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?
Answer: Like charges repel so that the charge on the cloud is not attracted toward the charge on the ground.
Why: Lightning only strikes when the charges on the cloud and ground are opposite and are thus attracted toward one another.
(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.
Answer: The positive charge is repelled by the positively charged cloud and must be pushed upward as it moves upward. An upward force exerted on an object as it moves upward does work on that object.
Why: You do work whenever you exert a force on an object and that object moves in the direction of your force on it. Moving a positive charge toward a positively charged cloud takes work because you must push it toward the cloud as it moves toward the cloud.
(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.
Answer: The positively charged cloud attracts negative charge onto you and your hair. Your hair is then attracted toward the cloud and stands up. The negative charges on each hair also cause them to repel one another so that they stand up. (Either answer is acceptable and sufficient)
Why: The positively charged cloud polarizes the ground beneath it, attracting negative charge and repelling positive charge. As the highest object on the ground, you also acquire a negative charge. Your hair rises upward as the large negative charge on it is pulled upward and as the individual strands repel one another.
(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?
Answer: The positively charged cloud attracts negative charge onto the lightning rod. Because of its sharp tip, this negative charge is able to flow into the air and up toward the cloud in a steady stream.
Why: Sharp tips are particularly good at emitting excess charge into the air. When a highly charged cloud passes overhead, the tip of a lightning rod becomes covered with opposite charge and begins to spray that opposite charge into the air. This charge moves upward and helps to neutralize the cloud. It also limits the amount of opposite charge that can accumulate on the ground and reduces the chance of a lightning strike.
(E) If the lightning rod had a large metal ball on top rather than a sharp point, it wouldn't work properly. Why not?
Answer: A smooth or rounded surface has difficulty emitting charge into the air so that a ball will stop the build up of opposite charge on the ground when a highly charged cloud passes overhead.
Why: When a highly charge cloud passes by, it polarizes the ground. The ground acquires a large opposite charge. A lightning rod with a large ball on it will also acquire a large charge but will not encourage that charge to flow into the air. The charge can build up until a huge lightning strike occurs.
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?
Answer: The electromagnet requires a current flow to work and you cannot sustain a current flow with only one wire.
Why: With only one wire going to the electromagnet, you could send current to it only briefly. Charge would build up inside the electromagnet and would soon repel any additional charge. The second wire is needed to extract charge back from the electromagnet so that you it can sustain a steady current flow.
(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.
Answer: When current flows through the coil, it becomes magnetic but with a north pole at one end and a south pole at the other.
Why: Magnetic poles are never found by themselves. They always occur in equal but opposite pairs. When you magnetize a coil by sending a current through it, it acquires both a north pole and a south pole and these two poles have equal strengths. Which end becomes north and which end becomes south depends on the direction of current flow through the coil.
(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.
Answer: Steel is intrinsically magnetic at the atomic level but that magnetic character is hidden because the tiny magnetic domains are randomly oriented. When the electromagnet is activated, the domains shift so that the steel becomes obviously magnetic and it is attracted toward the electromagnet.
Why: A few metals and alloys are intrinsically magnetic at the atomic level and carbon steel is one of them. However this magnetic character is usually hidden because it appears inside many tiny magnetic domains, each of which is randomly oriented. When the car roof is exposed to a strong magnet, its magnetic domains line up and the steel becomes strongly magnetic. It is then attracted toward the electromagnet.
(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?
Answer: When AC current flows through an electromagnet, its poles reverse with each reversal of the current. The changing magnetic field creates an electric field and induces AC currents in nearby metal. That metal becomes magnetic with a like pole always pointing toward the electromagnet's nearest pole. The two objects repel one another.
Why: An AC electromagnet induces currents and magnetism in nearby metal. In our universe, the resulting magnetic poles in the nearby metal repel the poles of the electromagnet. That repulsive behavior is called Lenz's law.
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?
Answer: The charges are opposite one another
Why: Since the socks cling to the shirt, you know that they are experiencing attractive forces. For an electrostatic force to be attractive, it must be between opposite charges.
(B) As you pull the socks off the shirt, you are doing work on the socks. What form is your energy being transformed into?
Answer: Electrostatic potential energy
Why: As you pull the items apart, they are clearly storing energy because they will snap back together if you let go. The potential energy between charged particles is electrostatic potential energy.
(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?
Answer: The voltage increases.
Why: Voltage measures energy per charge. As you pull the items apart, the electrostatic potential energy increases. Since the number of charges involved remains constant, the energy per charge is increasing so the voltage is increasing.
(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?
Answer: The increasing voltage makes it possible for the charges to leap off the fabric and produce sparks.
Why: As their voltage increases, the charges on the objects experience stronger forces pushing them off the objects. They leap toward your hands or other nearby surfaces as sparks.
