Tag Archives: magnetic field

#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.

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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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#modphys 18/180: Magnetic Field

I’ve been using the Electricity & Magnetism modeling materials with my grade 12 physics class, pulling out segments and re-arranging models to fit our fields-based approach. Today I introduced the magnetic field using the paradigm lab(s).

One conceptual difficulty we hit (not for the first time!) is the conceptual hurdle involved in using  quantity that is proportional to the quantity we are trying to measure. Here, we measured the angle between north and the direction a compass points near a current-carrying wire. Then, through a trigonometric argument, we used the tangent of that angle as a stand-in for the strength of the magnetic field caused by the wire at that point. Thus, our linearized graph eventually showed that tan(angle) is directly proportional to the inverse of the distance.

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I’m not sure there is a better way to do this paradigm lab, but we did play around a bit with ways to visualize the sum-of-two-magnetic-field-force-vectors.

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We found a bag of tiny compasses, which seemed to work pretty well (although they sometimes needed a tap to remind them of their profession!). Here, they are aligned with no current flowing through the wire. I accidentally deleted my photo of the compasses with current flowing — I’ll try to recreate it next week.

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I think next week we’ll work with the apparatus again, to try to eek out F=qvB (although, more usefully but I think less conceptually clear, we’ll probably have to aim for F=LiB).