Physics Test Ch. 25

transformers

A device that increases or decreases the voltage of alternating current with very little waste of energy

how do generators work?

The generator has wire loops that rotate between a magnetic field to create EMF (Since only current that is moving perpendicular to the magnetic field produces EMF, the current is strongest when the loop is horizontal to the magnetic field because then the motion of the loop is completely perpendicular to the magnetic field. When the loop rises to the vertical position, the current drops until it reaches zero because the motion is now completely parallel to the magnetic field. The loop keeps rotating, and current increases again. So that's how you get the wave graph for the current.)

self-inductance

The property of a wire, either straight or in a coil, to create an induced EMF that opposes the change in the potential difference across the wire.

(true/false) you need a change in the magnetic field to produce a current

True (that's electromagnetic induction)

what uses induced EMF to produce electrical energy?

generators

alternator v. generator

in a generator, you are creating electricity by moving a wire armature (imagine a square wire loop) within a fixed magnetic field. Now in an alternator, a magnetic field spins in a series of windings called a stator. In effect, the opposite of what happens internally in a generator. The net result is the same, just an alternator uses a much more efficient method. (the magnetic field spins around loops of wire in an alternator but is stationary in a generator. In a generator the wire is what spins)

Lenz's Law

the magnetic field produced by the induced current is in the opposite direction to the original field

AC v. DC currents

DC currents (like in batteries) only move in one direction and slowly die out as they continue along since the resistance slowly decreases the current until it is gone. In contrast, AC current oscillates and moves in two directions, so that the current alternates from positive to negative and does not die out due to resistance over time.

Formula for EMF

EMF=BLV(sinx)

Michael Faraday

English chemist who created the first simple electric motor and the first dynamo. He discovered electromagnetic induction.

(true/false) transformers can use DC current

FALSE! (Transformers can only use AC current)

War of Currents

a series of events surrounding the introduction of competing electric power transmission systems in the late 1880s and early 1890s including commercial competition, a debate over electrical safety, and a media/propaganda campaign that grew out of it. Basically, Thomas Edison had patents for DC current and was making tons of money on it, so when Tesla came out with the new and obviously better AC current, Edison tried to stop Tesla by taking drastic measures to make AC current appear super dangerous. (FYI Tesla was partnered with George Westinghouse)

How transformers work

a transformer has two coils that are insulated from each other but attached to the same iron core. One coil is connected to the AC source and is called the primary coil. The changing currents in this coil cause a magnetic field that is then carried through the core and to the other coil, which is the secondary coil. In the secondary coil, the changing magnetic field produces a new EMF and current. (sometimes the new current is larger, which is in a step-up-transformer, and other times it is smaller like in a step-down-transformer)

alternator

an electrical generator that converts mechanical energy to electrical energy in the form of alternating current

electric generators

converts mechanical energy into mechanical energy

Eddy currents

currents generated in any piece of metal moving through a magnetic field; the magnetic field they produece is opposite to the motion that caused the current. The force on the metal opposes the motion of the metal in either direction, but does not act when the metal is still.

Induced Electromotive Force (EMF)

the potential difference across a wire that results from the production of an electric field (When a wire moves perpendicular to a magnetic field, there is a force on the charges in the wire. The force causes negative charges to move to one end of the wire, leaving positive charges at the other end. This separation of charges produces an electric field and therefore a potential difference across the wire! The potential difference is the EMF)

Electromagnetic induction

the process of generating a current through a circuit due to a changing magnetic field or to the relative motion between a wire and a magnetic field