(E) The farther a sock is from the shirt, the less they attract one another. Explain.
Answer: The forces between charges decrease with distance.
Why: The electrostatic force or Coulomb force between two charges is inversely proportional to the square of the distance separating them. As the socks move away from the shirt, the distances between charges increase and the forces between them decrease.
(F) After removing them from the shirt, you find that the socks repel one another. Explain.
Answer: The socks have like charges.
Why: What type of charge accumulates on a particular piece of clothing depends on its fabric. Since the two socks are made of the same fabric, they tend to accumulate similar charges. Once they are away from the shirt, their like charges repel.
(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?
Answer: Separated charges (opposite charges) attract one another and will flow toward one another whenever possible. Because softened clothes conduct electricity, charges will move through them until they are electrically neutral throughout. [The lubricating effect of the softener may also reduce the charging effects of sliding friction]
Why: To sustain a charge separation, you must have an electrical insulator. Electrical conductors permit charge to move so that any charge separation is quickly neutralized. If you were to fill the drier with conducting aluminum foil instead of non-conducting clothes, it would not build up any static charge. The fabric softener helps to make the clothes more conducting so that they don't build up much static 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?
Answer: Iron and steel are magnetic at the atomic scale while aluminum is not intrinsically magnetic.
Why: Iron and steel are ferromagnetic metals. They are magnetic at the microscopic level, although this magnetism is often hidden by the random orientation of the tiny magnetic domains. When you bring a magnetic pole nearby, these magnetic domains align with the field and the metal exhibits its magnetic character. But aluminum, like most metals, is truly non-magnetic. It has no microscopic magnetic character so bring a pole nearby slowly does not have any effect.
(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?
Answer: The movement of aluminum through a magnet induces currents in the aluminum and it becomes magnetic.
Why: As the aluminum moves through the magnet, it perceives a changing magnetic field. From its perspective, the magnetic is moving about it. Thus it experiences an electric field (changing magnetic fields create electric fields). This electric field causes charges in the aluminum to accelerate (since aluminum is a metal) and current flows in the aluminum. Although aluminum itself is not a magnet, currents are magnetic so now the aluminum is a magnet and it interacts with the magnetic field through which it is traveling. It slows down.
(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?
Answer: The changing magnetic field induces currents in the hidden metal and the metal becomes magnetic.
Why: The changing magnetic field creates an electric field. While there are no mobile charges in the paper bundles themselves, the charges in hidden metal are free to move. They accelerate and currents flow in metal. As the magnetic field reverses back and forth, so does the current in the hidden metal. This alternative current in the metal creates an alternating magnetic field. This technique is used in most metal detectors.
(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?
Answer: The higher the conveyor belt, the more work it does on each item it carries. Since the motor provides this work, it must obtain more energy from the power line in order to lift each item. Thus it needs more electrical power.
Why: Motors draw power from the power line according to how much work they must do each second. In effect, the power line provides the motor with lots of power and the motor return the power that it does need back to the power line. It returns this energy by acting as a generator. If you load down the motor by making it lift object way up in the air, it will have little power to return to the power line and will end up consuming a large amount of power.
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?
Answer: It will flow around the circuit in the reverse direction (or possibly not flow at all, if the circuit board blocks current in one direction).
Why: The battery pumps current in one direction: from its negative terminal to its positive terminal. If you reverse it in the circuit, it will pump current backward around the circuit. If the circuit board were a light bulb, current would certainly flow backward around the circuit and the bulb would still light. However, a circuit board may not respond equally to current flowing backward through it. Some current will probably flow (backward, of course), but probably not the normal amount that would flow if the battery were inserted properly.
(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?
Answer: In the off position, the switch should open the circuit. In the on position, the switch should close the circuit.
Why: The switch's job is break the circuit when the radio is turned off. Without a complete path all the way around the circuit, charge accumulates on the two ends of the switch and current stops flowing. When you turn the radio on, it completes the circuit so that current can flow continuously around the loop.
(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?
Answer: You can connect the two wires at the back of the switch.
Why: When you connect the two wires at the back of the switch, you complete the circuit so that current can flow continuously around the circuit. In effect, you are turning the switch "on."
(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?
Answer: The broken switch wasn't completing or closing the circuit (it was leaving the circuit open).
Why: If the switch were working, turning it on should have the same effect as making electrical contact between its two wires. Since connecting those two wires with a paper clip worked but turning the switch on didn't, the switch must not be working.
(E) Why doesn't electrical power flow from the battery to the circuit board through the red wire alone?
Answer: Current cannot flow continuous through only a single wire.
Why: Without a return route (through the black wire), current can flow out to the circuit board but cannot return. Charge accumulates in the circuit board and the current stops flowing.
