Given Wednesday, March 8, 1995, from 1:00 PM to 1:50 PM
PART I: MULTIPLE CHOICE QUESTIONS
Please mark the correct answer for each question on the bubble sheet. Fill in the dot completely with #2 pencil. Part I is worth 67% of the grade on the midterm examination.
Problem 1:
A company that makes electric doorbells has been making electromagnets by wrapping copper wire around steel bolts. When current passes through the coil of copper wire, the steel bolt becomes magnetic. The electromagnet then attracts a clapper that rings the bell. But a new employee has used copper bolts as cores for some electromagnets. These copper-bolt electromagnets don't become very magnetic when current passes through their coils because
(A) unlike the atoms in steel, the atoms in copper are not intrinsically magnetic.
(B) both the bolt and the coil around the bolt are made of copper so that no magnetic dipole can be created.
(C) copper is a better conductor of electrical current than steel so that the copper bolt short-circuits the magnetic field.
(D) copper is not a metal and only metals can become magnetic.
Answer: (A) unlike the atoms in steel, the atoms in copper are not intrinsically magnetic.
Why: Copper has no microscopic magnetic structure so that there is nothing for the external magnetic field to align. The coil of copper wire might as well be wrapped around a piece of plastic or paper or even air. Steel, on the other hand, does have a microscopic magnetic structure that can be aligned by an external magnetic field. This steel augments the external field to create one that is strong enough to attract the clapper and right the bell.
Problem 2:
When a television set is displaying the image of a red wall, so that the entire screen appears red,
(A) the electrons inside the picture tube are only striking those screen phosphors that emit red light.
(B) the electrons inside have relatively low energies so that the screen phosphors glow with red heat.
(C) the electrons inside have relatively high energies so that the screen phosphors glow with red heat.
(D) only the red lamp inside the picture tube is illuminated.
Answer: (A) the electrons inside the picture tube are only striking those screen phosphors that emit red light.
Why: There are three different types of phosphors on the inside of the picture tube. These phosphors emit either red, green, or blue light. When the picture tube is trying to display a red object, it sends electrons only at those phosphor dots that emit red light when struck.
Problem 3:
The aurora borealis or "northern lights" is produced when electrically charged particles emitted by the sun spiral downward toward the earth's north pole. They are guided in that direction by the earth's magnetic field because
(A) moving charged particles experience forces when they pass through a magnetic field.
(B) positively charged particles are attracted toward north magnetic poles.
(C) positively charged particles are attracted toward south magnetic poles.
(D) positively charged particles are repelled by both north and south magnetic poles.
Answer: (A) moving charged particles experience forces when they pass through a magnetic field.
Why: Moving charged particles experience forces when they pass through magnetic fields. From their perspectives, the magnetic fields are moving/changing so they perceive electric fields. They thus experience forces that change their velocities.
Problem 4:
A typical lawnmower has an electromagnetic device called a magneto that produces a brief pulse of very high voltage electric charge. This charge runs through a short wire to the spark plug, where it jumps across a gap to produce the spark that ignites the gasoline. Unfortunately, the lawnmower interferes with the reception on your portable radio because
(A) when the charge accelerates during each ignition pulse, it emits radio waves.
(B) the magnetic field from the magneto attracts charge out of your radio's antenna so that it becomes electrically neutral.
(C) the alternating current used in the lawnmower's ignition system is incompatible with your portable radio, which requires direct current for its operation.
(D) the presence of electric charge in the magneto prevents current from flowing up and down your portable radio's antenna.
Answer: (A) when the charge accelerates during each ignition pulse, it emits radio waves.
Why: Accelerating electric charge always emits electromagnetic waves. The rearranged charge creates electric fields, the moving charge creates magnetic fields, and the acceleration of that charge ensures that the electric field will create a magnetic field and that the magnetic field will create an electric field. Many electrical devices cause rapid accelerations of charge and thus emit radio waves. These waves often interfere with radio reception.
Problem 5:
You have a pair of fabulous stereo speaker cabinets. Each cabinet contains 5 individual speakers. Unfortunately, one of the individual speakers is broken, so you buy an identical replacement and install it yourself. In the process, you reverse the two electrical connections to the speaker so that current flows backward through the speaker coil. The speaker cone moves the wrong way and makes the entire stereo sound odd. The speaker cone moves the wrong way because
(A) the current flowing backward through the speaker coil magnetizes it backward so that it is pushed in the wrong direction by the speaker's permanent magnet.
