Phys104 - How Things Work
University of Maryland, College Park
Spring 2013, Professor: Ted Jacobson

examples of notation:
E1.4 means "Exercise 4 of Chapter 1"
P1.8 means "Problem 8 of Chapter 1"
C1.11 means "Case 11 of Chapter 1" - for these see Cases, at 4th edition Textbook Companion Website
S5.1 means "Supplementary problem 1 for homework 5, written out here".

Your solution must explain your reasoning and show your method of computation in order to earn credit.
Explanations can be very brief as long as they are clear.
Numerical answers without explanation will receive no credit.

HW11 - due at the beginning of class, Thursday 5/09/13

On material already covered by Thursday, 5/02
E14.36 (Maser-1)
E14.37 (Maser-2)
E14.41 (LED-1)
E14.42 (LED-2)
C15.7 (optical fibers) [See p. 495-6]

On material to be covered Tuesday, 5/07. You may ask about them in class.
E16.21 (lead and X-rays)
E16.24 (MRI electromagnetic radiation)
E16.25 (MRI and bone)
E16.28 (magnetic field strength and MRI)

S11.1 (a) How do X-rays and gamma rays for radiation therapy differ from X-rays for imaging?
(b) How do radiation therapy photons kill cancerous cells?
(c) Name two ways that radiation therapy photons can be produced.

HW10 - due at the beginning of class, Thursday 05/02/13

E14.10 (refraction of diamond)
E14.13 (colored oil films) [See p. 451-2]
E14.20 (yellow paint)
E14.24 (light emission from excited state of sodium)
E14.28 (incandescent vs. neon lamp colors)

E15.28 (color of DVD surface) [See p. 489; also, this is basically the same as the diffraction grating I explained in class.]

C14.1 (color of sky) [See section 14.1]
C14.2 (electronic flash) [See section 14.2]
C14.4a,c,e only (interior house paint) [See section 14.2]

S10.1 In order to fold a long optical path in binoculars into a small space the light must be reflected several times. This is done, for example, by a "double Porro prism" shown here: (taken from
There are no mirrored (metallic reflecting) surfaces --- only glass prisms are used. How are the reflections accomplished without mirrors? [See p. 495]

HW9 - due at the beginning of class, Thursday 04/25/13

E13.14 (spinning magnet) Make this two parts, (a) in the horizontal plane in which the magnet spins, and (b) above the plane. (Hint: Think of how the magnetic field wiggles to infer the direction of the magnetic field in the wave, by analogy with the electric case (see the applet in the notes for 04/11/13). Use the relation between electric and magnetic fields in an electromagnetic plane wave, p. 428, to find the direction of the electric field.)
E13.16 (AM vs. FM fadeout)
E13.22 (oven vs. microwave cooking of a potato) Take the question to be this: how is heat deposited in the potato using the two cooking methods?
E13.28 (synchrotron radiation)

C13.6   (cordless microphone)

S9.1 Polarizing sunglasses, or a polarizing filter over a camera lens, can enhance the contrast between the sky and clouds, by darkening the sky more than the clouds. This and other uses of polarizing filters in photography is explained here: It states but does not explain in this article why the light from the blue sky is somewhat polarized. Try to explain why. Consider the following situation:

sunlight --------------molecule

The dashed lines represent sunlight that scatters from a molecule into the perpendicular direction towards the viewer. Explain why the light that reaches the viewer is 100% linearly polarized,
and say what the direction of polarization is.
(Hint: Remember this key fact about polarization: the electric field vector is always perpendicular to the direction the light is traveling. Consider two different cases for the incoming sunlight: (i) polarized vertically on this page, and (ii) polarized perpendicular to this page. Think of the molecule as an antenna whose charge is shaken by this incoming electric field, and then emits its own waves like a dipole antenna.)

S9.2 View (a) the LCD Monitor Teardown video, and (b) the "Creating Electronic Images" and Components of an LCD" sections of the Optics101 link (use the Navigation button), linked below. Then write a brief description (a few sentences) of how an LCD works. (I will also discuss this briefly in class on Tuesday.)

LCD Monitor Teardown

HW8 - due at the beginning of class, Thursday 4/18/13

E11.29 (compass "motor") (The answer is in the back of the book...)

C11.12 a,b,c only (electric shavers)

S8.1 An induction stovetop puts heat directly into a cooking pan, without even making the stovetop hot (a small amount of heat transfers by conduction from the pan to the stovetop). Read the Wikipedia article, and then explain how the induction stovetop works.

S8.2 Read the section in the textbook on induction motors, and watch this video: Then write a brief explanation of the principle of operation of an induction motor. (A few sentences will suffice.) 

S8.3 Read (all four panels of) this explanation of the Hall effect: You can ignore the equations.
[I had a copy of this web page, in case this site is unavailable...]

