The Transformer

The transformations involved are

changing current	®	changing magnetic field	®	induced emf (changing)	®	induced current (changing)
a.c. in the primary coil		conveyed from the primary coil to the secondary coil by a laminated soft iron core The core does not have to be an electrical conductor - there is no electricity involved during this phase of the operation.		the flux goes straight down the middle of the secondary coil, thereby creating flux linkage. The a.c. ensures that it is changing all the time, so the conditions for Faraday's Law are always satisfied		The induced emf causes a current in the 'load' resistor because of Ohm's Law

In the formulae, the subscript 'p' stands for primary and 's' for secondary, while 'n' stands for 'number of turns' and V & I have their usual meanings. So, for example, V_p means 'the voltage across the primary coil', while n_s means 'the number of turns on the secondary coil'

When people talk of a 'step-up' transformer, they mean that the voltage is stepped up, rather than the current. And it is the primary quantity that is stepped up (or down) to produce the secondary quantity. So, for example, in a step-up transformer

the voltage increases: V_sis bigger than V_p
the current decreases: I_s is smaller than I_p.

Exactly the reverse is true for a step-down transformer. You need to keep a clear head to get these the right way round.

Notice that the primary and secondary powers have to stay the same - you can't create or destroy energy. So the product V_s I_shas to be equal to the product V_p I_p. So if one of voltage or current goes up, the other has to go down.