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Practice Problems A horizontal wire is moving vertically upwards in a horizontal magnetic field of strength 0.150 tesla which is perpendicular to the length of the wire. Calculate the voltage induced in the wire if its length is 50.0 cm and it is moving with a speed of 30.0 m s-1. A coil of 1000 turns of mean area 20.0 cm2 is placed with its plane perpendicular to a uniform magnetic field of strength 1.50 T. Calculate the emf induced in the coil if the field falls uniformly to zero in 1.50 x 10-2 s. 2.25 V; 200 V Transformers Introduction to Inductance Transformer Theory and Notation Efficiency of Transformers Mutual Inductance If two coils are placed side by side, and the current in one coil is changed, the magnetic flux in the second coil will change, inducing a current in the second coil. Demonstration Transformers A transformer consists of two coils wound onto an iron core. Circuit Symbol Transformer Theory Because the input current is changing direction (AC), the magnetic flux in the iron core is changing. This results in an induced current in the secondary coil. Transformer Equation The voltage induced in the secondary coil is related to the voltage supplied to the primary coil by the following equation: N represents the number of windings in the coils. Vs N s Vp N p Problem: A battery charger contains a transformer to convert 240V to 12 V. If the primary coil has 1200 turns, how many are there on the secondary? 60 turns Three Types of Transformers A step-up transformer Higher voltage in the secondary coil An isolating transformer Same voltage in both A step-down transformer Lower voltage in the secondary coil Transformer Efficiency In an ideal transformer the power input is the same as the output. V p I p Vs I s If a transformer steps up the voltage there is a corresponding decrease in the current supplied In reality, some energy is lost (as heat) in the wire used and in the core* Vs I s input power 1 V p I p output power Problems Complete Questions 1-4 on page 247 of the study guide. Questions 1 and 2* in worksheet 4

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