How do transformers obey ohm's law?

[Philip Wood]

Suppose the secondary is open circuit. A very small amount of energy is taken (from the primary circuit) when the changing current in the primary induces an emf in the secondary forcing a small number of electrons through the secondary, depleting one end/terminal of the coil of electrons and giving the other end a surplus. But relatively few electrons flow in this way before the electric field set up in the coil by the deficit and surplus (positive and negative charges) at either end of the coil stop any more electrons flowing. So the flow is 'one-off' and the amount of energy required is negligible. What's more it's given back on the next part of the cycle.

In your football analogy, the energy needed to raise one football is negligible. But in a complete circuit the footballs keep being raised, requiring non-negligible energy per unit time. The energy is dissipated when they fall through a viscous medium (through the external circuit).

 

[Entanglement]

But relatively few electrons flow in this way before the electric field set up in the coil by the deficit and surplus (positive and negative charges) at either end of the coil stop any more electrons flowing. So the flow is 'one-off' and the amount of energy required is negligible.

I don't understand why only A FEW electrons will flow that way ??

 

[Philip Wood]

It doesn't take much charge to set up an electric field in the wire of the secondary coil which is equal and opposite to that due to the induced emf from the changing magnetic flux. After the electric field has reached that size, no more charge will flow (or wouldn't flow if the emf stayed the same!). THAT is the key thing to understand.

The reason why it doesn't take much charge is that, thinking of the wire (open circuit) as a capacitor, the plates (formed by the ends of the wire) will have a very small area, and are well-separated , so the capacitance will be extremely small. This is one way of looking at it. There will be others.

 

[Philip Wood]

ElmorshedyDr: You don't seem to have posted your daily transformer question...

 

[Entanglement]

I've bothered you a lot lately, I'm so sorry.

 

[Philip Wood]

You shouldn't have apologised: I was only teasing.

Some of your recent questions have made me think hard. Not a bad thing.