(B) the charge on the speaker coil becomes positive rather than negative so that it is pushed in the wrong direction by the speaker's permanent magnet.
(C) the charge on the speaker coil becomes negative rather than positive so that it is pushed in the wrong direction by the speaker's permanent magnet.
(D) the current flowing backward through the speaker coil polarizes its electrical charges backward so that they are attracted when they should be repelled and repelled when they should be attracted.
Answer: (A) the current flowing backward through the speaker coil magnetizes it backward so that it is pushed in the wrong direction by the speaker's permanent magnet.
Why: The speaker cone's coil becomes magnetic when current flows through it and it either attracts or repels the nearby permanent magnet. If you accidentally reverse the current flow through the coil, the speaker cone will move opposite to its intended motion. This can be a problem in complicated systems involving multiple speakers because those speakers can end up interfering with one another's sound rather than aiding one another.
Problem 6:
You are watching a child is flying a kite at the park. The kite is hovering motionless in the sky, about 100 m above the ground. The wind is blowing smoothly toward the east. The net force on the kite is
(A) zero.
(B) in the upward direction.
(C) toward the east.
(D) toward the west.
Answer: (A) zero.
Why: The kite is motionless so it is not accelerating. Anything that is not accelerating is experiencing zero net force.
Problem 7:
You are riding on a swing at the local playground. As you swing back and forth, you begin to think about your speed and kinetic energy (this is obviously a fictional story). These two quantities clearly change between the top of each swing (when you are reversing directions) and the bottom of each swing (when you are passing directly beneath the supporting beam). You wonder when each of these two quantities is at its maximum value. Actually, your speed is at its maximum
(A) at the bottom of a swing and your kinetic energy is at its maximum at the bottom of a swing.
(B) at the bottom of a swing and your kinetic energy is at its maximum at the top of a swing.
(C) at the top of a swing and your kinetic energy is at its maximum at the bottom of a swing.
(D) at the top of a swing and your kinetic energy is at its maximum at the top of a swing.
Answer: (A) at the bottom of a swing and your kinetic energy is at its maximum at the bottom of a swing.
Why: You come to a stop as you reverse directions at the top of each swing. At that moment, your speed is zero and you have no kinetic energy at all. All of your energy is stored as gravitational potential energy. As you swing through the bottom of each swing, you are moving as fast as possible. You have converted your gravitational potential energy into kinetic energy.
Problem 8:
The presence or absence of a few electric charges on the gate of a MOSFET transistor can dramatically change that transistor's
(A) ability to conduct electrical current.
(B) temperature and the color of the light it emits.
(C) magnetic field and its ability to attract nearby magnetic poles.
(D) frequency and the number of radio waves that it emits each second.
Answer: (A) ability to conduct electrical current.
Why: An MOSFET transistor's entire purpose for existing is that it can change its electrical resistance under the control of a small amount of charge on its gate. This feature allows a small charge (placed on the gate) to control the flow of a substantial electrical current (between the source and drain).
Problem 9:
Many mail order catalogs sell lamp dimmer disks. These disks, which you insert in the sockets of incandescent lamps, promise to reduce your electrical bills while making the light bulbs last virtually forever. Unfortunately, these disks simply reduce the power delivered to the bulbs' filaments so that they operate below their rated temperature and waste energy producing infrared light. The dimmer disk actually contains a single diode that forms a series circuit with the light bulb. AC current from the power line must flow through the diode and then through the filament (or vice versa). Overall, this diode reduces the power delivered to the filament by about a factor of two because
(A) the diode only allows current to flow in one direction so that during one half of each cycle of the AC power line, no current flows through the light bulb.
(B) the diode divides the current in half and only allows one half of that current to flow through the filament.
(C) the diode behaves like a filament so that there are two filaments in a series, with each of them receiving half the power that a single filament would receive.
(D) the diode halves the voltage of the current passing through the light bulb so that the power it receives is also halved.
