If a transformer is described as ideal, we mean

i)	the coils have zero resistance
ii)	all the magnetic flux, , produced by the primary current, Ip, is linked with the secondary coil.

When I_p changes, changes.

From Faraday’s (Neumann’s) law, we have

Combining these two statements gives

With an ideal transformer, when the secondary coil is open circuit (not connected to anything), there is no net energy taken from the supply. Energy is stored in the magnetic field during the time the current is increasing but is recovered when the field "collapses".

• Electricity and Magnetism Chapters Index
• Return to top of page