3) Next try the following configurations of bar magnets (one-fourth of the class does each case). In each case sketch the magnetic field lines and indicate the direction of the field lines everywhere on your sketch.
(a) Place two bar magnets end to end with like poles several centimeters apart.
(b) Place two bar magnets end to with unlike poles several centimeters apart.
(c) Place two bar magnets side by side several cm apart with like poles near each other.
(d) Place two bar magnets side by side several cm apart with unlike poles near each other.
(4) Finally, use the small solenoid instead of the permanent magnet. Place the iron rod inside the coil and place the small coil inside the large coil (which functions only as a stand in this experiment). Connect the red and black wires from the DC power supply to the small coil. Start with the Current knob set to zero.
Turn the Voltage knob to the 12 o’clock position. Dial up the Current knob to approximately 5 amps (Note: the coil will become warm to the touch after awhile.)
Place the paper plate on top of the large coil, sprinkle it with iron filings, and sketch the magnetic field. Use a compass to determine the direction of the field.
TURN OFF THE POWER SUPPLY WHEN YOU ARE FINISHED.
(5) Data analysis/Conclusions.
(a) Describe the general characteristics of the field that you observe.
(b) On your sketches label by some means the regions where the magnetic field is strongest and weakest for each configuration. Note: the iron filings are not only oriented by a strong magnetic field, they are also attracted to the region of stronger field.
Are there any regions where the iron filings are attracted to the magnet?
Are there any points where the field is essentially zero? Identify these locations clearly as well. Be sure to include the reasoning behind your answers.
(c) Can you find any places where the magnetic field lines ever cross? If there were a spatial point where two field lines crossed, what would the direction of the field be at that point?
If there are fields from two sources present at some point in space, for instance the magnetic fields of Earth and the bar magnet, will some iron filings feel forces from one field and other filings feel forces from the other field, or will all filings feel forces from both fields simultaneously? Discuss/explain.
Are there any regions on the map that the field lines seem to avoid? What is the magnetic field at these points? Explain your reasoning. How many such points are there on your map?
Exercise 2:
Simulating Fields, Determining Field Strength and Distance
1. The set-up
a. Google search: physics applets Colorado, and click on the PHET link
b. Select “Electricity, Magnets & Circuits”
c. Select “ Magnets& Electromagnets” and run the Java program (Maximize the window)
2. Bar Magnet
a. On the tabs at the top of the screen select “BAR MAGNET”
b. On the right hand side menu set Strength to 100%, deselect Show Compass, select Field Meter
- Move the bar magnet all the way to the left of the screen.
- Move the Field Meter to 1cm away from the N end of the magnet (measured with a ruler taped to your computer screen)
- Make sure the By is as close to zero as possible, then record the total magnetic field strength (B) at that point.
- Move the Field Meter another 1cm away and repeat the process.
- Build a data table of Distance (in Meters) and Field Strength (in Gauss) for distances of 1cm to 20cm.
- Graph Field Strength (y-axis) vs Distance (x-axis) [North Pole Data]
- Move the bar magnet all the way to the right of the screen.
- Move the Field Meter to 1cm away from the S end of the magnet (measured with a ruler taped to your computer screen)
- Make sure the By is as close to zero as possible, then record the total magnetic field strength (B) at that point.
- Move the Field Meter another 1cm away and repeat the process.
- Build a data table of Distance (in Meters) and Field Strength (in Gauss) for distances of 1cm to 20cm.
- Graph Field Strength (y-axis) vs Distance (x-axis) [South Pole Data]
For both graphs add the trend-lines that best fit the data with appropriate equations and R2 value. Give a %Difference showing how well your trendline fits the data. What is the name for the type of relationship between Field strength and Distance?
- Electromagnet (solenoid)
a. On the tabs at the top of the screen select “Electromagnet”
b. On the right hand side menu set # of coils to 4, deselect Show Compass, select Field Meter, deselect Show Electrons
- Move the electromagnet all the way to the left of the screen.
- Move the Field Meter to 1cm away from the open end of the coils (measured with a ruler taped to your computer screen)
- Make sure the By is as close to zero as possible, then record the total magnetic field strength (B) at that point.
- Move the Field Meter another 1cm away and repeat the process.
- Build a data table of Distance (in Meters) and Field Strength (in Gauss) for distances of 1cm to 20cm. [4 Coil Data]
- Set the # of coils to 3
- Repeat d-f
- Build a data table of Distance (in Meters) and Field Strength (in Gauss) for distances of 1cm to 20cm. [3 Coil Data]
- Set the # of coils to 2
- Repeat d-f
- Build a data table of Distance (in Meters) and Field Strength (in Gauss) for distances of 1cm to 20cm. [2 Coil Data]
- Set the # of coils to 1
- Repeat d-f
- Build a data table of Distance (in Meters) and Field Strength (in Gauss) for distances of 1cm to 20cm. [1 Coil Data]
Graph all 4 sets of data on one graph of Field Strength (y-axis) vs Distance (x-axis). Add the trend-lines that best fit the data with appropriate equations and R2 value. Give a %Diff showing how well your trendline fits the data. What is the name for the type of relationship between Field strength and Distance?
- Measure the field strength of the 4, 3, 2, and 1 coil electromagnets right at the opening of the coil (0cm away) then answer the following questions
What is the relationship between number of coils and Field Strength? If we increased the number of coils to 5 how strong would the magnetic field be 0 cm away from the coil? Does the data from your graph of the coil data support these claims? Why or why not.
Summary and Conclusions
Summarize your results and make any final conclusions.