Magnetic Forces

The Force on a Current-Carrying Wire in a Magnetic Field * Current-carrying wires have magnetic fields and… * Magnets exert forces on other magnets. Therefore… Magnets exert forces on current-carrying wires. The magnitude of the magnetic force is given by F = iL x B Where B is the magnetic field, measured in Tesla, T i is the current, measured in amps L is the length of the wire, measured in meters. The current flowing through the wire and the magnetic field, B, must be perpendicular to each other. Only PERPENDICULAR components contribute to the magnetic force! Yes, we’re back to vector components!

Speaker and Magnets 1. There is a magnet in the back of most speakers. 2. The wires going into the back of a speaker carry current from the amplifier. 3. The coil of wire experiences a magnetic force which pushes it back a forth against the paper cone. 4. The cone vibrates producing sound.

James Clerk Maxwell Maxwell showed that electricity and magnetism were all part of the same phenomenon- the electromagnetic force! He wrote 4 very famous equations showing that relationship and describing electromagnetic waves

Maxwell’s four equations: Don’t these look like fun?

The Direction of the Magnetic Force, F = iL x B Force is a vector and always has a direction associated with it. The direction of the magnetic force is determined by a “right-hand rule” Point your thumb in the direction the current is flowing. Point your fingers in the direction of the magnetic field. Your palm will face the direction of the magnetic force on the wire.

A wire of length 2. 0 meters carries a current of 1 A wire of length 2.0 meters carries a current of 1.2 A and is at an angle of 30 degrees to a magnetic field of 0.008 T as shown above. What is the magnetic force on the wire? What is the direction of the force? F = iL x B = F = (1.2 A)(2.0 m) x 0.008 T sin 30˚ F = 0.0096 N Direction: use the right-hand rule… “perpendicular and down into the page”

Magnetic Fields created by current carrying wires If two parallel wires have currents traveling in opposite directions, the magnetic fields generated by those currents cause these wires to repel each other. However, if two parallel wires have currents traveling in the same direction, the magnetic fields generated by those currents between the wires results in the wires attracting each other. i, current F, force i, current F, force