HW7 - due at the beginning of class, Thursday 4/11/13.

Note: Some of these problems involve material that will be covered in class Tuesday. I suggest you read ahead in the book,
think about the problems, and come to class Tuesday with any questions you have.

E11.2 (distance dependence of magnetic force between button magnets)
: What this question is asking is why does the force between dipoles decrease FASTER than the inverse of the square of the distance between them?

E11.4 (why don't magnet and iron repel?) Expand question: Explain why they attract no matter which pole of the magnet is next to the iron pipe.
E11.6 (hammering or heating a magnet) Expand question: In the case of heating, address two cases:
(a) if the magnet temperature is raised above the Curie point, and (b) if the temperature remains below the Curie point.
E11.8 (net force on a compass in a uniform field)
E11.19 (magnetic strip reader)
E11.24 (transformer in amplifier)
E11.26 (current in transformer coil) Note: The book gives a formula for this, but I don't quite like the accompanying explanation.
             Perhaps a better way to say it is that the work done by the power supply on the charges in the primary coil is transferred
             via the magnetic field to the secondary coil, where the same amount of work is done on the charges there. As with DC
             circuits, the instantaneous power is VI, where V is the induced emf (see p. 361), which plays the same role as the voltage.
             Setting the power in the primary equal to the power in the secondary gives you the answer to this question.

C11.2 (electromagnetic trash sorter) (Hint for (b): See  Check your understanding #4 of section 11.2, p.362.)
C11.5 (audio speaker)

S7.1 A substation transformer steps down the 500,000 V AC transmission line voltage to 5000 V for delivery to neighborhoods.
If the primary coil has 2000 turns of wire, how many turns must the secondary coil have?
A) 20
B) 100
C) 250
D) 20,000
E) 200,000

HW6 - due at the beginning of class, Thursday 4/4/13.

E10.14 (car battery voltage) To be specific, compare the energy of one Coulomb of charge.
E10.20 (electric field at battery terminal)

E10.32 (half a plug)

E10.40 (battery testing)

P10.2 (electrostatic force on socks)

P10.23 (voltage drop in extension cord) (Answer at back of book - you must supply the reasoning.)
P10.24 (wasted power in extension cord) ("is wasted" means "goes into heating the wire instead of the oven") 

C10.2 (Van de Graaff generator)
C10.3 (spark lighters)

S10.1 A balloon rubbed on your hair will acquire negative charge, and will then stick to a neutral surface like a wall.  
          Explain the origin of the force of attraction between the charged balloon and the neutral wall.

S10.2 You stick two pieces of adhesive tape on a glass window and then pull them off suddenly. If you now hold the tape pieces
near each other, will they attract, repel, or do nothing to each other. Why?
(Try it! Use 3 inch pieces of tape.)
S10. 3 Suppose you have two identical metal spheres on insulating stands and a balloon. You rub the balloon on your hair and then touch it to one sphere, charging the sphere. Then you place the two spheres in contact. As a result of these operations, which of the following will happen, and why:
A) the two spheres will wind up neutral
B) the first sphere will remain charged and the second sphere neutral
C) the second sphere will wind up charged and the first sphere neutral
D) the second sphere will pick up a small fraction of the charge from the first
E) the two spheres will wind up equally charged

S10.4 Suppose you have the same two spheres of S10.3, both initially neutral. You place the spheres in contact, rub the balloon on your hair, and then bring the balloon near to but not touching one of the spheres. You then separate the spheres, and then remove the balloon. As a result of these operations which of the following will happen, and why:
A) the two spheres will each wind up neutral
B) the two spheres will wind up oppositely charged
C) the two spheres will wind up with equal charges
D) the sphere closer to the balloon will become charged and the farther sphere will not
E) the sphere farther from the balloon will become charged and the closer sphere will not

S10.5 One month PEPCO billed me for $112.46 for 920 kWh of electrical use. (a) How many joules of electrical energy did I use? (b) How much am I paying per million joules?
(Food for thought: A million joules is about equal to the work (mgh) it takes to vertically lift 100 kg (220 pounds) a distance of 1 kilometer (0.6 miles). Does this price for a million joules seem high or low to you?) (Note: 1 kWh = 1 kilo-watt-hour)

HW5 - due at the beginning of class, Thursday 3/7/13.

E8.8 (airplane air conditioning)
E8.20 (plant growth and the second law of thermodynamics)

P8.6 (freezer work) Modify the problem:
a. By how much does the entropy of the food decrease?
b. By how much does the entropy of the room increase?
c. How much heat is added to the room?
d. The answer to c is greater than 100 J, since 300K is greater than 260K. 
Energy is conserved, so the source of the extra heat must be the work done by the compressor. How much work is that?

