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Physics Gang Signs Review 1. If your were a positively charged particle headed toward the Earth’s equator from outer space which way would the Earth’s magnetic field force you? B-field = v= q= so the Force = 2. If you were moving toward the Earth at either pole then what would happen? • NOTHING. . .the direction of the velocity and the B-field are parallel to each other so NO FORCE is felt!! (This is what causes the Northern and Southern Lights.) • Electrons from solar wind interact with the nitrogen and oxygen in the atmosphere anywhere from 20 to 200 miles above the surface of the Earth COLORS of auroras: O2 = Green (up to 150 miles) Red (above 150 miles from surface) N2 = Blue (up to 60 miles) Purple (above 60 miles from surface) Michael Faraday • The discovery that an electric current in a wire produced magnetism was a turning point in history. If a Current creates a Magnetic Field ... BI ... Can a Magnetic Field cause electric current? I Faraday’s Contribution • Discovered the electric current could be produced in a wire by simply moving a magnet in or out of a wire coil. • No battery or other voltage source was needed. • The amount of voltage produced depends on how quickly the magnet passes through coil of wires. – It doesn’t matter if it’s the magnet moving through the coil or visa versa. Electromagnetic Induction • ANY CHANGE in a MAGNETIC FIELD, will create an ELECTRIC CURRENT • Inducing voltage by changing the magnetic field around a conductor is electromagnetic induction. Faraday’s Law Faraday’s Law says that you can create (induce) more voltage by 1) moving the B-field faster 2) moving it into and out of more coils of wire **the amount of current produced by electromagnetic induction depends not only on the induced voltage but the resistance of the coil and circuit to which it’s connected.** Generator and AC current • A GENERATOR uses electromagnetic induction, it turns mechanical energy into electrical energy. • As the coil enters the magnetic field, voltage is induced one direction, as the coil leaves the magnet, voltage is induced the other direction. – This produces current that travels one direction in a wire, and then switches directions as the magnet leaves the coil = AC current Transformers • Uses Faraday’s Law and ferromagnetic properties of an iron core to raise and lower AC voltages. • Used to transfer electric energy from one circuit to another. How it works • Current passes through primary, as a result, current passes through the secondary. • Iron core guides B-field lines from primary through the secondary. • Voltage of primary is passed to EACH loop of secondary. – Twice as many loops, twice the voltage produced in secondary – Three times the loops, three times the voltage produced Equation • Relationship between the voltages is with respect to the number of turns of the wire. Vp Vs np ns Ip Is Energy transferred • The coil transfers energy from one coil to another • The rate at which energy is transferred is POWER – The primary gives no more power than the second uses (COE) REMEMBER: P = VI IN TRANSFORMERS PIN = POUT VIIN = VIOUT Step Up and Step Down transformers • Transformers are also used to raise and lower the voltage to meet certain operating requirements • The ratio between the number of turns in the primary and the secondary is called the Turns Ratio I1 PRIMARY I2 SECONDARY PRIMARY SECONDARY