# Physics Tutorial: Inductance and Transformers

A transformer is an apparatus for changing a given electrical current into another current of different

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voltage. There are two kinds of transformers: step down and step up.  Step up transformers increase the voltage where step down transformers decrease the voltage.

Explanation:  We call this device a transformer because it transforms electrical energy into magnetic energy, and then back to electrical energy again.  A transformer consists of two coils of wire both wrapped around the same core. The primary coil is the input coil and the secondary coil is the output coil. The relationship between the number of turns in the coil, voltage, and current in is Vs / Vp = Ip / Is = Ns / Np.  The V represents voltage, I represents current, and N represents the number of turns of the coil.  The s represents secondary and p represents primary.

A transformer’s basic operating principle is that of mutual inductance.  Mutual inductance occurs when two coils are so close together that the magnetic field of one coil links with the magnetic field of the other coil.  Current is induced in the second coil when the magnet field produced by the first coil changes. A transformer only works with alternating current.  Direct current would cause a magnetic field in the core, but not a changing one.  This would cause the voltage induced in the second coil to be equal to zero.

A transformer is used because they cause almost no energy loss.

Application:

These ideas benefit us every day.

Power is supplied to houses everywhere in the developed world.  In the power grid, voltage can be as high as 765000V.  This power is stepped down to 72000V at your local substation.  From here, the power is stepped down to about 220V at a transformer on a utility pole.  The voltage is so high in the beginning so it can travel long distances.  It is stepped down so often so it can be used in the home.  Certain appliances like air conditioners and stoves use about 220V where smaller appliances use less.  If such a high voltage were applied to these appliances they would need step down transformers installed in them.

Question: A transformer in the electrical grid is supplied with 72000V, and contains 9818 turns.  The secondary coil puts out 220V.  What is the number of turns on the secondary coil?

Vs / Vp = Ns / Np

220/72000=Ns/9818

Ns=30 turns

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Melissa Lagace