#modphys 20/180: Magnetic Fields

Today I took the bull by the horns and tried to develop a model of the magnetic field with my grade 12 physics class. I was excited about this, since my students typically have trouble visualizing magnetic fields, and we often resort to mnenonics such as blindly applying the Right Hand Rule.

We were able to get quite a bit out of simple set-up. A 40 cm vertical wire was supplied with 0 to 2 A of current. I used a rheostat as a resistor and a multimeter to measure the current.


1. On a piece of cardstock, we used a small compass to find the direction of the magnetic field near the wire by comparing (vector subtracting) the directions of the compass arrow in the background magnetic field of Earth, and the direction othe arrow in the additional field generated by the wire. This is well-explained in the E&M section of the modeling materials.

2. We repeated this on two more pieces of cardstock. This established the form of the magnetic field near a straight wire. By looking at the negative and positive terminals of the power supply, we estblished the straight wire Right Hand Rule. We switched the leads and saw that the RHR still held.

3. We rolled the wire into a single coil and talked about the structure of the magnetic field that would result. We checked that prediction and then saw that the RHR from the straight wire turns into the RHR for a coil.

4. I set out some small whiteboards and we used the compass to determine the direction of the magnetic field near a solenoid. This led to drawing field lines, and at this point we agreed on some conventions for drawing field diagrams for magnetic fields, and went back to draw them for coils and wires.

5. I held couple strong magnets on opposite sides of the vertical wire. With the current switched on, the wire moves. We pretended the force resisting the movement was spring-like, so the displacement was proportional to the force. Then, we varied the current to find that F = kI.

6. By holding lines of magnets beside the wire, we were sort of able to convince ourselves that F = kL. We ran out of time, so I asserted the other two dependencies, giving us F=ILBsinx.

Pretty successful! Here’s a better view of the field around solenoid.



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