Answer / blueraja

I'd also like to mention (sorry, there is no "edit" button)
that transformers absolutely **DO** obey Ohm's law - this
apparent contradiction does NOT occur because, as some have
stated above, "transformers are non-ohmic devices." This is
simply not true.

Answer / blueraja

@Nishant: I apologize, the blog post I mentioned above
(answer #13) is down - I promise I will get it back up
within the next week to answer your question.

The quick-and-dirty answer is that Ohm's law tells you how
voltage and current relate in a single circuit, while the
power-conservation-law-for-transformers tells you how a
transformer relates voltage and current in two separate
circuits.

For a more in-depth explanation, refer to my blog post in
answer #13 in about a week :)

Answer / nandakishore

Transformers obeys ohm's law.transformer consists of two
circuits primary and secondary both circuit obeys ohms law
but when the are magnetically coupled they obeys power
conservation law.the variation in the current or voltage is
due to resistance value

Answer / manmohan

but you did not mention about the change in temparature?to
satisfy the ohms law,temparature must be constant?how do you
justify ur answer?

Answer / ad patel

In ohm's law resistance is constant.
In transformer resistance is change with changing in voltage,because voltage is change by changing the length of winding.....
So in transformer R is change so here the Joule's temperature law is apply..
Where Joule's law is follow there ohm's low is not follow....

Answer / n.rajasekhar

thats because of induced voltage in the winding,but finally
while applying ohms law see how much net voltage appearing
across the impedance.just take one numerical example and see
you will get it.

Answer / surender rawat

A transformer is a device that transfers electrical energy
from one circuit to another through inductively coupled
electrical conductors. A changing current in the first
circuit (the primary) creates a changing magnetic field.
This changing magnetic field induces a changing voltage in
the second circuit (the secondary). This effect is called
mutual induction.

The e.m.f. required to drive electrons across inductive
and capacitive loads vary depending upon the powerfactor
at which the load is designed. The power factor in case of
resistive load is 1 and always remains so. Also, for change
in e.m.f., L and C loads receive varying responses. In
case of resistive loads, the response is steadier for a
longer duration compared to capacitive or inductive loads.

Answer / daher

ohms law applied to electric circuits wathever it AC or DC
CIRCUITS,V=IR or V=IZ where Z=(R^2+omegal^2)^1/2.the
equivalent circuit of transformer is composed of resistance
and inductance and ohms law helps to solve the problem

Answer / krish

Ohms Law applicable in constant Temperature condition only. But in Transformers we can't maintain constant Temperature in primary and secondary so it doesn't obey Ohm's law

Answer / dr h

I think there is a fundamental misunderstanding here. Ohm's law is just a statement that says that current is proportional to the applied voltage across a resistor at constant temp.

The resistance is defined as the ratio of voltage to current at any instant and this happens to be constant for all current voltage values at constant temp. For a filament lamp this ration changes as temp changes.

With a transformer secondary coil you cannot just consider the resistance of the output coil as you are dealing with alternating current, which leads to capacitive and inductive reactance. You are generating whats called back emf, defined by Lenz's law, which is ultimately a statement which results from the conservation of energy.

Considering the case of a step up transformer, somebody with a basic knowledge of electricity will question how that can be since the resistance is the same and the voltage has increased but yet the current has decreased. This is because the output resistance is still exactly the same but its imput impedence has increased. When dealing with AC voltages, you need to talk about the impedence, which only is partly due to the resistance.

It can be very confusing, but studying the physics of AC circuits should help you understand. Ultimately it is due to the conservation of energy, which will hold at all times and in all situations.

Dr H

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