# IB Physics 12 Mr. Jean December 5 th, 2013. The plan: Video clip of the day Lenz’s Law Investigation Solenoids Important Definitions AC & DC engines.

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IB Physics 12 Mr. Jean December 5 th, 2013

The plan: Video clip of the day Lenz’s Law Investigation Solenoids Important Definitions AC & DC engines

Lenz’s Law Investigation Complete the Lenz’s Law investigation This mini-lab should only take about 30 minutes.

Lenz’s Law Mini-lab:

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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.

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

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