Electric Power Question

[SDPHD]

I can't seem to get my head around this question.



Thanks for any help.

[Lasercookie]

The street pole transformer is the transformer right before the house.
The voltage in the secondary is 110 V RMS.

The voltage in the primary will be less than 9 kV RMS. The current in the primary will be less than 1.5 A.

It'll be less because there's probably a power loss in the lines. We are told that it's very close to 9 kV, so I'll just use that value.

The other possible trick is that the 1.5 A quoted may not be the RMS value. We know that the voltage's are RMS, as it is explicitly stated.

We are also told the transfomer is ideal, so the primary power will be the same as the secondary power (and the transformer equations will also apply).



We are trying to find current in the secondary ()
We also know that it's a step down transformer. So a decrease in voltage will result in an increase in current, as power is constant (I=P/V). That can give us a clue if our answer is correct.





Does that answer make sense? I don't know.

I want to try another method:

We are given the primary current and voltage for the primary circuit.
We can figure out the power in the primary. This will be equal to the power in the secondary.
We know the voltage in the secondary (and now the power in the secondary). Therefore we can find out the current in the secondary.








Same answer, more intuitive reasoning. It makes sense the answer is the same, after all the transformer equations are derived from the power relationship.

Not entirely sure if the value for current is in RMS though, but I'm pretty sure it is.

[SDPHD]

Thanks for the reply, I can't believe I didn't get it...

Not the confidence boost I was looking for a couple of weeks off the exam.

[ARMYMAN0010]

Thanks for the reply, I can't believe I didn't get it...

Not the confidence boost I was looking for a couple of weeks off the exam.

Don't feel so bad. The question was pretty much all fluff, with the only relevant bits being the last few words! There's pretty much one of these questions every year, and they're basically all the same. Do the transformer question on some of the old papers and it will all become pretty intuitive, if it isn't so now

[Lasercookie]

Thanks for the reply, I can't believe I didn't get it...

Not the confidence boost I was looking for a couple of weeks off the exam.

Nah dude, don't kick yourself too hard about it. These power transmission questions are the ones I tend to screw up myself - they present you all this information so it looks all complicated but it's really quite simple if you look at it at the right perspective. In my opinion the right perspective is to start off by thinking of every single one of these power questions in terms of power.

I tend to forget this, but you'll notice I did approach it power first in my second way of working it out. It resulted in much simpler working out in my opinion.

We still have 4 weeks till the physics exam - plenty time to boost your confidence.

[Bozo]

Lol, more than enough time. Taiga did 15 prac exams in one day and ripped out A+       Class.

[Lasercookie]

Lol, more than enough time. Taiga did 15 prac exams in one day and ripped out A+       Class.

O.o 15 in one day, that's insane - I barely have the motivation to do more than 3 exams in one day :/

[paulsterio]

Laseredd, you are right, the current is RMS because you derived it from a value for Power

[Lasercookie]

Laseredd, you are right, the current is RMS because you derived it from a value for Power

I know that if you plug in RMS values you get out an RMS value, but I was unsure if that current value they give you for the primary coil (1.5A) was actually an RMS value. It doesn't explicitly state that's it is an RMS value - I just assumed that it was.

If it was the peak value then you'd have to convert to RMS, which would reduce the answer. This is probably just another case of me reading way too much into a simple question.

Do you know what we're supposed to assume if it's not stated? (this was explained at the revision lecture I went to on the weekend :| forgot what was said). Either way, VCAA is a lot less dodgy with this RMS stuff, they usually explicitly state stuff.

[paulsterio]

Considering that EVERYTHING in the question is RMS, I would assume that it's RMS, but VCAA want to be as specific as possible, because they don't want arguments and people pointing the finger at them and their incompetencies, so they will generally be quite clear with that sort of stuff