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ANSWERS TO: PHYSICS 109 B (Closed Book) Tuesday, 17 April 2001

Please remember to show appropriate units where needed.

1. reappear
2. change its polarization so that the glasses are still effective
3. become dimmer but the fish beneath the water will also become dimmer
4. remain almost completely absent.

Answer 1. The glare will reappear when you turn your head(answer 1)

 

2. Ultraviolet light can cause chemical damage to your skin while infrared light does not because:

5. Ultraviolet light has longer wavelength than infrared light
6. Ultraviolet light has more photons than infrared light.
7. A photon of ultraviolet light carries more energy than a photon of infrared light.
8. Ultraviolet light is more intense than infrared light.

3. A resistor is essentially a poor conductor of electricity. When you send current through a resistor, that current always experiences a voltage drop and never a voltage rise. One way to understand this effect is:
1. Less current leaves a resistor than enters it so the voltage must drop as well.
2. More current leaves a resistor than enters it so the voltage must drop to compensate.
3. Current can only flow forward through a resistor, never the other way.
4. It is easy to turn electric energy into thermal energy but not the other way around.

Answer 4, .The voltage drop results when electrical energy is turned into heat. Current, however, is always conserved in a resistor.

1. If you were to remove an audio cassette from a tape recorder and examine the tape microscopically, you would find that the tape is coated with a layer of:
1. Tiny permanent magnets.
2. Material that magnetizes easily when a magnet is near it but demagnetizes quickly when the magnet is removed
3. Electrically charged particles that become neutral as the tape moves through the cassette.
4. Electrically neutral particles that become charged as the tape moves past the magnetic recording head. (Answer is 1. The tape contains tiny permanent magnets that can be oriented by the recorded head.

 

2. You install a diode (rectifier) in one of the two wires connecting your desk lamp to the electrical outlet=s alternating current. Any current passing through the bulbs filament now must also pass through the diode. With the diode in place, the bulb will glow only about half as bright as usual because the diode:
1. Turns half the current into voltage, which has no effect on the bulb.
2. Turns half the voltage into current, which has no effect on the bulb.
3. Only allows current to flow half the time.
4. Only allows half as much current to flow at every instant in time.

Answer 3. Current can flow in only one direction in a diode and thus only half the AC cycle.

 

3. The split iron ring of your tape recorders play back head is made of a magnetically soft material that does not retain much magnetism in the absence of an external field so that:
1. It doesn’t scratch the tape.
2. Its ends can become electrically charged easily.
3. Its magnetization follows the tapes magnetization easily.
4. A large current can flow through it easily.

Answer 3. We want the tape head to follow the magnetism of the tiny magnets on the tape

 

4. When you drop a permanent magnet with its north pole pointed downward onto an aluminum sheet, it falls slightly slower than normally. The magnet is being repelled by the aluminum because:
1. Aluminum is positively charged and repels approaching magnetic poles.
2. Aluminum usually has a net north magnetic pole which repels any approaching north pole.
3. Current induced in the aluminum by the approaching north pole produces a south pole on the aluminum’s surface which repels the magnet.
4. Current induced in the aluminum by the approaching north pole produces a north pole on the aluminum’s surface.

Answer 4. Eddy currents in the aluminum produce a magnetic field which resists the approach of the magnetic north pole.

 

5. A step down transformer transfers power from the 120-volt alternating current passing through its primary coil to the 12-volt alternating current passing through its secondary coil. If you interchange the primary coil and the secondary coil and send 120 volts through this new primary coil:
1. A 12-volt alternating current will pass through its new secondary coil.
2. No current at all will pass through its new secondary coil.
3. A 1200-volt alternating current will pass through its new secondary coil.
4. A 12-volt direct current will pass through its new secondary coil.

Answer 3. When we reverse the transformer’s connections, it will become a 10:1 step-up transformer.

 

6. A 60-watt, 120-volt light bulb is designed so that it reduces the voltage of a 2 ampere current by 120 volts. If you screw this bulb into a different fixture that is powered by very long, thin wires, it will glow more dimly than intended because the voltage drop across the bulb will be:
1. Less than 120 volts and the current through the bulb will be less than 2 ampere.
2. 120 volts but the current through the bulb will be less than 2 ampere.
3. 120 volts with the current through the bulb 2 ampere.
4. Less than 120 volts and the current will still be 2 ampere.