Answer: (A) the diode only allows current to flow in one direction so that during one half of each cycle of the AC power line, no current flows through the light bulb.
Why: A diode is a one-way device for electrical current. The pn junction in the diode prevents electrons from moving from the p to n side of the diode's internal structure. When you put a diode in series with a light bulb, current can only flow through that light bulb half the time; when the AC current is flowing so that electrons move from the n to p side of the diode. As a result, the light bulb only receives electrical power half the time and averages about half its normal power. It glows dimly.
Problem 10:
The back of your UVA ID card has a strip of magnetic tape on it. Like a music tape, this strip stores information as a pattern of magnetized patches. The machine that reads this information is essentially a tape player. When someone uses the reader to read your ID card, they pull the card quickly through the reader. It's important that the card move through the reader because the playback head can only respond to moving or changing magnetic fields. That is because moving or changing magnetic fields
(A) produce electric fields that can cause currents to flow in a coil of wire.
(B) produce temperature fluctuations that can easily be detected with a bimetallic strip thermometer.
(C) generate light in photocells; making it possible to detect the pattern of magnetization on the strip.
(D) can change the weight of a small steel ring so that it accelerates up or down.
Answer: (A) produce electric fields that can cause currents to flow in a coil of wire.
Why: Virtually all magnetic tape readers detect the magnetization on a tape by the currents it induces in the read head as it moves by. For this reading process to work, the tape must be moving because only moving or changing magnetic fields create the electric fields that induce current flows in the read head. There are some very modern tape read heads that can handle non-moving tapes, but these read heads use very sophisticated solid-state effects to detect the tape's magnetization.
Problem 11:
The aluminum rotor of an AC induction motor has no electrical connections and is not a permanent magnet. Nonetheless, it becomes magnetic during the motor's operation and is dragged around in a circle by the magnetic field around it. The rotor becomes magnetic because it is
(A) exposed to a changing/moving magnetic field that causes currents to flow in it.
(B) subject to vibrations which create an electrical dipole in it and give it a magnetic polarization.
(C) made of aluminum, an intrinsically magnetic metal. When you bring a magnet up to a piece of aluminum, the aluminum develops a north and south pole and is attracted to the magnet.
(D) full of magnetic north poles that migrate to the surface when the rotor is spinning.
Answer: (A) exposed to a changing/moving magnetic field that causes currents to flow in it.
Why: An induction motor can move its aluminum rotor by inducing currents and magnetism into that rotor. As the magnetic field around the rotor moves, it causes currents to flow inside the rotor and the rotor becomes magnetic. The rotor's magnetic poles are repelled by the moving poles around it and the rotor ends up moving along with those moving poles; chased around in a circle endlessly.
Problem 12:
If you were to build a motor using only permanent magnets, its rotor might spin briefly but it would soon come to a stop as friction turned its energy into heat. To keep the rotor turning, you must replace one of the magnets with an electromagnet and control that electromagnet so that it always does work on the rotor, increasing its energy. To do work on the rotor, the electromagnet should be adjusted so that it always
(A) attracts each magnet that moves toward it and repels each magnet that moves away from it.
(B) attracts each magnet that moves toward it and attracts each magnet that moves away from it.
(C) repels each magnet that moves toward it and repels each magnet that moves away from it.
(D) repels each magnet that moves toward it and attracts each magnet that moves away from it.
Answer: (A) attracts each magnet that moves toward it and repels each magnet that moves away from it.
Why: The electromagnet's magnetic poles must always do work on the permanent magnets by attracting them as they approach and repelling them as they move away. If instead it does negative work on those permanent magnets, the motor's energy will diminish (creating electrical energy instead) and everything will stop moving.
Problem 13:
The blades of a fan do work on the air in blowing it across the room. An electric motor keeps those fan blades turning. If you remove the fan blades from the motor, the motor will
(A) keep turning but consume less electrical power.
(B) keep turning but consume more electrical power.
(C) keep turning but consume the same amount of electrical power.
(D) stop turning.
Answer: (A) keep turning but consume less electrical power.
Why: The motor will have almost nothing to do work against so it will turn easily and extract very little power from the electrical power line. The unloaded motor may turn faster than it did with the blades attached but it will still do much less work against friction than it did against the fan blades and the air so it will still use less power. Actually, most fans are powered by induction motors and these motors turn at a relatively fixed rate, whether they are doing work or not.
