Friday, May 9, 2008

Answered one question on the electric circuit problem sheet 1.

Introduction to magnetism.

Showed that a magnet exerts a stronger force on the paper clips than the Earth.
Dick Tracy - He who controls magnetism controls the universe.

Sprinkled iron filings on paper over magnets (used simple bar magnet and horseshoe magnet under plastic crate). Showed patterns. Just as electric charges affect space, so do magnets. The filings align along magnetic field lines. Talked about North and South poles of magnetics. Opposite poles attract, like poles repel.

Showed maps of declination for US and world. Declination is the deviation between magnetic north and geographical north. Note, since the north pole of a compass points north and opposite poles attract, the magnetic north is actually a south magnetic pole. Also mentioned magnetic dip.

Showed with demo of wire and compasses that currents create a magnetic field. Showed Right Hand rule for current carrying wires. Point the thumb of the right hand in the direction of conventional current and the fingers curl in the direction of the magnetic field. Magnetic field lines are circles around the current carrying wire. The magnetic field = uo * I/(2 * pi * r).

k = 1/(4 * pi* eo)
Students calculated sqrt (1/(uo *eo) to get c.

Currents create magnetic fields. Changing magnetic fields in coils of wires create currents. Demonstrated with wires and galvanometer with horseshoe magnet and then more dramatically with the magnets in the coil of wire. Therefore changing magnetic fields create currents.

Showed using TV that magnets affect charges. Showed using oscilloscope that when you move the magnet left and right, the dot moves up and down. When you move the magnet up and down, the dot moves right and left. Must work in 3-D to do magnetism.

Showed second Right Hand Rule. Point the fingers of the right hand in the direction of velocity, curl the fingers to the direction of the magnetic field. The thumb points in the direction of force on a positive charge. If the charge is negative, flip the direction.

F = q V X B
Went over several examples finding direction of force. If v and B are co-linear, or the charge is at rest (v = 0), then F = 0.

Another way of writing the magnitude of the force is F = q v B sin (theta)

Showed examples of forces of one wire on another. First wire sets up a magnetic field at the location of the second wire. That magnetic field affects the moving charges. Wires with currents in the same direction attract, opposite direction and they repel.

Showed split ring commutator example for a motor.

Handed out RA 23.1 due Monday.