# Induction in Transformers

## Main Question or Discussion Point

Hi,

I would like to understand something pertaining to transformer action.

How does a changing magnetic flux, assumed to be bound by an 'ideal' magnetic core (with zero leakage flux), induce an EMF in the secondary winding of a transformer......without the flux ever shearing or cutting the winding conductors? Can someone plz explain the actual physics behind statically induced EMFs and currents under these conditions?
Thanx.

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The applied voltage (EMF) induces a flux in the iron, with a reactive magnetizing current (dI/dt = V/L, where L = the inductance), leading the applied voltage by 90 degrees. The flux change in the iron in turn induces a voltage (EMF) in the secondary winding (Faraday's law), again shifted by 90 degrees, so it is 180 degrees out of phase (remember the minus sign in Faraday's Law) with the applied voltage. If you connect a resistance to the secondary, the resistor current is in phase with both the input and output voltages.

The applied voltage (EMF) induces a flux in the iron, with a reactive magnetizing current (dI/dt = V/L, where L = the inductance), leading the applied voltage by 90 degrees. The flux change in the iron in turn induces a voltage (EMF) in the secondary winding (Faraday's law), again shifted by 90 degrees, so it is 180 degrees out of phase (remember the minus sign in Faraday's Law) with the applied voltage. If you connect a resistance to the secondary, the resistor current is in phase with both the input and output voltages.
Dear Bob,

I'm aware of Faraday's laws of Electromagnetic Induction and transformer action in general. However, my interest is basically to understand how a magnetic flux confined to a magnetic circuit be able to induce an EMF in a coil surrounding the magnetic core. Flux leakage is considered to be zero so that there is no cutting of secondary winding conductors......basically I need to understand the physics behind statically induced EMFs.
Thanx

Dear Bob,

I'm aware of Faraday's laws of Electromagnetic Induction and transformer action in general. However, my interest is basically to understand how a magnetic flux confined to a magnetic circuit be able to induce an EMF in a coil surrounding the magnetic core. Flux leakage is considered to be zero so that there is no cutting of secondary winding conductors......basically I need to understand the physics behind statically induced EMFs.
Thanx
There is no requirement for the flux to physically pass through the wire, just that there is a time varying flux passing through the area of the loop made by the wire.

It's a strange concept.....

Try looking at Green's theorem (leading onto stokes theorem). To accept that green's theorem is correct requires the same leap of faith you need to accept that flux through a loop can induce a voltage in the loop. I would recommend typing 'green's theorem MIT' into youtube and watching the lecture by Prof. Auroux.