Problem 14:
An electrical insulator can't carry an electrical current because
(A) its electrons can't respond to an electric field.
(B) it contains no electrons.
(C) its electrons are positively charged rather than negatively charged, as they are in an electrical conductor.
(D) it contains no atoms.
Answer: (A) its electrons can't respond to an electric field.
Why: The electrons in an insulator fill the valence levels complete but leave the conduction levels unoccupied. Because these two types of levels are widely separated in energy, it is very difficult for a valence level electron to move to a conduction level. Thus, when you apply an electric field to the insulator in hopes of sending a current through it, the electrons cannot respond. They continue to move in their original paths and there is no net flow of electric charge across the insulator.
Problem 15:
The huge steam-powered generators found in electrical power plants produce electricity by
(A) moving magnets past coils of wire.
(B) moving electric charges up and down capacitors.
(C) turning iron cores inside of transformers.
(D) rubbing copper disks against sheets of glass.
Answer: (A) moving magnets past coils of wire.
Why: The most efficient and cost-effective way of turning mechanical work into electrical energy is to move magnets past wires. That is what is done in virtually all electrical power generation. The one significant example is photoelectric cell generation of electricity, but that technique doesn't involve steam.
Problem 16:
Your pet rabbit has chewed the cord to your desk lamp and has created a short circuit; an electrical connection from one wire to the other inside the cord. When you plug the lamp into the electrical socket,
(A) current will bypass the bulb and the bulb will not light up.
(B) excessive current will pass through the bulb and the bulb will glow very brightly.
(C) the current will begin to flow backward through the bulb so that it glows at half its normal brightness.
(D) current will flow alternately through the bulb and through the short circuit, so that the bulb will blink on and off rapidly.
Answer: (A) current will bypass the bulb and the bulb will not light up.
Why: The direct connection between the two wires of the cord will provide an easy path for the electrical current. It will skip the lamp entirely so the lamp will not light. This failure should blow the circuit breaker because too much current will flow through the short circuit. If it doesn't, the wires may overheat and start a fire.
Problem 17:
The principal advantage of sending electrical power across the country on very high voltage transmission lines is that
(A) the electrical power lost in the wires is greatly reduced.
(B) these transmission lines are less likely to get in the way than low voltage transmission lines (which are much closer to the ground).
(C) they carry much more current than low voltage transmission lines.
(D) they carry less energy per charge than low voltage transmission lines.
Answer: (A) the electrical power lost in the wires is greatly reduced.
Why: Since power wasted in an electrical conductor is proportional to the square of the current flowing through it, the best approach to minimizing waste is to reduce the current in the transmission lines. That is done by sending a small current of very high voltage charges through those transmission lines. The number of charges pass through the lines each second (the current) is small but the energy carried by each charge (the voltage) is large. Overall, the power is still transferred but less of it is squandered in the transmission process.
Problem 18:
A battery
(A) pumps positive charge from its negative terminal to its positive terminal.
(B) pumps positive charge from its positive terminal to its negative terminal.
(C) creates positive charge.
(D) creates negative charge.
Answer: (A) pumps positive charge from its negative terminal to its positive terminal.
Why: A battery adds energy and power to a current flowing through it. It does this by creating a voltage rise: as current passes into its negative terminal, the battery pumps that current (of positive charges) to its positive terminal. The battery does work on that charge and adds energy to it.
Problem 19:
Ball bearings permit a wheel to turn freely on an axle without creating any heat because they form a mechanical system that involves
(A) no sliding friction.
(B) no static friction.
(C) no friction of either type.
(D) no electricity.
Answer: (A) no sliding friction.
Why: As the balls turn, they touch the wheel hub and the axle but do not slide across either one. The static friction that occurs as these objects touch one another does not waste energy as heat but sliding friction does. Since there is no sliding in a ball bearing, there is essentially no heat production.
Problem 20:
A gymnast doing a double back flip leaps off the floor with her arms and legs extended and then pulls herself into a very compact position. In her compact shape, she rotates very rapidly and completes two full rotations before opening back up to land on the floor. During the time that she is not touching the floor, the one aspect of her motion that is constant is her
(A) angular momentum.
