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Physics Lesson Plan Teacher Howard Unit Title Length Goal(s)/PLO(s): Course Grade Level Block/Period solve problems that deal with an ideal transformer and that involve – primary voltage – secondary voltage – number of primary windings – number of secondary windings – primary current – secondary current – identify a transformer as step-up or stepdown Phys 12 12 Date Class Size Lesson #, of 21-07 give examples of the use of transformers in the home, workplace, and community Materials: Transformers Timeline Class Activities Introduction Body Notes 21-07 Closure Questions 17-19, Problems 31-37 odd 21-07 Transformers and Transmission of Power A transformer is a device for increasing or decreasing an AC voltage. faraday_en.jar • found in many electrical devices, on utility poles and at the generator sites • consists of two coils of wire o primary (input voltage) o secondary (output voltage) Transformers are designed so that “all” the (really > than 99%) magnetic flux produced by the alternating current in the primary coil passes through the secondary coil. The frequency of the induced emf in the second coil is the same as in the primary coil but if the number of coils is different then, by Faraday’s Law: Vs = N s Where N is the number of coils and ∆Φ ∆t ∆Φ ∆t is the rate of change of flux. The input voltage in the primary coil is Vp = N p ∆Φ ∆t If we assume that little or no flux is lost then we can divide the two equations and ∆Φ disappears. ∆t Transformer Equation The secondary (output) voltage of a transformer is related to the primary (input) voltage as the ratio of the numbers of turns in the secondary and primary coils. Vs N s = Vp N p Transformers only work with AC because the alternating current produced a change in the magnetic flux. DC current does not produce a change in flux while it is flowing. If the secondary coil has more turns than the primary coil (N s >N p ) then the transformer is a step up transformer. • the secondary voltage is higher than the primary voltage If the secondary coil has fewer turns than the primary coil (N s <N p ) then the transformer is a step down transformer. • the secondary voltage is lower than the primary voltage The power output (in a perfect world) is the same as the input so P p =P s IpVp= IsVs 2nd Transformer Equation The secondary (output) current of a transformer is related to the primary (input) current as the ratio of the numbers of turns in the primary and secondary coils. I s Vp N p = = I p Vs N s Example 21-11 There is a time when a DC current can work with a transformer. The moment the circuit is closed there is a change in flux • Ignition systems use this to create high voltage spikes and produce a spark across a gap in a internal combustion engine • 12V battery creates a 30000V spike across the spark plug The main reason our electrical grid uses AC instead of DC is that transformers work with AC. If power can be transported long distances at low currents (high voltage) then the power loss from P=I2R is minimized. But high voltage is dangerous in town or in your home because the electricity is more likely to arc so the voltage is stepped down in town and then further stepped down before it enters your house. • safety trumps efficiency Example 21-12 Questions 17-19, Problems 31-37 odd