Answer 1, The long thin wires, having more resistance than a normal fixture, will introduce a voltage drop enroute to the bulb. The overall resistance will thus have risen and therefore there will be less current through the bulb and less than the expected 120V voltage drop across it as well.

 

 

7. Your new toaster has two separate toasting units, each of which consumes 600 watts of power when in use. When you operate one unit, a current of 5 amperes flows through the wiring in your home and the wires waste about 1 watt of power carrying that current. If you now operate both toasting units at once, your toaster consumer 1200 watts and the current flowing through the wires in your home doubles to 10 amperes. How much power will the wires in your home waste now?

1. About two watts.
2. About 0.5 watts
3. About 4 watts.
4. About 16 watts.

11. One use of a resistor in an amplifier or similar kind of electronic circuit is to:

 

1. Restrict the flow of current to only one direction.
2. Limit the current flow for a given voltage.
3. Amplify the current.
4. Store charge.

Answer 2. Limit the current for a given voltage.

 

12. All microwave ovens:
1. Cause water molecules to come to rest in the food.
2. Use a frequency of electromagnetic waves that resonates with water molecules.
3. Heat water inside the food by ultrasound waves.
4. Are able to directly heat many types of molecules in the food.

Answer 2. Microwave ovens radiate at the resonant frequency of water molecules.

 

13. Electromagnetic waves have:
1. Only electric fields.
2. Only magnetic fields
3. Neither electric nor magnetic fields.
4. Both electric and magnetic fields.

Answer 4. Both E and M fields.

 

14. Very high voltages are used to transmit power over long distances because:
1. For a given amount of power, the resistance is lower.
2. For a given amount of power, the current is higher.
3. For a given amount of power, the current is lower.
4. For a given amount of power, the resistance is higher.

Answer 3. The same power can be transmitted by high voltage at low current. Since power loss depends on I²R, that allows the same piece of wire to dissipate less power enroute to the user.

 

15. For an organ pipe with both ends open, when air is blown into it:
1. The fundamental frequency will be sounded.
2. The second harmonic will be sounded.
3. The fourth harmonic will be sounded.

16. In an ordinary small flashlight with one battery generating 1.5 volts and a light bulb with a resistance of 10 ohms, the current flowing will be:
1. Cannot be determined from the information given.
2. 15 amperes.
3. 15 milliamperes.
4. 150 milliamperes.

Answer 4. 0.15 Amps or 150 milliamps as I=V/R.

 

17. The lines in the discharge tubes containing helium and neon, which we viewed with diffraction gratings in class, provide direct evidence that:
1. The energy of photons is directly proportional to their wave length.
2. Light waves have very long periods.
3. An electron in an atom can have virtually any energy at all.
4. An electron in an atom can have only certain energies.

Answer 4. The specific colors we see result from the very specific energy differences in the atom itself.

 

18. If I blow across the open end of a soda bottle, I get a sound whose wave length is:
1. The length of the bottle.
2. 2 the length of the bottle.
3. 1/4 the length of the bottle.
4. None of the above.

Answer 4. A closed end organ pipe has a fundamental that fits one-fourth a wavelength in the pipe. Thus the wavelength is four bottle lengths. The second harmonic has a wavelength 4/3rds the length of the bottle. Etc.

 

19. In the sound interference demonstration we performed in class using two loud speakers emitting the same frequency, the distance you would have had to move your head to go from a loud spot to a soft spot would be about:
1. 100 wave lengths.
2. 5 wave lengths
3. 2 wave length.
4. The distance is independent of wavelength.

Answer 3.Half a wavelength. Moving the head (or a speaker) by about ½ wavelength can change constructive to destructive interference.

 

 

20. The wave length of the second harmonic frequency (first overtone) for a string on a guitar is _________ times its length:
1. One.
2. Two.
3. .2.
4. 1/4

Answer 1. The fundamental fits ½ wavelength on the guitar string and the second harmonic fits a full wavelength on the string.