(B) momentum.
(C) velocity.
(D) angular velocity.
Answer: (A) angular momentum.
Why: While the gymnast is in the air, the only force she experiences is gravity. Gravity changes her velocity and momentum (she travels up then down) but it cannot exert any torque on her about her center of mass. That is because gravity acts at her center of gravity, which is coincident with her center of mass. With no lever arm over which to act, the force of gravity produces no torque. Since she experiences no torque, her angular momentum cannot change. Her angular velocity may vary if she changes her shape.
Problem 21:
Even when you are driving at a constant 60 miles-per-hour along a straight, level road, your car's engine must be running. As the engine turns the car's wheels, friction between the ground and the tires exerts a forward force on the car. The car needs this forward force from the ground because
(A) air drag (air resistance) exerts a backward force on the car.
(B) an object that is moving requires a net force to keep it moving. In the absence of any net force, objects are motionless.
(C) an object's velocity points in the direction of the net force on that object.
(D) the car has a velocity and is thus accelerating. In order to accelerate, the car must be experiencing a net force.
Answer: (A) air drag (air resistance) exerts a backward force on the car.
Why: As an object moves through the air, the air pushes backward on that object with a drag force. Drag forces always oppose relative motion and thus resemble frictional forces between surfaces. To keep a car moving forward against the backward force of air resistance, the ground must push the car forward.
Problem 22:
There is a large bar magnet built into the table in front of you. The north pole of that magnet is exposed and points upward. You have a small bar magnet in your hand. As you move the small magnet toward the table, its south pole is attracted toward the north pole of the table's magnet and the two poles stick together tightly. However, the north pole of the small magnet is repelled by the north pole of the table's magnet. The reason why this repulsion doesn't push the small magnet away from the table is that
(A) the two north poles are relatively far apart and the forces between magnetic poles decrease quickly as the distance between them increases.
(B) the north pole of the small magnet is weaker than its south pole.
(C) the south pole of the small magnet shields its north pole so that the magnetic forces do not affect the north pole as much. (The north pole lies behind the south pole.)
(D) the attractive forces between opposite magnetic poles are inherently stronger than the repulsive forces between like magnetic poles.
Answer: (A) the two north poles are relatively far apart and the forces between magnetic poles decrease quickly as the distance between them increases.
Why: As you move the magnet in your hand toward the north pole on the table, both poles of your magnet experience forces. However the strength of the forces involved depend on the distances separating the poles. When the south pole of you magnet is very close to the fixed north pole, the attraction is very strong. The north pole of your magnet is still being repelled by the fixed north pole, but since their separation is larger, that repulsive force is relatively weak. Overall, you little magnet is attracted toward the fixed north pole.
Problem 23:
You are doing exercises at the gym. When you lift a weight over your head, you push upward on it both as you lift it and as you lower it. However, when you work out with a particular exercise machine, you push upward as you lift its bar but must pull downward to lower that bar. When you use that exercise machine,
(A) you do work on the bar as you raise it and as you lower it.
(B) you do work on the bar as you raise it but it does work on you as you lower it.
(C) its bar does work on you as you raise it and as you lower it.
(D) its bar does work on you as you raise it but you do work on it as you lower it.
Answer: (A) you do work on the bar as you raise it and as you lower it.
Why: In this exercise machine, you must do work on the bar whether you move it up or down. You must push it upward so you do work on it as you lift it (force in the direction of motion). You must pull it downward so you do work on it as you lower it (force in the direction of motion).
Problem 24:
A quick trip down a plastic playground slide has caused a child's thin white-blond hair to begin standing straight up so that she looks like a dandelion puff. During the trip down the slide, sliding friction has
(A) transferred electric charge to her body and hair so that the like charges on her hair are repelling one another strongly.
(B) polarized her body so that she has a north pole near her head and a south pole near her feet. Her hair is following the lines of magnetic force that extend between the two poles.
(C) turned her kinetic energy into heat and given her hair a blow-dried look as a result.
(D) aligned the magnetic poles inside her body so that they all point in one direction. Her hair is levitated magnetically.
