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Home Q & A Board Science-Related Homework Help Physics
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fireman5 fireman5
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Posts: 35
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12 years ago
The voltage has stepped, and the current hyas been stepped down. Ohm's law says that increased voltage will produce increased current. Is there a contradiction here,or does Ohm's law not apply to transformer?
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wrote...
#1
12 years ago
Ohm's law applies to resistors, not transformers.
wrote...
#2
12 years ago
You can't get more power out of a transformer (or anything else) than you put in - in fact there will always be some loss.

Let's say we have a step-up transformer which gives a secondary voltage twice that of the primary.

And let's say we apply 50V to the primary.  The secondary voltage will therefore be 100.  

The power rating of the transformer is, say, 100W.   The maximum current which can be drawn from the secondary is, therefore, 1A.  

Ok, let's do it.  Connect a 100 ohms load to the secondary.  We now have 100 watts out.

The power in is also 100W, plus a little (because of small losses, but we'll ignore them).

100 watts in, at 50V, means 2 amperes in.  


100 watts in, 100 watts out.

100 watts in by means of 50V at 2A.  100W out by means of 100V and 1A.

No contradiction.
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lepurplebirdlepurplebird
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#3
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12 years ago
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wrote...
#4
12 years ago
Ohm's law apparently does apply to transformers. A  statement that the losses are small and ignored, is not entirely true in practice. Actual transformers do require some means of cooling because of those various heat losses which are predictable. Regardless of the effects of induction some RI^2 heating will still occur according to Ohm's law due to the resistances of the transformer windings. True, small transformers will usually take care of themselves as- is ,where convective or forced air currents will dissipate the small amounts of heat. However, larger power transformers have various cooling means included within their designs, one style being convective oil with finned, ambient air-cooled heat exchangers.
wrote...
#5
12 years ago
ohms law is for direct current, transformers work with alternating current, the difference being that besides having a resistance they also have reactance which is like resistance but it changes was the frequency changes.reactance also can be inductive or capacitive which have opposite effects.
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