____________________________
Name
(Print clearly in block letters, please!)
Physics 6B
Quiz #7
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Perm Number
Winter, 2012
Instructions:
This is a closed book quiz. That means no notes, no books, no cell phones, and no
calculators. Just you, the quiz, and a pen or a pencil! Pen is much easier to grade, so if you
have one please use it.
Second, do not open your quiz until I say it is OK to begin. Once we begin, you will have 25
minutes, which should definitely be enough time.
Third, while waiting to begin, please PRINT your name and perm number CLEARLY on the
front page of your quiz.
Put all of your work on the quiz, even if you have done something on scratch paper.
Advice:
There are three problems, and the last one has 3 parts. None of the problems is very difficult
or tricky.
Being NEAT, REALLY NEAT, in what you write down on the quiz will help your grade,
although in a perfect world maybe it shouldn’t. Being neat will certainly increase your
chances of solving the problems though, so please don’t insist on being messy!
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1. (30 points) In an experiment, two protons collide head on. Initially the protons were very
far apart and each one was traveling with speed VO directly toward the other one. Calling the
mass of a proton m, and its charge +q, find out how close the two protons come to each other,
i.e. find their minimum separation distance. Do not plug in any numbers.
2. (15 points) A parallel plate capacitor with vacuum between its plates is connected to a
1,000 Volt battery. This capacitor has a capacitance of 12 µ F . How much energy is stored
in the capacitor?
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3. The following experiment is performed using the capacitor of problem number 2. The
battery is left connected, which means that the electrical potential difference between the
plates is held fixed at 1,000 Volts at all times. Then the plates are allowed to move toward
each other until the distance between them is half of the initial separation.
a. (15 points) Find the ratio of the electric field between the plates after they have come
toward each other to the electric field that existed between the plates in their original
position.
b. (15 points) Find the ratio of the energy stored in the capacitor after the plates have come
toward each other to the energy stored in the capacitor when the plates were in their original
position.
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c. (25 points) How much electrical charge flowed from the negative plate of the capacitor to
the positive plate of the capacitor during this process? (It flows through the battery.)
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