21.2

Magnetic Filed Produced by Long Straight Current-Carrying Wire

Student Notes

With this simulation, you can investigate several topics dealing with magnetic fields, specifically magnetic fields created by currents in long straight wires. Here you are viewing two wires head on, as they carry current either out of the page toward you or into the page away from you.
Here are some things to consider. The default settings, with one current set to zero and the field vectors off, allow you to focus on the magnetic field from a single current-carrying wire. The direction of the magnetic field lines can be found using your right-hand; if you point your thumb in the direction of the current and curl your fingers, your fingers show the direction of the magnetic field. The direction of all the field lines is shown by the simulation on the outer field line - using your right hand, you should be able to determine which current direction the simulation takes to be positive. When the current is positive, is it directed out of the page or into the page? What else indicates the direction of the current in the wire? Is magnetic field a vector or a scalar? To test this, with one current set to zero and the other at 2 A, use the mouse to measure the magnetic field at a point between the wires. Now reverse the current settings, setting the first one to zero and making the second one 2 A. What is the field at the point now? Measure the field a third time with both currents set to 2 A - is the field there simply the sum of the original two field readings? Turning on the field vectors to show the net field should help you see how the fields from the two wires are combined. When there are currents in both wires, the wires exert forces on one another. The wires will either attract or repel, depending on how you set the currents. Under what conditions do the wires attract one another? Under what conditions do the wires repel? You can also use your right hand to determine the direction of the force exerted by one wire on another. Wire 2, for instance, feels a force because of the magnetic field generated by wire 1. Note the direction of the field generated by wire 1 to the right of wire 1, where wire 2 is. Hold your right hand so the fingers point in the direction of the current in wire 2, and your palm points in the direction of the magnetic field generated by wire 1. If your thumb is 90 degrees to your fingers, it should point in the direction of the force experienced by wire 2. Does it?

Self-test

Consider the point exactly halfway between the two wires. Can you adjust the current so that, with current passing through each of the wires, the net field is zero here? Note the distance between the field lines from one of the wires. Are they equally spaced? Explain this. With currents of different magnitudes passing through the two wires, how do the forces on the two wires compare? What is the reason for this?

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