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IB Physics 12
Mr. Jean
December 5th, 2013
The plan:
• Video clip of the day
• Lenz’s Law Investigation
• Solenoids
• Important Definitions
• AC & DC engines
Faraday / Lenz’s Law:• Brightstorm: Faraday Lenz’s Law
– http://www.youtube.com/watch?v=qWu82nJS42I
Lenz’s Law Investigation
• Complete the Lenz’s Law investigation
• This mini-lab should only take about 30 minutes.
Lenz’s Law Mini-lab:
CHECK OUT THIS!
• This person has some excellent material with regards to E & M.
• http://www.youtube.com/user/lasseviren1/videos
Electro-magnetic Induction:
• Brightstorm: Electro Magnetic Induction: – http://www.youtube.com/watch?
v=BJcH7PELbqI
Lenz’s Law:
• http://www.launc.tased.edu.au/online/sciences/physics/Lenz%27s.html
• https://www.boundless.com/physics/induction-ac-circuits-and-electrical-technologies/magnetic-flux-induction-and-faraday-s-law/faraday-s-law-of-induction-and-lenz-law/
• http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html
Magnetic Field in a Solenoid:
• A long straight coil of wire can be used to generate a nearly uniform magnetic field similar to that of a bar magnet.
• Such coils, called solenoids, have an enormous number of practical applications.
• In the above expression for the magnetic field B, n is the number of turns per unit length, sometimes called the "turns density". The expression is an idealization to an infinite length solenoid, but provides a good approximation to the field of a long solenoid.
Solenoids:
• Brightstorm: Solenoids– http://www.youtube.com/watch?v
=r7EZb3MXJBM
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.
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.
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– iv. Electric motor
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