physics 12 mr. jean december 14 th, 2012. the plan: video clip of the day facts about magnets...
TRANSCRIPT
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.
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).
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