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We khow that due to Ohm's Law,

$I$ is proportional to $V$

So if $V$ rises, $I$ should rise too. But in a transformer when $V$ rises, $I$ lessens. Why does this happen? It seems like transformers don't follow Ohm's Law. Where am I wrong?

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  • $\begingroup$ Ohm's law applies across a piece of (conducting) material. It is not a feature of a "machine" of "device". A transformer is a technical and more complicated device. The individual wires and material parts inside it might show Ohmic relationships but it does not make sense to talk about Ohm's law "on" the device itself. $\endgroup$ – Steeven Feb 14 '18 at 7:46
  • $\begingroup$ @Steeven Thanks. I didn't khow that. $\endgroup$ – Theoretical Feb 14 '18 at 7:48
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When power flows through a transformer, the relationship between current and voltage in the input (the primary winding) is changed in going to the output (the secondary winding).

If the secondary has more windings in it than the primary, the voltage in the secondary will be larger than that in the primary and the current in the secondary will be smaller than that in the primary. But power is always conserved:

(voltage x current) in the primary = (voltage x current) in the secondary.

If the secondary has fewer windings in it than the primary, the voltage in the secondary will be smaller than that in the primary and the current in the secondary will be larger than that in the primary. Again, power is always conserved:

(voltage x current) in the primary = (voltage x current) in the secondary.

the amount by which the voltage in the secondary is stepped up or down relative to the primary is set by the ratio of the number of windings in the secondary to the number of windings in the primary: if the secondary contains twice the number of windings as the primary (2:1 ratio), the secondary will put out twice the voltage (and half the current) that is being fed to the primary.

If the secondary contains half the number of windings as the primary (1:2 ratio), the secondary will put out half the voltage (and twice the current) that is being fed to the primary.

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