NSC 101 Laboratory for 4/14/97

This week's exam will cover chapters 11 through 16. Below are a few sample questions from an old test. Good luck!

NOTE: Answers to Ch 15 and 16 are at the bottom!

1. As a car speeds by you, what can be noticed about the sound from the car radio?

2. When rain just begins to fall, you can see many colored swirls coming from the asphalt oil floating on the water. Explain this effect.

3. Order the following from low frequency to high frequency; x-rays, radio waves, visible light, microwaves, and ultraviolet.

4. It is said that matter is mostly empty space. Explain.

5. Blowing over the top of an empty vita-malt bottle produces a low note because
   1. the speed of sound is about equal to the height of the bottle.
   2. a standing wave is created in the bottle.
   3. because 2000 Hz sound waves are typical in human speech.
   4. sound energy is created from vibrations of the glass bottle.

6. White light passing through a glass prism emerges with colors separated because
   1. the speed of light in the glass is different for each color.
   2. the photons of light are unequally absorbed by the glass.
   3. the amount light diffracts is inversely proportional to the energy.
   4. the polarization is different for each color of light.
   5. All nucleii are composed of protons and neutrons (nucleons).

7. A friend claims the Egyptian stone pyramids have been radio-carbon dated to be 5,000 years old. You know he is misinformed because
   1. radio-carbon dating is valid only to about 3,000 years.
   2. it would be impossible to obtain a large enough sample for accurate dating.
   3. radioactive decay is strongly attenuated in stone.
   4. the pyramids are made of stone, which cannot be dated by this method.

8. You are floating at Magens Bay and notice that a wavecrest passes you every 4 seconds. The distance between crests is 8 meters. What is a) the wavelength, b) the wavespeed, and c) the frequency of the waves at Magens?

9. Draw two rays from the top of the object in front of the flat mirror and show where and how the image is formed.

10. Complete the following nuclear reactions
    1. ¹⁴C ----> beta + _______
    2. ²³⁸U ----> alpha + _______
    3. ²³⁵U + n ----> 3n + ⁹¹Kr + ________

11. You measure the radioactivity of a sample of ²⁴Na to be 1000 counts/sec on your geiger counter. Thirty (30) hours later, the count rate is 250 counts/sec. What is the half life of ²⁴Na?

1. Due to the doppler effect, the frequency changes as the car moves past you.
2. Different colors result from constructive interference at different angles and at different thickness of the oil (thin film interference).
3. radio, microwave, visible, ultraviolet, xray
4. The nucleus is massive but very tiny. The electrons orbit about the nucleus at radii much larger than the size of the nucleus.
5. 2
6. 1
7. 4
8. 8 meters, 8m/4s = 2 m/s, 1/4 Hz
9. Check your text or notes for a ray diagram
10. ¹⁴N, ²³⁴Th, ¹⁴¹Ba
11. 1000 --> 500 --> 250 means two half-lives in 30 hrs. So one half-life must be 15 hours.

15.3 No. An alpha is composed of 2n and 2p.

15.4 They are oppositely charged, so the magnetic force reverses direction. The gamma is a photon and has no charge at all!

15.5 The alpha is deflected less because it so much more massive than a beta.

15.7 Alpha decay gives the greatest change in both atomic mass and atomic number.

15.10 Alphas are much larger and more likely to collide with the nucleii in the material.

15.13 1/4 after 2 years, 1/8 after 3 years, and 1/16 after 4 years.

15.14 160 --> 80 --> 40 --> 20 --> 10 cts/min. This is 4 half-lives over a period 8 hours. So one half-life must be 8/4 = 2 hours.

15.16 Alpha decay is ²³¹Bi --> alpha + ²²⁷Tl,

whereas for beta decay ²³¹Bi --> beta + ^231PoTl

15.19 1p + 1n, 6p + 6n, 26p + 30n, 79p + 118n, 38p + 52n, 92p + 146n

15.23 The unstable elements below Uranium are constantly being re-populated by the decays of the radioactive isotopes above them.

15.30 Radio carbon dating can be done only for matter that was once living and hence had taken in the ¹⁴C being produced in the upper atmosphere at that time. Once the plant or animal dies, the cargbon-14 is no longer replenished.

16.1 The uranium ore is not nearly concentrated enough to start a chain reaction. In fact, even after you melt the ore to isolate the uranium, most of it is the isotope U-238, which decays via alpha decay. The isotope U-235 is the one that will fission, and it is only a fery small fraction of the total sample. Isolating the U-235 is a difficult and expensive process.

16.4 The neutron is neutral, so it doesn't experience the coulomb repulsion a proton would from the protons in the nucleus. An electron must make it through all the electrons in the atom. (Once in the nucleus, if it interacts at all, it will probably just change a proton into a neutron.)

16.6 A sphere has the lowest possible surface to volume ratio. (That's why balloons inflate into a spherical shape.) So, the neutrons will be more likely to collide with another nucleus in a spherical shape.

16.8 The Pu will have a smaller critical mass. That is, with more neutrons per fission, less are needed to sustain the chain reaction.

16.9 Probably the Pu will be faster since chain reaction has slightly less distance to move with a smaller sample. But there are other factors, such as the average time it takes the nucleus to fission once it has absorbed the neutron.

16.13 The mass of any nucleus is always less than the sum of the masses of the individual nucleons. This "missing mass" can be thought of as being tied up in the bonding energy. The more the mass converted to bonding energy, the more stable the nucleus.

16.15 From the table of nuclear masses, we calulate the difference in total mass energy before and after the fission. The mass that is missing will appear as kinetic energy of the outgoing fragments. This mass times c squared will be the energy.

16.23 Fusion would use deuterium for fuel..it is safe and cheap. The end products of fusion are also relatively safe.

16.24 The sun is an ongoing fusion bomb! Long live fusion!