University of Maryland
Department of Physics
David Hammer
Physics 122
SAMPLE Exam #1
Answer all questions on these sheets. Please write clearly and neatly: We can only give you
credit for what is on the paper! Put your name and section number on every page.
There are nine problems. The first six are all multiple choice. There’s no partial credit for
these – just choose the best answer and indicate it clearly.
For the last three problems, you need to show your reasoning, in words, mathematics, and/or
diagrams. These problems all require explanation, whether the word “explain” appears in
the question or not! Full credit is for a correct answer with a clear explanation. You may get
partial credit, but only if we can follow your reasoning. But you’ll get no credit at all,
regardless of the answer you give, if we can’t.
Please: Think about the physics, not about the psychology of how I write exams!
__________________________________________
Name (printed)
_______________
Section #
At the end of the exam, rewrite and sign the pledge: I pledge on my honor that I have
not given or received any unauthorized assistance on this examination.
___________________________________________________________________________
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Signature and date
Physics 122
Midterm Exam 1 – p. 2
Name_____________________
Section____________________
Multiple choice questions.
Just the answer counts for these. (8 points each)
1) These are two different sized metal spheres connected by a wire. Somehow we manage to
put the same excess positive charge on each sphere, and then we connect them with a wire,
with a sensitive ammeter to measure current. What should we expect for the current
through the wire?
a) There should be no current in the wire.
A
b) There should be current in the wire for a short time toward the
smaller sphere (increasing the excess charge there)
c) There should be current in the wire for a short time toward the larger sphere (increasing
the excess charge there)
d) There should be continuous current in the wire due to the potential difference between
the spheres.
e) There is not enough information to answer the question.
2) These are two identical metal spheres, connected by a wire with a sensitive bulb. We bring
a strong charge Q near the sphere on the left, hold it there for a while, and then we take it
away. If the bulb is sensitive enough to respond to tiny amounts of current, when would
the bulb glow?
a) While the charge Q is near.
b) While the charge Q is far away.
c) Once briefly when we bring the charge Q near.
Q
d) Twice briefly, once when we bring the charge Q
near and once when we take the charge Q away.
e) Not at any time.
3) A charge Q is a distance R from one end of a metal bar with no net charge. The metal bar is
length d. How would the size of the net electrical force by the charge Q on the metal rod
depend on R and d?
R
Q
d
a) There is no net electric force, no matter the values of R or d.
b) There is a net electric force, but its strength does not depend on R or d.
c) Increasing R would make the net force weaker, but increasing d has no effect.
d) Increasing R or d would make the net force weaker.
e) Increasing R would make the net force weaker; increasing d would make it stronger.
f) Increasing R would make the net force stronger; increasing d would make it weaker.
Physics 122
Midterm Exam 1 – p. 3
Name_____________________
Section____________________
4) A 4 Ohm resistor is in series with a 400 Ohm resistor, as part of some circuit. With that
information, we know (Pick one from each box, 4 pts each)
4Ω
a) I4Ω = I400Ω.
400 Ω
b) I4Ω = 100 I400Ω.
c) 100 I4Ω = I400Ω.
The rest of the
circuit…
d) Nothing about the currents.
e) ΔV4Ω= ΔV400Ω.
f) ΔV4Ω= 100ΔV400Ω.
g) 100ΔV4Ω= ΔV400Ω.
(I4Ω is the current through the 4 Ohm
resistor, ΔV4Ω is the potential
difference across the 4 Ohm resistor;
etc for the 400 Ohm resistor.)
h) Nothing about the potential differences.
5) All of the bulbs are identical. Rank them in order of brightness.
A
a) bulb D would be the brightest, then A, then B and C which
would be equal.
D
B
C
b) bulb A would be the brightest, then B and C, which would
be equal, then D would be the dimmest.
c) bulb A would be the brightest, then D, then B and C, which would be equal.
d) bulbs A and D would be equal and brighter than B and C, which would be equal.
e) bulb A would be the brightest, and B, C, and D would all be equally bright.
6) I charge a 5 mF capacitor to a potential difference of 10 V and then I put it in series with a
switch and a light bulb as I’ve shown in the first circuit. I then turn on the switch and the
light bulb glows brightly at first then fades to nothing in 3 seconds. Next I use two 5 mF
capacitors. I connect them in parallel, charge the pair to a potential difference of 10 V, and
put them in series with the same bulb and switch. I then turn on the switch.