P8.8 (heat pump work) Modify the problem:
a. How much does the entropy of the room increase?
b. How much does the entropy of the outdoor air decrease?
c. How much heat is extracted from the outdoor air?
d. The answer to c is less than 1000 J, since 260K is less than 300 K. The source of the extra heat must be the work done by the compressor. How much work is that?

P8.10 (airplane engine work) Modify the problem:
a. What fraction of the heat leaving the burned gases is discarded as heat to the air?
b. What fraction 
of the heat leaving the burned gases is converted to work?
(Hint: Let Q represent the heat that flows from the burned gases, and use algebra...)

C8.2 (refrigerator)

S5.1 If a car engine operates at 600 K in ambient air at 300 K, what is the maximal work it could possibly obtain from 1000 J of heat at 600 K?

Note on P8.6,8,10,S5.1: Solve these problems using the fact that the entropy change is given by Q/T,
the fact that in the operation an ideal heat pump or engine, the total entropy is unchanged.
Refer to the lecture notes of 2013 and 2010 for a discussion of these things. (The textbook has equations
(8.1.2) and (8.2.1) that refer to work consumed in pumping heat and work provided by a heat engine,
depending on the temperatures involved. The equations are derived using the entropy change Q/T. I
think it is more instructive to solve these problems making direct use of the entropy changes.

HW4 - due at the beginning of class, Thursday 2/28/13.

E9.2 (period of swinging clothing rack)

E9.9 (pitch of guitar string) Modification: Explain, in terms of inertia and/or restoring force as appropriate, why the pitch is higher for the cases of
(a) smaller mass, higher tension, and (c) shorter length.

E9.14 (organ pipe filled with helium)

E9.30 (gong overtones)

E9.32 (string bass body)

C9.9 (trumpet) Omit part (d). (Part (d) is interesting, but difficult to answer well, I think.)

S9.1 What is the role of the escapement mechanism in a pendulum clock or spring wound wristwatch?

S9.2 A bat can hear sounds at 100,000 Hz. (a) What is the period of one sound vibration at this frequency? (b) Approximately what is the wavelength of this sound?

S9.3  Figures 9.2.3,4 illustrate the string motion in the first three vibrational modes (the fundamental and the next two) of a vibrating string. Draw similar diagrams for the air pressure deviations in the first three modes of (a) a pipe open at both ends, and (b) a pipe closed at one end and open at the other.
Bonus extra credit part
for those who want a challenge: (c) Suppose the pipes in (a) and (b) have the same length, and let f_0 denote the frequency of the fundamental mode of the pipe that is open at both ends. What are the frequencies of the first three modes of the pipes in (a) and (b)? Explain your answer.

S9.4 A guitar string that normally vibrates with a fundamental frequency of 110 Hz is also capable of vibrating at 330 Hz without changing the length or tension. When that higher frequency vibration occurs, the string is vibrating
A) with 3 times its normal amplitude of oscillation.
B) with 1/3 of its normal amplitude of oscillation.
C) as the fundamental mode of a string that is 3 times as long.
D) as the fundamental mode of a string that is 1/3 as long.

S9.5 If you blow across the end of a tube open at both ends it will sound a tone with some characteristic pitch. If you then cover one end with your hand and again blow across the other end the pitch will be
A) one octave higher because the closed end becomes a pressure node
B) one octave higher because the closed end becomes a pressure anti-node
C) one octave lower because the closed end becomes a pressure node
D) one octave lower because the closed end becomes a pressure anti-node
E) the same because the length is the same

HW3 - due at the beginning of class, Thursday 2/21/13.

E5.16 (lowest thermometer readings)
E7.7 (fireplace convection)
E7.12 (how space shuttle dumps heat)
E7.20 (wine bottle in ice water)
E7.24 (steamed vegetables)

E7.30 (disappearing ice)

P5.6 (net force on submerged log)

C7.3 (electric oven) and/or
C7.8 (duck warmth) (It's important for this case that both fat and oil are relatively poor heat conductors, compared to water or body tissue.)
C7.9 (tight-fitting metal parts) (See page 229 for a discussion of thermal expansion, and the demo,  I1-11 THERMAL EXPANSION - BALL AND HOLE)

S3.1 Air is mostly composed of oxygen molecules and nitrogen molecules. The mass of a nitrogen molecule (N_2) is 7 times the mass of a helium atom. If the average speed of a nitrogen molecule at room temperature is v, what is the average speed of a helium atom at room temperature?

S3.2 Why do metals tend to conduct heat better than non-metals?

HW2 - due at the beginning of class, Thursday 2/14/13.