 

21. If a cello and a saxophone both play the same note, they will sound different because:
1. The notes would have different overtones or harmonics.
2. The notes would have different fundamental frequencies.
3. The speeds of sound in brass and wood are different.
4. The impedance matching in the two instruments is different.

Answer 1. It is the harmonic content that makes the same note sound different when played by different instruments.

 

22. Which of the following are electromagnetic radiation?
1. Gamma rays
2. X-rays.
3. Microwaves
4. All of the above.

Answer 4. All are EM waves.

 

23. If I have two resistors in series, one with a value of 10 ohms and one with a value of 2 ohms, the effective resistance will be:

1. 20 ohms.
2. 12 ohms
3. 8 ohms
4. Less than 2 ohms.

24. The sky is blue because:
1. Red and green light are absorbed by the atmosphere.
2. Blue light is not scattered but red light is scattered by the atmosphere.
3. Blue light is scattered more strongly by the atmosphere than red light.
4. Molecules of air are larger than the wavelengths of red light.

Answer 3. Nitrogen and oxygen molecules scatter blue light best of the visible wavelengths.

 

25. If I have a 10 ohm resistor and I put a 2 ohm resistor in parallel with it, I will expect the effective resistance of the pair to be:
1. Greater than 10 ohms.
2. Between 10 and 2 ohms.
3. Equal to 8 ohms.
4. Less than 2 ohms.

Answer 4. Two or more resistors in parallel will always have an effective resistance less than the smallest resistance in the group.

 

26. If the current in a part of a circuit is found to decrease, we can assume:
1. The resistance of the circuit has decreased.
2. The voltage across that part of the circuit has increased.
3. The voltage across that part of the circuit has increased and/or the resistance of the circuit decreased.
4. The voltage across that part of the circuit has decreased and/or the resistance of the circuit increased.

Answer 4. Either V decreased or R increased or both.

 

27. If I pluck a guitar string and then reduce its length by a factor of 2 and pluck it again, I will hear a tone which is:
1. An octave lower.
2. An octave higher.
3. Two octaves lower.
4. Two octaves higher.

Answer 2. Shortening the string shortens the wavelength of the fundamental by the same fraction.

 

28. When an object becomes charged (by rubbing it with some other object, for example):

0. Protons have been created on the object..
1. Protons have been caused to move from one object to the other.
2. Electrons have been caused to move from one object to the other.

29. When white sunlight strikes a diamond or a cut-glass pitcher we see colors coming to our eye. Those colors must be resulting from:

1. similar wavelengths in air can have different wavelengths in the material

2. the same wavelengths can have different colors

3. different wavelengths travel at different speeds in most transparent materials

4. the same frequencies in air can have differing wavelengths in the materials.

Answer 3. The spectral colors result from splitting white light into a range of wavelengths. The refraction which causes this is the result of differing speeds for different light frequencies in the glass or diamond.

 

 

30. The tubes in fluorescent lights have a white powder on their inner surfaces. This powder:

1. scatters the white light produced by the gas atoms in the tube so that it is both vertically and horizontally polarized and cannot cause glare.

2. conducts electricity from one electrode to the other emitting white light as the current passes through that powder.

3. slows down the white light produced by the gas atoms inside the tube so that less of the white light reflects as it passes through the tube.

4. absorbs ultraviolet light from the gas in the tube and uses that energy to produce white light.

Answer 4. The white phosphor powders absorb UV photons emitted by mercury atoms and reradiate visible light.

 

31. T F When two waves interfere the result is always smaller than the two original waves.

F. The resulting wave can have smaller or larger amplitudes.

32. T F Polaroid sunglasses filter out all types of light equally well.

F. Polaroid material filters out specific polarization of light. The sunglasses are made to exclude much of the light scattered from water or road surfaces and this scattered light will be mostly horizontally polarized.

33. T F Radio waves cannot propagate in a vacuum.

F. All EM radiation can propagate *(fastest) in a vacuum.

34. T F A red sweater looks red because its pigment absorbs red light.

F. The red sweater scatters the red light but absorbs most other colors.

35. T F The wavelength of FM radio waves at 91.3 MHz is shorter than those at 106.5MHz.