Compared to the first circuit, how brightly and how long will the bulb glow in the second ?
a) The bulb will glow more brightly at first and go out in 3 seconds.
b) The bulb will glow more brightly at first and go out in 6 seconds.
c) The bulb will glow more brightly at first and go out in 12 seconds.
d) The bulb will start out at the same brightness and go out in 3 seconds.
e) The bulb will start out at the same brightness and go out in 6 seconds.
f) The bulb will start out at the same brightness and go out in 12 seconds.
Physics 122
Midterm Exam 1 – p. 4
Name_____________________
Section____________________
Short answer questions, with explanations. For these, you do need to explain.
7) (12 points) One of Kirchoff’s rules says that the current in to any point in the circuit must
be the same as the current out of that point in the circuit: Iin = Iout. So, for example, if
there’s a wire with current toward
Does the current in to
a point in a circuit and two with
this plate equal the
current away from the point, we
current out of it?
have I1 = I2 + I3.
But if you look at a capacitor plate,
there’s only one wire.
How can Iin = Iout for a capacitor plate, or does it?
Explain.
Physics 122
Midterm Exam 1 – p. 5
Name_____________________
Section____________________
8) (20 points)
a) Find the total electric force by the two charges Q on a third charge, q, placed a distance r
from each of the first two.
Q
r
q
60°
r
Q
b) Think of those same two Q charges again, in the same positions, but this time think of
them as lying along a line (as I’ve drawn). I’ve measured the distance of q from that
line, and I moved it in half as far away.
Someone who’s been paying attention gives the following argument: “That means the
kq q
total force is 4 x as large, because F = 12 2 . You’re bringing it twice as close, and that
r
makes the denominator 4 times smaller.”
Give your response to that argument, either to agree or disagree.
€
Q
q
Q
q
Physics 122
Midterm Exam 1 – p. 6
Name_____________________
Section____________________
c) Physicists calculate the force due to more complicated arrangements of charges in the
same way as you did in part a (plus some calculus tricks, but it’s basically the same
thing). That would be a way to find the force on a charge q due to the charge distributed
on the two plates of a capacitor.
But it’s often easier to find the force using other
information.
Suppose the positive plate of the capacitor is at a
potential 2000 V higher than the negative plate, and
suppose the plates are 10 cm apart.
Use that information to estimate the size of the force on a
charge q = 2 x 10-7 Coulombs moving between the plates.
q
Physics 122
Midterm Exam 1 – p. 7
Name_____________________
Section____________________
9) (20 points) The Van de Graaff raises a small amount of charge to pretty high potentials.
Working perfectly on a dry day, it can get to be as high as 100,000 Volts above the potential
of ground.
But there’s very little charge involved in that: At 100,000 V, the Van de Graaff has only
about 10-6 Coulombs of excess charge. And if you let that charge leave, it takes a long time
for the Van de Graaff to get back to 100,000 V — the Van de Graaff is very slow to get
another 10-6 Coulombs back on the dome.
a) You’ve seen me put my hand near the Van de Graaff and get a little shock—it can sting
just a bit. Supposing I do this on a dry day with the Van de Graaff working at its best,
about how much energy would you expect to go into my body?
b) Usually I just touch the Van de Graaff with one hand, without touching anything else.
Sometimes, though, my other hand is holding a wire or some other metal that connects
me to ground. When would I get a worse shock, when my other hand is connected to
ground or when it is not?
Physics 122
Midterm Exam 1 – p. 8
Name_____________________
Section____________________
In the US, the two sides of an electrical wall outlet have a potential difference of about 110
to 120 V; estimating for this problem you can just call it 100 V. But the amounts of charge
involved are much, much higher than on the Van de Graaff—it’s easy to have a several
Coulombs of charge go by every second, without any drop at all in the potential difference.
c) If I were to touch one side of a wall outlet with one hand and the other side with the
other hand (or, more likely, if I touched wires that are connected to either side of the
outlet), I’d have about 100 V potential difference across my body. Suppose my
resistance is 10,000 Ohms. How much energy would go into my body per second?