See precisioninfo comment in relation to this video:
https://www.youtube.com/watch?v=KM1XFOCzKDE&feature=youtu.be
The comment reads as follows:
Responses to Question 18
In terms of the HSC theory and step-up / step-down the correct answer is (b) - as the narrator initially identifies. Anything beyond that seems beyond the scope of the HSC course. In reality, the secondary current is determined by the input power (voltage and current), the ratio of the windings and, importantly, the load on the transformer. Hence, with effectively no load, the point of the question seems moot. The “correct answer” is put to put a meter on the unloaded transformer and see with a given transformer.
In attempting to justify returning an answer of (A) the narrator experiences a number of difficulties and argues inconsistently with the requirements of an ideal transformer. The narrator is undoubtedly correct in that this is not the best question (nor answer) and consequently justifications are not readily available.
Nevertheless, the application of Ohm’s law to the input coil here is inadmissible. Ohm’s Law is a Law of Conduction in name only. It is not a law that is embedded in a mechanism or a deeper theory of guiding principles such as Newton’s Laws of motion.
Ohm’s law is a statement of observation of the behaviour of a thermal resistor over the range of (constant input) voltages for which resistance is constant. To achieve anything approximating Ohms law over a range of applied (DC) voltages it is typically necessary to control other variables including temperature (using water baths etc).
Ohm’s law does not apply in semi-conduction or super-conduction.
Ohm’s law does not apply in any cases where current is limited by some (external) factor(s).
Ohm’s law does not apply in high frequency AC applications such as operational motors, generators and transformers. In all of these cases Back EMF and or inductive loads come into play.
To claim, via Ohm’s law, that by removing coils from an input loop, effective input current increases in an “Ideal” transformer, misconstrues the facts that “ideal” transformers cannot have (thermal) resistance. Secondly, removing loops in the input wire does not require shortening it (as the video implies in claiming reduced resistance on decreasing the number of turns). Particularly, in the case where the transformer is stated to be ideal then the length of the wires should make no difference as thermal resistance would be zero.
The only way to develop transformers problems even close to properly in the HSC is to start from the premise that Faraday’s Law is written in terms of magnetic flux and voltage, use the argument that changes in magnet flux will induce a voltage from which current may then follow (depending on circuit resistance). Further, transformers have a range of operational input voltages and currents for which they are optimised. It is only at these voltages for which they are optimised that highly efficient voltage and current transformations will be achieved.
A transformer (such as a computer charger) can be left plugged in with no load of it for days and consume very little power (and nor will it get particularly hot). Nevertheless, the transformer will get quite warm when a battery is plugged in for charge. Ohm’s Law cannot explain this situation as it would predict massive currents in a loop regardless of whether or not there is a transformer core there.
In an unloaded transformer input AC current is limited by non-ohmic factors (principally back EMF). As the load on the non-input (output) side of the transformer increase the transformer will carry more current and will be observed to heat up since the loops are then carrying more current.
Similarly, using Ohms law, it is not possible to explain why a fast running motor draws little current; but if you slow it down (by putting a load on it) then current draw increases dramatically. Other major factors are at play.