E2.12 (wire cutter)
E5.4 (grocery freezer displays)

P2.6 (nutcracker)
P5.4 (air compressor pressure)
P5.5 (fridge pressure change) To simplify this problem, instead of finding the change of the pressure, find the
         of the cold pressure to the room temperature pressure.
(Note: Don't forget to use the absolute temperature scale!)

C1.5 (takeoff and landing on aircraft carrier)
C5.2 (bass air bladder) Assume the fresh and saltwater bass have the same mass.

S2.1 If a car's engine does work W to accelerate it to a speed of 10mph, how much more work would be required to reach a speed of 30mph, assuming perfect efficiency, i.e. neglecting wasted heat, friction, and air resistance?

S2.2 In a classroom demonstration, I broke a 2x4 piece of wood using a hydraulic press. If the force applied to the wood must be 5000 N to break the wood, the point of application of the press on the wood moves through 5 mm, and my hand applying the force on the lever moves through 50 cm, how much force must I apply?

S2.3 The Mariana Trench is the deepest part of the earth’s oceans, and lies around 10,000 meters below sea level. How many times atmospheric pressure is the pressure at the bottom of this Trench? (Hint: In class we computed how many meters high a column of water must be if its weight produces atmospheric pressure at the bottom.)

S2.4 Consider a block of stone with a mass of 2500 kg used in building the Great Pyramid of Khufu in ancient Egypt. In order to float that block on a raft on water across the flooded Nile valley, what volume of water would have to be displaced, neglecting the mass of the raft itself?

S2.5 (melting icebergs) Ice floats on water because when water freezes and becomes ice, the density drops by about 10%. When a floating iceberg melts, the sea level does not go up (or down), but rather stays exactly the same. Explain clearly why this is so. (By contrast, if ice initially on land slides into the ocean and melts, then of course the sea level rises.)

HW1 - due at the beginning of class, Thursday 2/7/13.

E1.4 (toothbrush drying)
(Book: Why does tapping your toothbrush on the sink dry it off?)

E1.8 (carousel velocity)
(Book: Why is your velocity continuously changing as you ride on a carousel?)
Let's clarify and expand the question: (a) How is your velocity vector is changing?
(b) What is making your velocity change?

E1.10 (coffee grinder)
(Book: One type of home coffee grinder has a small blade that rotates very rapidly and cuts the beans into powder.
Nothing prevents the coffee beans from moving so why don't they get out of the way when the blade begins to push on them?)

E1.14 (falling ball)
(Book: A ball falls from rest for 5 seconds. Neglecting air resistance, during which of the 5 seconds does the ball's speed increase most?)

E1.22 (force on Metro train cars)
(Book: What is the net  force on (a) the first car, (b) the middle car, and (c) the last car of a metro train traveling at constant velocity?)
Hint : What is the acceleration of the cars?

E1.38 (roller skating uphill)
(Book: When you're roller skating on level pavement, you can maintain your speed for a long time.
But as soon as you start up a gradual hill, you begin to slow down. What slows you?)

Let's make this problem two parts: (a) What external agent exerts the horizontal force that decreases your horizontal velocity?
(b) What external agent exerts the vertical force that initially increases your upward, vertical velocity as you start rolling up the hill?

P1.8 (sprinter acceleration)
(Book: A sprinter can reach a speed of 10m/s in 1 s. If the sprinter's acceleration is constant during that time,
what is the sprinter's acceleration?)

P1.10 (mass and weight)
(Book: How much does a 60 kg person weigh on earth?)
(Give your answer in Newtons. Use the approximate value g = 10m/s2.)

(hydroelectric vs. human power)
(Book: As water descends from the top of a tall hydroelectric dam, its gravitational potential energy is converted to
electric energy. How much gravitational potential energy is released when 1000 kg of water descends 200 m to the  generators?)
Consider the book's problem to be part (a). Add two parts:
(b) If a human can do work at at rate of 400 watts over an extended period, how long would it take them to deliver
the same total energy as the ton of water falling off the dam? (c) How many pieces of cherry pie
(see page 30) would you have to consume to obtain the energy required to do this much work?

(work when sanding)
(Book: You're sanding a table. You must exert a force of 30 N on the sandpaper to keep it moving steadily
across the table's surface. You slide the paper back and forth for 20 minutes, during which time you move it 1000 m.
How much work have you done?)
Consider the book's problem to be part (a). Add part: (b) What is the average power you have supplied?

E2.22 (horse and cart)
(Book: A horse does work on a cart it's pulling along a straight, level road at constant speed. The horse is transferring energy to the cart,
so why doesn't the cart go faster and faster? Where is the energy going?)

E2.28 (friction on sled)
(Book: If you are pulling a sled along a level field at constant velocity, how does the force you are exerting on the sled compare
to the force of sliding friction on its runners?)
Make this two parts: (a) if you are pulling horizontally, (b) if you are pulling diagonally upward.