One last Physics Question...similar to AAMC Q 77 on test 6

[chandler742]

When a Step Up transformer is used would occurs afterward to the current, voltage, and power?

a) Current increased, Voltage decreased, Power constant
b) Voltage increased, Current decreased, Power constant
c) Current increased, Voltage increased, Power increased
d) Voltage increased, Current decreased, Power increased

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[limit]

I don't see anything close to this anywhere in AAMC 6.

Answer is B, however, I highly doubt anything of this nature will be asked without being in the context of an explanatory passage.

 

[chandler742]

Check out last question on AAMC 6. Physical sciences..
It was the very last free response question.

And yes, you are responsible for this.

 

[limit]

Now I see it, it's #142. The way this question is worded allows you to answer it by process of elimination, since they actually describe what a step up transformer is (ie, voltage increases). All but the correct answer make very little sense.

It's actually an interesting question, the answer states that the purpose would be to minimize heat loss, however a transformer actually wastes energy because it heats up.

Is my answer correct nonetheless? Voltage increases, current decreases, I know that... As far as power...?

 

[limit]

BTW, according to the difficulty rating, this was the most difficult question in the physical sciences section.

 

[chandler742]

Here is the rationale behind transformers.

Power is lossed in transmission lines due to thermal heat loss(power lines have a fixed resistance). Heat loss is proportional to current and resistance.

P=I2R (Current squared times resistance)


The purpose of the transformer is to lower current so less power is dissipated in the transmission lines. This is done by increasing Voltage(10 to 1000 fold). Since, by conservation of energy you can't make energy something has to give. In the equation, P=IE. Current decreases when Voltage is increased, as long as the power remains constant.

As for step down transformers, this is the transformer you see on the neighborhood power poles. This decreases Voltage and increases current so you can use it run electrical appliances in your home.



Good luck on the MCAT. Feel free to ask questions. I am around about couple times a week.

 

[limit]

Yikes, you must scare a lot of Kaplan students. Let me just bring it down to common sense level so people a better understanding of this concept and actually go away with something more than a formula...

The real world purpose of transformers is to save electricity. Imagine a very long wire that has resistance (the longer, the more resistance). Although it can have a very low resistance, it really adds up with great distances, and this wastes energy. A low current requires less energy, so less energy is lost during the trip through the wire (this is the whole purpose of the high voltage / low current). Subsequently, the less energy is wasted, the more electricity for everyday appliances.

 

[chandler742]

Good point Limit, i'll buy that.

Although I would change one sentence of your explanation.

A low current "dissipates"(rather than requires) less energy, so less energy is lost during the trip through the wires.

As for our two styles, you are free to respond to my explanations in other concepts too. I think it would be a benefit to students if they saw two different interpretations of the same concept.

 

[freakazoid]

Hey chandler, feel free to post more questions. They do make me think, and I usually end up refining a concept I thought I had down pat before. Well, only if you have time. Thanks for what you've done so far, though.

 

[chandler742]

no problem, buddy. I'll cook something up for you.

 

[twesting2173]

how does heat affect resistance?