Due Monday, February 20, 1995, In Class
Please Answer Each Question As Briefly As Possible
You May Work Together, But Write Up Your Answers Separately
Question 1:
Lightning occurs when large amounts of electric charge flow between the clouds and the ground.
(A) Lightning is most likely to strike when a cloud and the ground beneath it have accumulated large opposite electric charges. Why is it unlikely to strike if they have large like charges?
(B) The atmosphere does a great deal of work in creating the separated charge that produces lightning. Show that it takes work to move positive charge from the negatively charged ground to the positively charged cloud overhead.
(C) When enough opposite charge has accumulated on the ground and the cloud above it, lightning will strike. One indication that this dangerous change accumulation has occurred is that your hair begins to stand up. Explain this effect.
(D) A sharp lightning rod reduces any local buildup of electric charge and prevents nearby lightning strikes. How does the lightning rod get rid of local electric charge and allow it to flow gradually to the clouds overhead?
(E) If the lightning rod had a large metal ball on top rather than a sharp point, it wouldn't work properly. Why not?
Question 2:
A metal scrap yard uses a crane with a large electromagnet attached to it to move old cars about. The operator lowers the electromagnet onto the car's roof, activates it, and then lifts the electromagnet upward. The car clings to the active electromagnet and also moves upward.
(A) The electromagnet contains a coil of wire and both ends of this coil are connected to the electrical power source. Why is it important that two wires connect to the power source rather than one?
(B) When current flows through the coil, it becomes a magnet. Does the electromagnet become a north pole or a south pole, or is it more complicated than that? Explain.
(C) The steel top of a car does not appear to be magnetic: your car does not attract or repel someone else's car. Yet when the electromagnet is activated, the top does become magnetic. Explain.
(D) The electromagnet is powered by direct current. If the magnet used alternating current instead, then it would have a tendency to repel metal. Why?
Question 3:
As freshly laundered clothes tumble about in a hot dryer, they rub against one another. Sliding friction transfers electric charges from one piece of clothing to the other so that some items become positively charged and others negatively charged. These charge accumulations produce static cling. Fabric softener prevents static cling. Fabric softener is a soap-like chemical that binds to wet fabric fibers and lubricates them. This lubrication is what softens the clothes. But fabric softener also attracts moisture, which in turn reduces static cling.
(A) As you unload the dryer, you find several socks clinging to a shirt. How do the charges on the socks and shirt compare to one another?
(B) As you pull the socks off the shirt, you are doing work on the socks. What form is your energy being transformed into?
(C) Suppose that your socks are covered with positive charge. As you pull them away from the shirt, what happens to the voltage of the charges on the socks?
(D) As you pull the socks away from the shirt, you hear and feel sparks leaping from the socks to your hands and to parts of the shirt. Why do the sparks occur as you separate the shirt and socks, rather than before?
(E) The farther a sock is from the shirt, the less they attract one another. Explain.
(F) After removing them from the shirt, you find that the socks repel one another. Explain.
(G) The next time you do your clothes, you add fabric softener. Your clothes emerge from the dryer with a thin layer of moisture on them and this moisture permits charge to move freely about the clothes. Why does this mobility prevent the build-up of separated charge?
Question 4:
A recycling plant is trying to separate metals from other trash automatically. It grinds the trash up into small pieces and then sends these pieces across the poles of a strong electromagnet.
(A) Iron and steel scraps in the trash are attracted to the poles of the electromagnet but aluminum scraps are not. What is different between the two types of metals?
(B) If they are moving quickly enough, aluminum scraps experience a magnetic drag force that separates them from the trash. Where does this force come from?
(C) To look for metal hidden inside bundles of paper, the recycling plant exposes the bundles to an alternating magnetic field (one that reverses directions many times a second). If it finds that something inside a bundle creates another alternating magnetic field in response, it knows that the bundle contains metal. How does metal create this second magnetic field?
(D) The company uses electric motors for its conveyor belts. It finds that the higher a belt lifts the trash before dumping it onto a heap, the more electrical power that belt's motor consumes. Why should a motor's electrical power depend on the height of the conveyor belt it drives?
Question 5:
The small personal stereo that you wear while jogging isn't working. After paying last month's phone bill, you can't afford to have someone else fix it. So you open it up and take a look yourself. You find a circuit consisting of a 9V battery, an on-off switch, and a complicated circuit board. The battery has two wires attached to it: a red wire attached to "+" and a black wire attached to "-". The black wire connects to the switch and the red wire connects to the circuit board. Another wire connects the switch to the circuit board.
(A) Your first suspicion is a dead battery so you replace it with a new one, carefully attaching the red wire to "+" and the black wire to "-". What would happen to the current in the circuit if you reversed the battery's connections?
(B) The stereo still doesn't work. You can't see anything obviously wrong with the circuit board, so you check the switch. Its sliding button feels very loose so you think it might be broken. How is the switch's position (on or off) supposed to affect the circuit?
(C) To test whether the switch is at fault, you decide to try operating the stereo without the switch. You have a paper clip in your hand. What can you do with that paper clip to get current flowing through the stereo's circuit without involving the switch itself?
(D) The paper clip works and the stereo begins to play. The switch is evidently broken, so you replace it. What wasn't the broken switch doing for the stereo's circuit?
(E) Why doesn't electrical power flow from the battery to the circuit board through the red wire alone?