Answer: (A) transferred electric charge to her body and hair so that the like charges on her hair are repelling one another strongly.
Why: Sliding friction often transfers electric charge. In this case, it put a large charge on the child and some of this charge ended up on her hair. Since the charge in each hair is the same, the individual charged hairs repel one another and her hair stands up like a dandelion puff.
Problem 25:
The small negative ion generators that are sold in appliance stores as a way to "improve your health" create negative ions in the air by transferring negative charge from a metal surface to passing air molecules. They generally use a number of sharp metal whiskers rather than a smooth metal ball to transfer the charge to the air because negative charges
(A) placed on sharp points repel one another more strongly than those placed on smooth surfaces.
(B) flow best through wires and the metal whiskers are essentially wires.
(C) the whiskers can touch one another to form complete circuits and charge only flows when there is a complete circuit present.
(D) cannot accumulate on a metal ball without repelling one another.
Answer: (A) placed on sharp points repel one another more strongly than those placed on smooth surfaces.
Why: While you can put a large amount of charge on a round ball, you cannot do the same with a sharp whisker. On the whisker, the charges are too close together and the forces they exert on one another a very strong. They can become so strong that some of the charges are pushed right out into the air, where they charge the air molecules or dust particles.
PART II: SHORT ANSWER QUESTIONS
Please give a brief answer in the space provided. Part II is worth 33% of the grade on the midterm examination.
Problem 1:
You're trying to open a wide-mouthed jar of pickles. You hold the jar tightly on one hand while twisting the lid with the other hand. But try as you might, you cannot get the lid to turn.
(A) What is the net torque on the lid? zero.
Why: The lid is not turning and it is also undergoing no angular acceleration. According to Newton's First Law of Rotational Motion, the lid must be experiencing zero net torque.
(B) Your grip loosens and your hand begins to slip across the lid. Your skin feels warm as a result. Why? Your skin is experiencing sliding friction and sliding friction turns work into heat.
Why: Work done against the forces of sliding friction is turned into heat, making the surfaces involved warmer.
(C) You find a jar opening device that grips the lid tightly in its jaws and has a long handle that extends outward from the side of jar. By exerting a modest force on this handle, you can exert a very large torque on the lid. Why does the long handle increase the amount of torque you can exert? The torque you exert is the product of the force you exert at right angles to the handle times the handle's length (the lever arm). The longer the handle, the more torque.
Why: When you use a force to produce a torque, that torque depends on where you exert the force. Since the torque that might open the pickle jar is exerted around the center of the lid, the distance between the center of the lid and the location of your force is very important. If you double that distance by using a long handle, you double the torque without changing the force at all.
(D) The jar still doesn't open so you throw it out the window and into the woods behind your house. As it flies, you notice that the jar continues to rotate at a steady rate about a fixed axis, up until the moment it hits a tree. Why doesn't its rotation slow down in flight, as it would have if you had spun the jar on the kitchen counter? The rigid jar is experiencing no torque so it spins with constant angular velocity.
Why: When the jar is in the air, nothing exerts a significant torque on it (the torque of air resistance is very small). As a result, its angular momentum remains constant. Since the jar is rigid, its moment-of-inertia is constant, so its constant angular momentum gives it a constant angular velocity. That is Newton's First Law of Rotational Motion (which applies to a rigid object that experiences zero torque).
Problem 2:
The battery in your friends' car is dead so you are using jumper cables to help them get started. You are going to use the fresh battery in your car to power the starter motor in their car. If you can send enough current through the starter motor, it will make their engine turn.
(A) You connect the first jumper cable from the positive terminal of your car's battery to the positive terminal of the battery in your friends' car. One of them then tries to start their car but nothing happens. Why isn't the first cable helping? With only one cable attached between the cars, the circuit is open and current cannot flow continuously.
Why: For current to flow continuously through your car battery and your friends' car, there must be a complete circuit present. With only one cable connecting the two, positive charge can flow briefly to your friends' car, but there is no route through which it can return to your car battery to pick up more energy. Current soon stops flowing and no more power is delivered to your friends' car.
