# The current of primary and secondary windings

• fawk3s
In summary, the textbook states that when the resistance on the secondary winding circuit is infinite and no current runs through it, then the power on the transformer is zero. However, current still ought to go through the primary coil as a result of the inductance of the windings.

#### fawk3s

So I was just reading about transformers from my textbook, and the textbook seems to state that when the resistance on the secondary winding circuit is infinite and no current runs through it thus the power on it P=VI is zero, then, according to the energy conservation law, no current runs through the primary winding as well.
But I can't really grasp the idea. If the resistance on the secondary winding is infinite, shouldn't the transformer just act as a simple inductor for the primary winding? Current still ought to go through it, like in a simple powersource-inductor circuit.
I can totally see that the power on it is zero due to only the reactance of the transformer/inductor, but shouldn't still current pass through it?

Please tell me where I am wrong.

You're not using conservation law right. A real transformer has one input and TWO outputs. The second output is heat due to inefficiencies.

I can't really see what you're saying. Are you saying that if the resistance on the secondary winding is infinite, then the energy released is in the form of heat on the transformer/primary winding?

No "if": there is always heat loss on the primary.

I think that you are not talking about the resistance. You are, I think, talking about the Reactance, due to the Inductance of the windings. A primary coil with infinite resistance would never let any current through it!
The primary inductance would normally be chosen to be high enough to reduce the primary current to an 'acceptably low value' -not zero - with the secondary unloaded.

[PLAIN]http://img202.imageshack.us/img202/239/transformeru.png [Broken]

Im not sure if you understood my problem correctly, so I'll try and explain again.
In the picture above, P is the primary coil circuit and S the secondary. The secondary is open, or unloaded, giving it infinite resistance. So no current goes through it.
Now what my textbook states is that when there's no current in the secondary circuit, there is no current in the primary either. And I can't understand why that is.
What I would understand is that there is current going through the primary, but since no work is done on the secondary coil, the transformer would act only as an inductor for the primary circuit. Leaving the resistance of the wire beside, there would only be reactance in that circuit and thus the current would be, well, very big. But certainly not 0 like in the secondary.

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Why would the current be big?
If you have enough turns on the primary, the current can be arbitrarily small, surely.
I=V/Z
Lots of current in the coil you've drawn, though! LOL

sophiecentaur said:
The primary inductance would normally be chosen to be high enough to reduce the primary current to an 'acceptably low value' -not zero - with the secondary unloaded.

Ofcourse !
How could have I been so slow ! Oh my!

At last it struck me, while I was taking a bath. The energy in the coil is determined by the inductance and current. The bigger the inductance, the smaller the current needs to be for the same energy value.

Thank you so much for saying that sentence mate! This thing has been giving me a headache since last night.

Any time!

## What is the purpose of primary and secondary windings?

The primary and secondary windings are used in transformers to transfer electrical energy from one circuit to another. The primary winding receives the input voltage and current, while the secondary winding outputs a different voltage and current based on the ratio of the windings.

## What is the difference between primary and secondary windings?

The primary winding has a larger number of turns and is connected to the input voltage source. The secondary winding has a smaller number of turns and is connected to the output circuit. The difference in the number of turns results in a difference in voltage and current between the two windings.

## How does the current in the primary and secondary windings affect the output?

The current in the primary winding creates a magnetic field, which induces a voltage in the secondary winding. This induced voltage results in a current flow in the secondary winding. The ratio of the currents in the primary and secondary windings determines the voltage and current output.

## What factors affect the current in primary and secondary windings?

The current in the primary winding is affected by the input voltage and the number of turns in the winding. The current in the secondary winding is affected by the induced voltage from the primary winding, the number of turns in the winding, and the load connected to the output.

## How do you calculate the current in primary and secondary windings?

The current in the primary winding can be calculated using Ohm's law, where current equals voltage divided by resistance. The current in the secondary winding can be calculated by multiplying the current in the primary winding by the ratio of the number of turns in the secondary winding to the number of turns in the primary winding.