The Official AP Physics C Thread

Test Preparation AP Test Preparation
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<p>^ In this case, the potential energy of each particle is changed into kinetic energy. So K(final)=qV. It would be a ratio of K lithium / K helium = (3m)(Vo)/(2m)(Vo).</p>

<p>Is it E) 3/2 ?</p>

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<p>yes redfire, E is right?</p>

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<p>okay, so you know that U=Vq
so you can find the kinetic energy both particles by multiplying the voltage difference by their charges. </p>

<p>so i think its just E</p>

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<p>What exactly does grounding do?</p>

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<p>Grounding something makes its net charge zero</p>

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<p>grounding sort of balances charges. Say you have an excess of negative charge on a sphere. When you ground the sphere, the electrons will be attracted to the nuetral ground, and they will travel through your grounding device. That is why I wouldn’t recommend touching a highly charged sphere while your feet are on the ground. The current will travel through your body.</p>

<p>Someone else on the baord can probably explain this better than me, though.</p>

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<p>Ok thanks :)</p>

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<p>another one, involving work:</p>

<p>Two protons and an electron are assembled along a line. The distance between the electron and each proton is a. WHat is the work done by an external force in assembling this configuration of charges?</p>

<p>A) -2ke^(2)/a
B) -3/2 ke^(2)/a
C) 1/2ke^(2)/a
D) 3/2ke^(2)/a
E) 3ke^(2)/a</p>

<p>I am lost, plz help?</p>

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<p>…If anyone’s interested, the curves for both C tests are here:</p>

<p><a href=“http://faculty.trinityvalleyschool.org/hoseltom/handouts/Scoring%20the%20AP%20Physics%20C%20Exam.pdf[/url]”>http://faculty.trinityvalleyschool.org/hoseltom/handouts/Scoring%20the%20AP%20Physics%20C%20Exam.pdf&lt;/a&gt;&lt;/p&gt;

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<p>the work question:
so basically this is asking, “if all of these charges are infinitely far apart, what would be the work required to bring them into this position?”</p>

<p>Just this definition allows you to see that what they are really asking for is the negative of the magnitude of the potential energy of the system. You can compute the PE by adding the individual potential energies between each pair of particles(remember that the sign of the charge matters)</p>

<p>then just take the negative</p>

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<p>im still not getting it =(</p>

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<p>^so the answer is A right?</p>

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<p>F= kqq/a + kqq/a (for both protons) - Coulombs law
= 2ke*e/a
Work done= -2ke^2/a</p>

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<p>nopee, answer is B</p>

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<p>

</p>

<p>you need to have three terms, not two, since you have three particles</p>

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<p>is it D? i just did it and thats what i got</p>

<p>Edit: lol nvm jsut saw your other post</p>

<p>AHHHH I was wrong, you dont take the negative. Because the work required to move something from infinity to the position IS the definition of PE</p>

<p>its jsut -ke^2/a - ke^2/a + ke^2/2a</p>

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<p>I guess this matters according to Collegeboard, but the E&M curve now is about 51/90 instead of around 49/90 if anyone wants to know. It was raised after a college comparability study.</p>

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<p>I heard from someone that they kinda remade the exam this year because it was too hard? So the curve will be harsher, but the exam will be easier…anyone else know about this?</p>

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<p>

</p>

<p>all of this doesnt matter at all, they have a certain percentage that they want to get 5s, 4s, etc. So just do your best and don’t worry about the curve</p>

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<p>^ Good advice. Just study what you can and do what you can. Don’t worry about others.</p>