Fall 2001
Today's Lecture (10/8):Summary:
- A current loop in a magnetic field has a torque, just like a magnet would. This would suggest that maybe a current creates a magnetic field. It turns out there is a magnetic field that is wrapped around the wire.
- There are two ways to calculate the magnetic field created by a current. One method, called the Biot-Savart Law, is analogous to Coulomb's Law. The other method, which we will do later, is similar to Gauss's Law.
- According to the Biot-Savart Law, the contribution to the field, dB, at a point, r, caused by a current, i, on a bit of wire, ds, is given by the following formula.
- The law is difficult to use for most situations, but it does provide some useful insight. First of all, it gives us another right hand rule, namely, that if you curl your fingers around the wire with your thumb in the direction of the current, then your fingers point in the direction of the field.
- Two common situations where you want the magnetic field are straight wires and curved wires. If you know the fields for these two situations, you can use superposition to add them as vectors for more complicated situations. The formula for the magnetic field near a straight wire is
and for the point at the center of a curved arc, the formula is
.
Conceptests:
Assignments:Next Reading Assignment (due 10/10)
Next Homework (due 10/10)
Diversion-of-the-week:There are no end to the interesting physics experiments that you can find on the web. Here's another one involving a cd and a microwave oven. I can't guarantee this is safe, so you may just want to watch the video clip.
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