Overview

Concepts:

• Electromagnetic Induction
• "The induced voltage in a coil is proportional to the product of the number of loops and the rate at which the magnetic field changes within those loops."
• V = -n delta phi/delta t
• Relates induced voltage to # of turns of coil and rate of change of magnetic field.
• Moving a magnet in a coil induces a voltage in the coil.
• Induced voltage creates a magnetic field that opposes the motion of the magnet
• Energy is conserved
• If coil has no load no work is done to move the magnet
• If coil has a load, work to move magnet = work done in load.
• Generators & Alternating Current
• Converts mechanical energy to electrical energy
• Coil rotates in magnetic field
• Turbine
• Mech. energy from falling water or steam
• Motor & Generator Comparison
• Motor Effect
• Magnetic field exerts a force on a current-carrying conductor
• Magnetic Force (F=qvB) moves the wire perp. to charge motion.
• Generator Effect
• Generator Effect - Current is induced by moving a conductor in a magnetic field
• Magnetic force (F=qvB) moves the charges along the wire perp. to the wire motion.
• Both depend on magnetic force on moving charges
• Transformers
• 2 coils
• Primary (input)
• Secondary (output)
• Changes in V in primary change its magnetic field, which induces V in secondary (Faraday's Law)
• Iron core increases coupling of the coils
• V stepped-up or stepped down
• Ease of changing voltages is principal reason for AC power
• Ratio of voltages = ratio of turns (coils)
• Energy conservation applies
• IVprimary = IVsecondary
• Power Transmission
• High voltage & low current
• P=I2R
• Long distance transmission 120,000 V or more
• Stepped down to 2200 V in cities & towns
• Stepped down to 220 V for homes
• Induction of Electric & Magnetic Fields
• Changing magnetic field creates an electric field
• E field proportional to rate of change of B field
• E field at perp. to B field
• Maxwell (1860's)
• Changing E field creates a B field
• B field proportional to rate of change of E field
• B field perp. to E field
• Electromagnetic Waves
• moving charge creates changing E field
• changing E field creates changing B field
• move at speed of light
• discovered by Maxwell
• calculated speed of propagation from basic equations
• "c" is the only speed at which fields don't die out
• experimentally confirmed by Hertz