(B) You connect the second jumper cable from the engine of your car (which is electrically connected to the negative terminal of its battery) to the engine of your friends' car (which is electrically connected to the negative terminal of its battery). One of them then tries to start their car and the engine begins to turn slowly. Briefly describe the path through which electrical current is flowing in this situation. Current leaves your battery's positive terminal through the first jumper cable, passes through your friends' car's starter motor, and returns to your battery's negative terminal through the second jumper cable.
Why: Current travels through a complete circuit, which in this case includes the battery, the starter motor, and two jumper cables. The current (of positive charges) flows from the positive terminal of the battery, through the first jumper cable, through the starter motor, back through the second jumper cable, to the negative terminal of the battery. The battery pumps this current from its negative terminal to its positive terminal and the current keeps flowing around the circuit.
(C) Your jumper cables are inexpensive and light weight. They get warm as your friend operates the starter motor in their car and the starter motor turns slowly; too slowly to start the car. Why are these cheap jumper cables so ineffective? Their electrical resistance is too great or they waste too much electrical power as heat.
Why: Cheap jumper cables do not have enough good copper or aluminum in them to carry the large currents needed to power a starter motor. The current loses too much energy as heat in passing through the cables so that there is relatively little power left to operate the starter motor.
(D) A passing motorist loans you a pair of heavy duty jumper cables and these cables start your friends' car immediately. You then notice that your friends' head lights were on the whole time, running in a parallel circuit with the starter motor. Why did the head lights make it harder to start their car through your cheap jumper cables? Current from the jumper cable was being shared by the starter motor and the head lights so that the starter motor received even less power than if the head lights had been off.
Why: When two devices are wired in parallel, they share the current that reaches them through one wire. In this case, the head lights and the starter motor shared the current reaching them from the positive jumper cable. With that cable already limiting the current flow and sapping its energy, this sharing process made it even more difficult for the starter motor to get the power it needed to turn the engine. With the head lights off, it might have had a chance.
Problem 3:
A blender is a common kitchen appliance. It consists of a glass or plastic pitcher with a rotating blade at the bottom. The pitcher sits in a base containing an electric motor. When you push the on button, the motor spins very rapidly and turns the blade. The spinning blade stirs and liquefies the contents of the pitcher.
(A) If you put an ice cube into the pitcher and push the on button, the blade spins and chops the ice cube into small fragments. The bottom of the pitcher is smooth and the ice cube is slippery, so no outside forces keep the ice cube from moving and staying ahead of the spinning blade. Still the ice cube stays put and the blade slices through it. What holds the ice cube in place? The ice cube's inertia.
Why: It takes a force and some time to get the ice cube moving. If the blade approaches the ice cube quickly enough, the ice cube will not have time to accelerate enough to avoid having the blade cut through it. Although the blade exerts a huge force on the ice cube, the ice cube's inertia prevents it from getting out of the blade's way quickly enough. Thus the blade slices through the ice cube despite the ice cube's apparent freedom to move.
(B) The blender plugs into an electrical outlet. When you push the on button, the blender mixes. At a particular moment during its operation, which way is electrical current flowing through each of the two wires in the blender's power cord? It is flowing toward the blender through one wire and returning from the blender through the other wire.
Why: The blender cannot accumulate charge so it must accept charge through one wire and return it through the other wire. Although the direction of current flow through the circuit reverses many times each second, at any given moment, the current is flowing in through one wire and out through the other.
(C) The blender contains a universal motor and can actually run on either AC or DC electrical power. If you reverse the blender's plug, so that the two prongs trade places in the outlet, will the motor and blade continue turning in the same direction or will they now turn in the opposite direction? The motor and blade will continue turning in the same direction.
Why: A universal motor is designed to turn in one direction even though it is operating on AC current. It is used to having the current reverse directions every 1/120th of a second. When you reverse the electrical connections between it and the power company, you are merely adding one more current reversal. It doesn't respond and continues to turn in its usual direction.
(D) You leave a beverage mixing in the blender while you answer the telephone. When you return, you find that the beverage has become quite warm. How did the spinning blade heat up the liquid? The spinning blade is doing work on the beverage and that work is being converted into heat.
Why: Work done against a disappative force such as sliding friction or drag is converted into heat. As the blade pushes through the beverage, it does work on that beverage (force times distance) and the beverage becomes hotter and hotter.