I'm suspecting this may be a newbie question but I -have- to ask...

Can somebody explain in simple terms what are the electromagnetic effects that makes a transformer under small/no load to consume less current on the input ?

In simple terms - well... This is one of the questions where a proper answer is inordinately long compared to the length of the question. Nevertheless, the electromagnetic effects in play here:

First we need the concept of MagnetoMotor Force or MMF for short.

MMF is defined as the intensity of the magnetic field penetrating a closed path. Such a closed path being e.g. a transformer winding. Properly this is formulated in Ampere's law, taking the path integral around the path thus arriving at the intensity. The end result of that integration is what interests us now, and the result is simply:

MMF = Ni, where

N = number of turns in the coil

i = current in the coil conductor

So, instead of being a "true" force in the mechanical sense, the unit of MMF is ampere-turns. This is the "force" driving magnetic induction.

Next, the magnetic flux density B can be calculated. Again, we want to integrate the magnetic field intensity H over the surface area penetrated by that field, multiplied by the magnetic permeability of the medium. The density of the magnetic flux is an important quantity in determining the requirements for the magnetic core material.

Now finally, the voltage resulting from the MMF in a conductor (coil) can be determined from Faraday's law stating that the voltage over a closed path is the product of the flux density over the area enclosed by said path, times the rate of change of the flux density. The resulting voltage in the closed path is known as the ElectroMotor Force or EMF.

Further, Lenz's law talks about the directions of the quantities such that a time-varying flux induces currents in the conductor opposing the original flux. This is the mechanism causing inductive reactance in a conductor.

On to transformers.

Ampere's law states that the sum of MMFs over a closed loop is 0. Thus in an ideal transformer where the primary and secondary share the same magnetic loop, it follows that MMF

_{pri} + MMF

_{sec} = 0. Recalling the unit of MMF we can also write N

_{1}i

_{1} + N

_{2}i

_{2} = 0. From Faraday's law we can rewrite the previous in the form:

U

_{1} = N

_{1} * dPHI/dt, U

_{2} = N

_{2} * dPHI/dt where

U

_{1} = primary EMF

U

_{2} = secondary EMF

PHI = magnetic flux linking the coils

Eliminating common terms, we can write simply U

_{1}/N

_{1} = U

_{2}/N

_{2} and from there

U

_{1}/U

_{2} = N

_{1}/N

_{2}; I

_{1}/I

_{2} = N

_{2}/N

_{1}So, the answer to your specific question was embedded in the middle, in Lenz's law that introduces the opposing magnetic induction. The magnetic flux in the transformer core couples the coils together. Faraday's law states the resulting Electromotor forces (coil voltages), and Ampere's law the Magnetomotor forces (magnetic flux) In case there is a load connected to a secondary coil, the coil will be a consumer of magnetic flux energy. Balancing the equations results in increase of primary current until MMFs over the core loop again cancel out. When the load is disconnected, Lenz's law balances the primary MMF with little primary current.

That's it in a nutshell.

Edit: subscripts for readability.