Answer Posted / 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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