Official 2011 AP Physics B and C Thread

Test Preparation AP Sciences
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<p>This is from PR’s SAT Physics, Test #3:
After a process during which an ideal gas did 7 joules of work, the internal energy of the gas increased by 12 joules. How much heat was added to or lost by the gas?</p>

<p>I mean, I see why the math makes it 19 J added. (dU=dQ+dW, rearrange for dQ=dU-dW).
But, why is W equal to -7? I know the concept that if something does work on the environment it’s negative, but this does not seem logical to me. . .
If the internal energy increased by 12, but then it used 7 for work, it SEEMS like it should be 12-7 J left.
Do I just have to memorize it and learn it like that, even if it seems wrong?</p>

<p>Does it seem wrong/weird to you guys too, or is it just me?</p>

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<ol>
<li>A dielectric increase stored charge. Does that also mean it stores ENERGY?
I just took a sparknotes practice test (#3) and it said that it doesn’t. . . doesn’t energy = charge in this case?</li>
</ol>

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<p>@Abrayo, for the last one, taking the derivative gives you angular velocity. To find the speed, you need to multiply it by the radius of the circle, which is 2.</p>

<p>Ahh. I’m taking this tomorrow!</p>

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<p>@Abraya: it increases capacitance, and U = 1/2(C)(V^2) should make it so that U is greater 0___0 anyone explain?</p>