Physical World 1 - 11.30am

Test 4 - April 28, 2000

NAME: _________Answers to the blue test _________

(Last name, first name)

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Eighteen short answer questions. Each question is worth 5 points. (Total points = 90.)

1. Strictly speaking, will a penny be slightly more massive if it has a negative charge or a positive charge? Explain.

2. Magnetism is due to

a) the magnetic dipole of the nucleus.

c) the motion of protons around a nucleus.

d) oscillating positive and negative charges.

e) the motion of neutrons in the nucleus.

3. Like magnetic poles ______repel__________________________ .

Unlike magnetic poles _______attract_________________________ .

4. a) Are the headlights for a car connected in series or in parallel?

b) Explain how you know this fact.

5. The magnetic field around a wire is created by the motion of _electrons (or current) _ through the wire.

6. In a demonstration we have two identical plastic rods and a wool cloth.

a) If one rod is rubbed with wool and brought near the other (unrubbed) rod the two rods attract each other. Explain with a diagram why they attract each other.

b) If, however, BOTH rods are rubbed with the wool they repel each other. Explain why they repel each other in this case.

7. You have a magnet, a coil of wire and a light bulb. Draw a diagram of how you would hook up this equipment and explain what you would need to do in order that the bulb emit light.

8. A laser beam passes through a slit of width d as shown in the diagram below

(i) After the slit the laser beam has which of the following shapes?

a) b) c) d)

(ii) This effect is caused by __diffraction_____________________ .

9. In a demonstration white light was reflected from a soap film. In the thinnest part of the film the film appeared black because

a) black light was reflected from the soap film.

b) of diffraction effects within the soap film.

d) of constructive interference between waves reflected from the front and back surfaces of the soap film.

e) no light was reflected from this part of the soap film.

10. A double slit interference demonstration is shown below.

a) State the condition, with regard to the extra distance d traveled by beam 2 relative to beam 1, which determines the position of the first interference peak.

b) Which colored light has the first interference peak closest to the central peak? (Choose from i-iv below.)

c) Which colored light has the first interference peak furthest from the central peak?

11. When combing your hair, you scuff electrons from your hair onto the comb.

a) Is your hair then positively or negatively charged?

b) How about the comb?

12. In an electric circuit

a) electrons flow from the positive terminal to the negative terminal

c) protons flow from the positive terminal to the negative terminal

d) protons flow from the negative terminal to the positive terminal

e) current is the flow of both electrons and protons in a circuit

13. a) When you pay your household electric bill at the end of the month, which of the following are you paying for: voltage, current, power, energy?

b) Explain.

14. Circle the true statement.

a) A stationary electron always feels a force on it in the presence of a stationary magnetic field.

c) An electron always feels a force on it in the presence of a magnetic field whether it is moving or not.

d) The motion of an electron is never influenced by the presence of a magnetic field.

15.

In which of the circuits above does a current exist to light the bulb?

16. When two charged rods are moved three times as far apart the force of interaction decreases by a factor of

a) 1/3

b) 1/9

c) 1

d) 3

17. Circle the true statement.

a) If you continue to break a magnet in half you will eventually end up with an isolated north or south pole.

b) A stationary electron is always attracted to a permanent magnet.

c) Isolated electric charges cannot exist.

e) A stationary magnet can induce an electric current in a stationary coil.

18. Two 40W bulbs are hooked up in series to a 120V power outlet. Another two 40W bulbs are hooked up in parallel to another 120V power outlet.

Circle the true statement.

a) The bulbs in series are brighter than the bulbs in parallel.

b) The bulbs in series have more current flowing through them than the bulbs in parallel.

d) The bulbs in parallel use up less energy per second than the bulbs in series.

Four longer questions. Each question is worth 5 points. (Total points = 20.)

19. A current of 0.07 A can kill you. If you accidentally touch a 120 V outlet, what would the resistance of your body need to be in order that you received a fatal electric shock?

In the diagram above, bulbs A, B, and C are identical.

a) How does the brightness of bulb A compare with bulb B?

b) How does the brightness of bulb A compare with bulb C?

c) Which bulb draws the most current?

d) What will happen if bulb A is unscrewed?

e) What will happen if bulb C is unscrewed?

21. A 1200W hair dryer is connected to 120V mains outlet.

a) How much current does this hair dryer draw?

b) What is the resistance of the hair dryer?

c) How many hair dryers could safely be connected up in parallel without blowing a 25A fuse?

d) If it took 600 seconds to dry your hair how much electrical energy was used up during this time?

e) How much heat energy was generated during this time period?