Download PHYS 202 General Physics

PHYS 202 General Physics
Mid-term II review sheet
Winter 2010
The mid-term exam will be given on March 3, 2010. You may bring a 3 × 5 card with
appropriate equations (no text) written on it. Don’t forget to bring your scientific calculator,
as well.
The exam will be comprehensive, but will emphasize material covered since the first
midterm. This one will cover material up through simple circuits with light bulbs. There
may be a problem with series & parallel resistors. A sample examination will be posted on
the course web site.
Here are some study questions. Many of these have been given on exams in previous
years.
1. A meterstick is hung from one end and forms a physical pendulum. If the mass of the
meterstick is 0.150 kg and it is swinging back and forth with a total energy of 0.016 J,
then (A) what is the amplitude of oscillation, in degrees, and (B) what is the highest
speed at which the tip of the meterstick moves? (12.0◦ , 0.80 m/s)
2. A meter stick of mass 200 grams is balanced on a pivot in the middle of the stick, so
that there is no net torque. A mouse of mass 40 grams then jumps onto one end of the
stick and holds on.
pivot
(A) Find the inertia I of the system, about the pivot point. Treat the mouse as a
“point mass.”
(B) Find the angular acceleration, α, of the stick just after the mouse jumps on.
(C) Find the maximum velocity of the mouse. This will happen when he is at the
lowest point in his path of motion.
(D) Suppose the mouse is at rest and at equilibrium, so that the stick is vertical. If
we give the stick a little push, it will oscillate back and forth as a physical pendulum.
Find the period of oscillation.
q
3. As you know, he speed of a wave on a stretched string is given by v = F/µ, where
F is tension and µ is mass per unit length. Suppose the speed of a wave on a guitar
string is 120.0 m/s at a tension of 60.0 N. If we increase the tension to 65.0 N, what
is the change in the speed? Express the answer to the proper number of significant
figures. [125 m/s].
4. Shown below is a string on which two disturbances are traveling at a speed of 2 cm/ms,
in opposite directions. Draw the shape of the string (A) 1 ms later, and (B) 1.5 ms
later.
v
2
4
6
8
10
x, cm
V
x, cm
5. Pick one: waves do not transmit (a) information, (b) mass, (c) energy.
6. Sound travels through the air at a speed of 343 m/s at room temperature. Suppose a
flute plays a high “E” (f = 677 Hz). What is the wavelength of this sound in air? [0.507
m] Suppose this sound were transmitted into water. Would you expect the wavelength
to get shorter or longer? Explain.
7. A violin string 40 cm long vibrates at a frequency of 660 Hz, which is its fundamental
frequency. What is the velocity of a wave traveling along this string? [528 m/s.] If we
now touch the string 1/3 of the way from the end, so that a node appears there, what
is the frequency of the vibration?
8. The sound intensity of normal conversation is about 3.2×10−6 W/m2 . (a) What is the
sound power level in dB? The sound power level in Mac’s Grill at lunchtime is 70 dB.
What is the ratio of the intensities (in W/m2 ) of these two sound levels? [3.1 ]
9. A siren on top of a pole emits a sound at 300 Hz. A person 30 m from the pole hears
the sound at a sound level of 70 dB. (A) What is the sound power (in dB) at a point
120 m from the pole? Assume that the intensity obeys the inverse square law. [58 dB ]
(B) For a 70-dB sound, the amplitude PA of the pressure variation is 0.091 N/m2 . For
the 300 Hz, 70-dB sound, write an equation for pressure as a function of time, of the
form
P = PA cos (ωt) .
10. A tube 2.0 m long is open at both ends. What is the lowest frequency for a standing
wave in this tube? (That is, what is the lowest resonant frequency?) [86 Hz ] What
would the lowest resonant frequency be if the tube were closed on one end? Why are
organ pipes closed on one end?
11. Again, a 2.0-m-long tube is open at both ends. A standing wave is set up inside the
tube, so that there are three displacement nodes inside the tube. Draw a diagram
showing the node-antinode pattern, and find the frequency of the standing wave. [257
Hz ].
12. Two tuning forks are played together, and a beat frequency of 4.0 Hz is heard. One of
the tuning forks has a frequency of 256 Hz. What possible frequencies could the other
tuning fork have?
