Physics 12

Mr. Jean

December 10th, 2013

The plan:

• Video Clip of the day– http://www.youtube.com/watch?v=bI7AUgp5f

PI

• DC Motors

• AC Motors

• DC Generators & AC Generators

• Transformers

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 Motors:

• http://www.youtube.com/watch?v=_-WQZ1SGqi4

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.

To view on your own time: Introduction to DC Power & Motors:

• http://www.youtube.com/watch?v=OpL0joqJmqY

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.

AC Generators:

• Generators are essentially the same design as motors. – The mechanical energy input to a generator turns the

coil in the magnetic field. This produces an emf (voltage). A sinusoidal voltage output.

• The mechanical energy may come from:– i. Steam– ii. Wind– iii. Waterfall

DC Generator: • The commutator must change the AC

flowing into its armature into DC.

• Commutators keep the current flowing in one direction instead of back and forth.

Power Production

• Generators were built by Tesla to generate electricity reliably and in large quantities.

• Most of today’s energy sold is in the form of AC because it can easily be transformed from one voltage to another.

High Voltage with Low Current:

• Power is transmitted at high voltages and low current without much energy loss (heating of wire) because it can be stepped down from the plant to many cities, to a city, to the household.

• Household typical outlet is 120 V AC.

Transformers: • Transformers are used to transfer energy

from one circuit to another by means of mutual inductance between two coils.

Types of Transformers:

• Transformers consist of a primary coil (input) and a secondary coil (output).

• Step-up Transformer – Secondary has more turns – greater electric potential (V) induced, caused

by the lowering for current (I)

Types of Transformers:

• Transformers consist of a primary coil (input) and a secondary coil (output).

• Step-down Transformer – primary has more turns– Thus less electric potential (V) induced

causing a greater current (I).

Transformers:

• Transferring energy from one coil to the other OR the rate of transferring energy is the power.

• The power used in the secondary is supplied by the primary.

• LAW OF CONSERVATION OF ENERGY

Transformers:

• Brightstorm: Transformers– http://www.youtube.com/watch?v=4aYhAbzS

wvs

To do:

• Work on Reviews

• Continue to read– You should have read chapters 15 & 16 at

this point– Always read through the blue shaded

examples because these examples are very similar to much of the work we do in class.