Phys 132
Fall 2008
Electromagnetism and Optics: Class Exercise 3
Transformers
A transformer consists of two solenoids that are connected via an iron core. A current is
applied through the left hand solenoid (the primary coil) and the field that is produced by this is
transmitted through the iron core. This induces an EMF in the right hand solenoid (the secondary
coil).
The field throughout the iron core is uniform in magnitude and the core is constructed so that
it has the same cross-sectional area on the left as on the right. The following exercise will illustrate
the basic ideas behind which the transformer works.
1 Questions
Suppose that the length of each coil is L, their cross-sectional areas are both Aand the
primary coils has Npturns and the secondary coil has Npturns.
A first qualitative attempt to describe this is as follows:
a) Suppose that the potential difference across the primary coil, ∆Vp,varies with time.
Using the notions of magnetic flux and Faraday’s law describe qualitatively how this
can produce a potential difference across the secondary coil ∆Vs. Indicate how you
could attempt to use this to determine an expression for ∆Vsgiven ∆Vp.
b) Suppose that the current through the primary coil is as illustrated.
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