To see how currents running through wires can create magnetic fields, and how the strength of those magnetic fields depends on distance from the wire.
Even if you don't have a magnet, you can still create a magnetic field. The current passing through a wire will create a magnetic field near the wire. Ampere's Law can help you to figure out the strength of such a field in some circumstances.
Weigh down the long patch cord with tape, books or other items so that it won't move around on the table.
Turn on the power supply. Because the patch cords provide only a very small resistance, a large current will run through them. The power supply's built-in current limiter should activate -- you'll see the red bulb above "C.C." light up -- preventing the current from rising too high. Set the toggle button on the front of the power supply to the "Low" setting. Use the ammeter display on the power supply to read the current running through the wire. Adjust the power supply so that the current running through the wire is as close to 3 Amps as you can make it; do not go over 3 Amps.
Place the probe so that it lies parallel to the straight section of the wire. Twist the probe so that the white dot is pointing straight up, as shown. It may help to have one person look at the probe end-on in order to determine when the flat portion of the probe tip is exactly horizontal.
When the current is flowing, it will create a magnetic field around the wire. Which way will the magnetic field point? Indicate the direction of the magnetic field near the position of the probe in the figure above.
Measure the magnetic field when the power supply is turned off; you are detecting a "background" consisting of the Earth's magnetic field as well as components generated by electrical appliances in the room. Then turn the power supply on and measure the field again; your probe now detects the "background" plus the field generated by the current running through the wire. The difference between the two readings is the magnetic field created by the current in the wire alone.
In order to determine only the magnetic field produced by the wire, and not the changes in the ambient magnetic field, you need to make a differential measurement like so:
Start with the probe on the side of the wire away from the power supply. Place it a distance of r = 12 cm and measure the field strength B. Write down your results. Then acquire more measurements at distances r of 10, 8, 6, 4 cm.
Now, actually move the probe to the other side of the wire. Make sure that the white dot still faces straight up, and make the measurement. Was your prediction correct? If not, can you explain what changed as you moved the probe from one side of the wire to the other? Perhaps it would help to draw the direction of the magnetic field on both sides of the wire in the diagram above....
Make measurements on this other side of the wire at the same set of distances: 4, 6, 8, 10, 12 cm.
Start by drawing axes on the graph paper provided. Place "Distance from the wire" on the horizontal axis, and "Strength of magnetic field" on the vertical axis. Use the absolute value of the magnetic field strength, so that all your measurements become positive numbers; do the same for your distances, too, if you recorded distances on the two sides of the wire with different signs. Figure out how large you'll have to draw each axis to contain all of your measurements, plus leave a little room for clarity.
Plot all your measurements on this graph. You should have two values at each distance, one from each side of the wire.
Now, how can we compare the models to the data on this graph? We will normalize all three mathematical functions,
B = k1 / r^2 B = k2 / r - r B = k3 * eso that they match the graph exactly at one point, somewhere in the middle of the range. We'll use r = 4 cm as the point in common. Calculate the average value of your magnetic field strength at this distance. For each of the three functions,
If one of the mathematical models is a good match to reality, then it ought to follow the actual measured values pretty closely. All of the lines will run through the data at r = 4 cm, but which of them does the best job of matching the data at other distances?
The equation in your textbook includes a number of factors which your empirical function lumped together as a single constant (k1 or k2 or k3). In a perfect universe, those factors would exactly equal your empirical constant; in other words, if you plugged into the textbook equation the values for the current through the wire, etc., you'd end up with exactly the magnetic field strength you actually measured.
Last modified 5/13/2004 by MWR