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magnet wikipedia school physics rochester Chapter 24Section 1
Faradays Discovery
Michael Faraday and Joseph Henry discovered that if a wire moves through a magnetic field, an electric current can be induced in the wire.
To induce current, there must be relative motion of wire and of magnetic field. In order to generate current, the conductor can move through a magnetic field, or the magnetic field can move past a conductor. The process of generating a current through a circuit in this way is called electromagnetic induction.
Electromotive force The potential difference or voltage, given to the changes by the pump is called the electromotive force, or EMF Electromotive force is potential difference measured in volts.
The EMF depends on the magnetic field strength, the length of the wire, and its velocity perpendicular to the field.
Equation for Electromotive force
EMF=BLvA microphone is a simple application that depends on an induced EMF. The voltage generated is small, usually 10^-3v.
Electric Generators
Electric generator was invented bt Faraday and it converts mechanical energy to electric energy.
" When a generator is connected in a closed circuit, current flows that is proportional to the induced EMF".pg519
Generators and motors are almost the same in construction but they convert energy in different directions.
"A generator converts mechanical energy to electric energy but a motor converts electric energy to mechanical energy"pg 520
Alternating current Generator
The power produced by a generator is the product of the current and the voltage. Power is always postive and I and V are both positive or negative.
Vs=Ns
Vp Np
Section 2
Lenz's Law
The direction of the force was determined by a man named Lenz.
" Lenz's law states the direction of the induced current is such that the magnetic field is the resulting from the induced current opposes the change in the flux that caused the induced current". taking form the Physics book pg 523.
Lenz's law also can be applied to motors.
" When a current- carrying wire moves in a magnetic field, an EMF is generated."pg 524
Self inducatance Self inductance: Induced EMF produced in coil by changing current. pg 525.
The size of the EMF is proportional to the rate at which flux lines cut through the wires.
" Because of self inducatance, work has to be done to increase the current flowing through the coil."pg 525
Energy is stored in the magnetic field.
Transformer A transformer is a device used to increase or decrease AC voltages. Transformers are widely used because they change voltages with essentially no loss of energy.
Two coils of transformer
The secondary coil can also be expressed as the secondary voltage and the equation is
Electromagnetic Waves
If the changing electric field is produced by an accelerating charge, energy will be ratiated away from the charge in form
of electromagentic waves. In an electromagnetic waves, both the electric field and the magnetic field vary as sine wave and the two fields are
perpendicular to each other and to the direction of motion. All electromagnetic waves travel in space at the speed of light. Problem
A transformer steps up potential difference from 300. Volts to 600. The current in the primary coil is 6.0 amperes,
while the current in the secondary coil is 2.0 amperes. Calculate the effiency of the transformer. Sources
Secondary Voltage number of turns on secondary
----------------- = -----------------------------
Primary voltage number of turns on primary
If the secondary voltage is larger than the primary voltage, the transformer is called the step- up -transformer.
But if the voltage out of the transformer is smaller than the voltage put in, it is called step-down-transformer.
Solution
(600.v)(2.0A)
-------------- x 100% = 67 percent
300.V)(6.0A)