Physics 12

Mr. Jean

December 14th, 2012

The plan:

• Video Clip of the day

• Facts about Magnets

• Lenz’s Law

• E=mc2 Video part #2

Facts about Magnets:

• Permanent magnets are made of ALNICO. Aluminum, nickel, and cobalt. There are also rare earth elements, neodymium, that produce extremely strong permanent magnets for their size.

About Magnets:

• Permanent magnets are fragile. Dropping them will cause the domains to disorient.

• Heating them weakens the magnet. Once cooled, it will regain its strength unless heated past the “Currie point”. The magnet will be totally destroyed.– (p. 755 Currie point table)

Galvanometer

• Galvanometer - device to measure very small currents.– Consists of a small coil of wire placed in the

strong magnetic field of a permanent magnet. Each turn of the wire is a loop.

– The current passing through the loop goes in one side of the loop and out the other side.

– One side of the loop is forced down while the other side is forced up (third RH rule).

– The loop has torque and rotates.

Producing Current

• One way to produce an electric current through a conductor is to move a conducting rod through a magnetic field.

• When there is a voltage in the rod, it becomes part of an electric circuit.

Lenz’s Law (1834):

• For motion, Lenz’s law states that if a loop of wire moves while inside an external magnetic field then a current is produced in the moving wire that will produce its own magnetic field.

• This magnetic field will interfere with the external magnetic field in such a way that the two fields will try to oppose the motion (change in position of the moving loop of wire).

DC Electric Motor

• Has a coil with an iron core ARMATURE⇒• It is surrounded by electromagnets

Problems with DC Power:

• 1. The magnetic forces are aligned directly opposite each other and will no longer experience a torque.– If you could change the direction of the

current, the coil would again experience a torque.

• 2. If the coil keeps turning, the leads will twist and eventually break.

AC Electric Motor

• Uses slip rings as commulator.

• Since the current is alternating, the motor will run smoothly only at the frequency of the sine wave.

• The magnetic field is sinusoidally varying, just as the current in the coil varies.

AC vs. DC Motors:

• Electric motors are mostly AC because our electric energy for industry and home is transmitted as AC.

• DC motor – starter motor on a car.

Magnetic Field Demonstration:

• Video Demonstration of Ferrell Fluid

• Rare Earth Magnets

• AC & DC Motors