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Experiment 27
Magnetic Induction
Objectives
To explore the behavior of a transformer and to see the effect of ferromagnetic coupling.
Introduction
In this experiment we will examine the behavior of transformers. A transformer consists of two coils one of which supplies a changing magnetic flux that induces a current in the second. The emfs across the coils are related to the number of turns in each coil by the equation.
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where a, a positive constant, is a measure of the fraction of the flux from the primary that passes through the secondary. For perfect, non-ferromagnetic coupling, a is 1.
Procedure
1. Connect the circuit of the figure. The primary coil is the inner coil. Place the steel bar in the primary such that it just reaches the end of the primary. Put the primary all the way into the secondary. Measure the peak emfs of the primary and secondary using the oscilloscope. Change the primary emf in increments of ½ volt and measure each corresponding secondary emf up to a primary emf of 5 volts.
Repeat the measurement at 1.0 V primary without the steel bar. What is the meaning of your result?
2. Replace the steel bar and primary to the starting position of Procedure 1. Now move the primary (with steel bar) out of the secondary one tenth of the length of the rimary and measure the emf in the secondary for a convenient primary emf. Maintaining the same convenient primary emf, remove the primary out of the secondary a succession of tenths of its length measuring secondary emfs as you proceed until the primary no longer will stay inside the secondary.
Graphs
1. Graph the results of Procedure 1 plotting the primary emf on the x axis and the secondary emf on the y axis. What can you conclude from the appearance of the data?
2.
Graph the data of Procedure 2 by plotting the ratio of secondary emf to primary
emf on the y axis against the fraction of the length of
the primary remaining inside the secondary. What can you conclude from this
graph?
