Physics

Thursday, June 16, 1921.

1. Show that sound (a) takes time to travel through air, (b) can be reflected. Illustrate (a) dependence of pitch on frequency, (b) difference in quality, (c)

resonance.

Ans. (a) When a man is observed chopping wood at a distance of forty rods, the ax is seen to descend and some time afterwards the sound is heard. (b) Sound, reflected from a distant object as a forest, may return as an echo. (a) In a siren, the faster the wheel is rotated, the more frequent the vibrations and the higher the pitch. (b) It is shown by

the difference in tone between a violin and a tuning fork, owing to the overtones. (c) Resonance is the re-enforcement of sound due to the union of direct and reflected sound waves and is shown by holding a vibrating tuning fork over the mouth of a cylindrical jar. Change the length of the air column by pouring in water slowly. The sound will increase in loudness until a certain length of air column is reached, after which it becomes weaker.

2. Describe with the aid of a labeled diagram apparatus to illustrate one of the following: (a) heat convection in air, (b) heat conduction by a solid, (c) heat radiation from a solid. State (a) the facts observed, (b) the principles involved.

Ans. If one end of an iron poker is put in the fire, the other end will become hot. The molecular energy, or heat, is transmitted from molecule to molecule along the poker. This method of heat transmission is called conduction.

3. Describe in words or by labeled diagram any change that occurs in temperature, state or volume in each stage of

progress as ice below freezing is converted into steam.

Ans. When heat is applied to ice below freezing, the temperature rises until 0° C. is reached. At this point the temperature remains stationary until the ice is melted and then the temperature rises until it reaches 100° C. where the temperature remains unchanged until the water is changed to steam. The volume of melted ice decreases, if it is

warmed, till it reaches 4° C. and increases above that point. Ice below 0° C. requires about one-half a calorie of heat to raise it each degree till it reaches 0° C. where the ice becomes a liquid by the application of 80 calories of hcat,

called the heat of fusion. The further addition of 100 calories of heat raise the water to 100° C. and the addition of 537 calories more (heat of vaporization) will change the water to steam.

4. Find the candle power of a lamp which, 6 feet distant from a book, will give it the same illumination as one of 20 candle power 4 feet distant. State the law involved.

Ans. When a body is equally illuminated by two different sources of light, the intensities of the sources vary directly as the squares of their respective distances from the body. 20 X : (4)2 (6)2; 20: X 16: 36; X 45, the candle power.

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5. Mention two optical instruments each using a converging lens (a) one forming an enlarged virtual image, (b) one forming a diminished real image. State in each case the position of the object and of the image with relation to the principal focus.

Ans. (a) The simple The simple microscope forms an enlarged virtual image. The object is placed between the principal focus and the lens and the image is beyond the principal focus. (b) The lens

of a camera forms a diminished real image. The object is further than twice the focal length and the image will be on the opposite side and between the focus and twice the focal length.

6. Make a labeled diagram of apparatus to illustrate one of the following: (a) mutual action of magnetic poles, (b) magnetic dip, (c) magnetic induction in iron by the earth. Give procedure and observations. Explain results.

Ans. Answers will vary.

7. A metal ball is suspended by an insulating thread. Tell in detail how, using a glass rod bearing a positive charge of electricity, you could (a) give the ball a positive charge, (b) give it a negative charge.

Ans. (a) Touch the metal ball with the glass rod and the ball will be charged positively by contact. (b) Bring the charged rod near the metal ball and touch the ball with the finger to conduct away the positive electricity. The negative electricity of the ball is held bound by the positive electricity of the glass rod. Next remove the finger and then the rod and the ball will be negatively charged by induction.

8. What resistance has a water heater which allows a flow of 5 amperes from a 110 volt circuit? Find the cost of operation for 90 minutes at 12 cents a kilowatt hour.

Ans. (a) The direct current, because the anode and cathode are not permanent. (b) Either direct or alternating current may be used to change the voltage. (c) The A. C. would be used to operate a transformer. (d) D. C. should be used, since in a dynamo the polarity of the field should be constant in dircction. (e) D. C. should be used, since the metal is deposited on the cathode and the A. C. would have no permanent cathode.

10. A current is passed through a straight iron wire joined in series to a straight copper wire of the same diameter and length. Compare, giving reasons, the wires as to (a) resistance, (b) current transmitted, (c) surrounding magnetic field, (d) heat developed.

Ans. (a) Iron has the greater resistance, because it is a poorer conductor. (b) The current will be the same as in the iron wire, since the current equals that of the poorest conductor in the series. (c) The field is the same, since the field is proportional to the current. (d) The iron develops more heat, since heat is proportional to the resistance and the square of the current.

11. In a hydraulic press what force will be exerted by a piston of 300 sq. in. area when on the connected piston of 2 sq. in. area the force is 800 b? Make a labeled diagram of the press. State the law of the press.

Ans. P W as the area of the small piston to the area of the large piston; 800 : X 2:300; X = 120,000 lb. Pressure exerted upon any part of an inclosed liquid is transmitted undiminished in all directions. This pressure acts with equal force upon all equal surfaces, and at right angles to them.

12. Make a labeled diagram of a lift pump connected to water. State what occurs and why, when the piston (a) descends, (b) ascends.

Ans. See text-books for diagram.

When the piston descends, the valve at the base of the cylinder closes the pipe leading to the source of the water and the valve in the piston opens to allow the air or water to pass above the piston valve. When the piston ascends, its valve closes and, a vacuum being formed below it in the cylinder, atmospheric pressure on the liquid pumped causes it to force open the lower valve and rise in the cylinder below the piston. Repeated strokes cause the liquid to rise above the piston and pass out the spout above.

13. State the principle of Archimedes. Define specific gravity. Make the proper note book entry, indicating the computations, for a laboratory exercise to determine the specific gravity of an irregular piece of granite.

Ans. A body immersed in a liquid is buoyed up by a force equal to the weight of the liquid that it displaces. The specific gravity of any solid or liquid is the ratio of the weight of the body to the weight of an equal volume of pure water at 4° C. Note book entries will

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