13. Suppose two cars have horns which produce identical sounds with a single frequency
of 300 Hz. One of the cars is stopped beside the road, and other other car passes it
at some speed, coming toward you, the observer. When they are close together, both
cars sound their horns, and you hear a beat frequency of 8.0 Hz. At what speed is the
moving car traveling? Assume the speed of sound is 343 m/s. [8.9 m/s].
14. Point charges +Q and -4Q are separated a distance of 10 cm, and the force between
them is 6.0 N. We now increase the distance to 30 cm. What is the new force?
15. Suppose a charge -Q is sitting at the corner of a square of side ℓ = 1.00 m, and an
electron is sitting at the opposite diagonal corner, as shown. In addition, two positive
charges Q are at opposite corners of the square. The magnitude of Q is 3.00 µC. (A)
Determine the electric field (magnitude and direction) at the electron’s position. (B)
Determine the force on the electron. (24,600 N/C, 3.9×10−15 N)
+Q
-e
-Q
+Q
16. An electron located near a plate at potential -10 V in a vacuum is released and moves
toward a plate at ground potential (0 V). When the electron hits the second plate,
what is its kinetic energy in electron volts? What is its speed? (1.88×106 m/s). Is
there enough information here to determine how much time it took the electron to
move from one plate to another?
17. An electric alarm clock used a 5.0-W motor and runs all day, every day. If electricity
costs $0.10 per kWh, determine the yearly cost of running the clock. ($4.38)
18. A cigarette lighter in a car is just a resistor that, when turned on, is connected across
the 12-V battery. Suppose a lighter dissipates 33 W of power. Find (A) the resistance
of the lighter and (B) the current that the lighter draws from the battery. (4.4 Ω, 2.8
A)
19. In the circuit shown, identical light bulbs are connected to a battery which maintains
a constant potential between points 1 and 2. What happens to the brightness of each
bulb if bulb B is removed from its socket? The circuit is now restored to its original
condition. What happens to the brightness of each bulb if bulb A is removed from its
socket? Bulb A is now replaced. What happens to the brightness of each bulb if we
now connect a wire between points 2 and 4?
A
5
B
C
4
3
1
2
20. For the circuit shown, calculate (A) the total current supplied by the battery, (B) the
voltage drop across the 6- Ω resistor, (C) the power dissipated by the 10- Ω resistor,
and (D) the total power supplied by the battery.
6V
6Ω
10 Ω
3Ω
21. 30.0 m of thin platinum wire is found to have a resistance of 12.0 Ω. What would be
the resistance of 100 m of the same wire? What would be the resistance of the 30-m
piece if you folded it in half and connected the ends, to form a conductor 15.0 m long?
(3 Ω)
22. A certain battery is found to have a voltage of 28.0 V across the terminals when
measured with a voltmeter. (A voltmeter requires almost no current to operate.)
When a 120- Ω resistor is connected across the terminals of the battery, the voltage
drop across the resistor is found to be 25.5 V. Determine the internal resistance of the
battery. Predict what the voltage between the battery terminals would be if we hooked
a 40- Ω resistor between the terminals. (11.8 Ω, 21.6 V)
23. Why can a bird sit on a high-voltage power line and not be shocked?
24. Here is drawn a circuit involving a battery and a light bulb. What will happen if we
“ground” a terminal of the light bulb by hooking to a metal pipe stuck in the ground?
Explain.
PIPE
25. A typical NiCad cell has voltage of 1.28 volts and a charge capacity of 1400 mA-hours.
Suppose we hook two of these cells in series, and connect them to a light bulb which
has a “hot” resistance of 9.0 Ω. What is the current through the bulb? How much time
could the NiCad cells keep this bulb lit? How much charge in C would pass through
the bulb in that time? How much energy does the bulb dissipate in that time? (0.28
A, 4.9 hours, 5040 C, 12,900 J).
26. In the circuit below, all bulbs are identical. Rank the bulbs in order of brightness. If
two bulbs are equally bright, indicate so. The battery supplies a total of 0.80 amperes,
and the current through bulb B is 0.25 amps. Find the current through bulb D.
A
B
C
D
E