Physics Exam 3

Your friend says that, when jump-starting a dead battery, you should connect your live battery in parallel with the dead battery, which, in effect, replaces the dead one. Do you agree?

Agreed. When you jump start a car you actually use the live battery from the other car to start the other the dead car. So you connect the battery in parallel. Wait a few minutes while the car charges the dead battery and then take it off. The dead battery will continue to charge from the started cars alternator.

In what sense are all magnets electromagnets?

All magnetism originates in moving electric charges. An electron rotating around the nucleus of an atom is an electric current, Ampere's law shows that a magnetic field is created by a moving electric charge. Also the spin of the electron on its axis also creates a magnetic field.

Only a small percentage of the electric energy fed into a common lightbulb is transformed into light. What happens to the remaining energy?

All of the electrical energy gets "consumed "by the filament and it is converted to thermal energy. The temperature of the filament increases and gives off electromagnetic energy in the form of light. But only 5% of the thermal energy is converted to light.

All atoms have moving electric charges. Why, then, aren't all materials magnetic?

All of the moving charges in all material do create magnetic fields, however many times these fields cancel due to the randomness of each current loop. Sometimes the many of the current loops are oriented in the same direction - called domains. But the domains are often randomly oriented and their fields cancel. Remember the magnetic field is a vector (magnitude and direction).

What kind of force field surrounds a stationary electric charge? What additional field surrounds it when it moves?

An electric field surrounds a stationary electric charge. An electrical field and a magnetic field surround a moving electric charge. A gravitational field always surrounds an electric charge whether it is stationary or moving.

At the atomic level, what is meant by saying something is electrically charged?

An electrically charged material is something with too many or not enough electrons compared to the number of protons in the nucleus of it

The leaves of a charged electroscope collapse in time after the charge object is removed. At higher altitudes, they collapse more rapidly. Why is this true? (Hint: The existence of cosmic rays was first indicated by this observation.)

At high levels of the atmosphere, cosmic rays from space are more likely to produce ions. The ions are produced when the high energy cosmic rays knock away an electron. Ions are charged particles and are electrically conductive and bleed off the charge from an electrical at a faster rate than normally.

If you place a chunk of iron near the north pole of a magnet, attraction will occur. Why will attraction also occur if you place the same iron near the South Pole of the magnet?

Attraction will occur because the magnet induces opposite polarity in the chunk of iron. North will induce south, and south will induce north. This is similar to charge induction, where a balloon will stick to a wall whether the balloon is negative or positive.

To make a compass, point an ordinary iron nail along the direction of the Earth's magnetic field (which, in the Northern Hemisphere, is angled downward as well as northward) and repeatedly strike it for a few seconds with a hammer or a rock. Then suspend it at its center of gravity by a string. Why does the act of striking magnetize the nail?

Beating on the nail shakes up the domains, giving them enough energy to align in the direction of the Earth's magnetic field. The result is a net alignment of domains and a magnetic field. Note: if you drop a magnetic sometimes its magnetic field will become weaker - the kinetic energy from the drop is enough to un-align.

Are car headlights connected in series or parallel? Give an everyday observation that supports your answer. Draw a sketch of your answer.

Car headlights are wired in parallel, as we can tell by the fact that some cars have only one working headlight. When a circuit is wired in parallel if one component such as the light bulb fails and doesn't allow current to flow there is still a path for current to flow back to the power source or battery

Why do clothes often cling together after tumbling in a clothes dryer? What can you add to your dryer to prevent this from happening?

Clothes become charged when two different types of materials rub one another, one becoming positive and the other negative. The opposite charges cause the clothes to cling to eachother and the charge to stay longer because they are insulators.

Why do farmers feed cow's magnets?

Cow magnets are popular with dairy farmers and veterinarians to help prevent Hardware Disease in their cattle. While grazing, cows eat everything from grass and dirt to nails, staples and bits of bailing wire (referred to as tramp iron). Tramp iron tends to lodge in the honeycombed walls of the reticulum, threatening the surrounding vital organs and causing irritation and inflammation, known as Hardware Disease. The cow loses her appetite and decreases her milk output (dairy cows), or her ability to gain weight (feeder stock). Cow magnets help prevent this disease by attracting stray metal from the folds and crevices of the rumen and reticulum. One magnet works for the life of the cow!

Why does dust tend to accumulate on electronic components?

Electronics become hot so they need to be cooled by a fan and the plastic around the electronic is an insulator so the charged dust being moved around by the fan sticks to the plastic.

Why is a good conductor of electricity also a good conductor of heat?

For both electricity and heat, the conduction is via electrons, which in a metal are loosely bound and easy flowing.

At some automobile toll booths, a thin metal wire protrudes from the road, making contact with cars before they reach the toll collector. What is the purpose of this wire?

Friction "strips" electrons from the atmosphere as a car travels and builds up on the surface of the car. The wires are highly electrically conductive and the electrons transfer from the car to the wire so that the toll collector does not get shocked every time they are handed money.

What is one method by which lightning can form? Explain

Frictional charging. Clouds are known to contain countless millions of suspended water droplets and ice particles moving and whirling about in turbulent fashion.

What is lightning? What are 4 ways by which it may occur?

Lightning is a sudden electrostatic discharge that occurs typically during a thunderstorm. This discharge occurs 1) Within a cloud 2). From one cloud to another 3).From one cloud to the surrounding air 4)Cloud to ground

Why are the tires for trucks carrying gasoline and other flammable fluids manufactured to be electrically conducting?

Like the strips at the toll booth, Electrically conducting tires now is used to prevent the buildup of static charge when the truck travel through air and the friction strips electrons from the atmosphere. Static buildup could cause a spark which is especially dangerous for trucks carrying flammable cargoes.

A common pickup for an electric guitar consists of a coil of wire around a small permanent magnet, as shown in the lecture 16. Why will this type of pickup fail with nylon strings?

Magnetic induction does not occur in nylon since it has no magnetic domains. You must use steel or magnetic strings for an electric guitar

Is a current-carrying wire electrically charged? Explain your answer

No because for every electron that is moving in the wire there is a proton in the material. So the wire is electrically neutral. The electrons moving in the wire to create the current come from the wire itself and not the battery.

When an object that was neutral becomes charged, does the total charge of the universe change? Explain

No conservation of charge says that charge can not be created or destroyed in the universe

Your body is composed of electric charges. Does it follow then that you produce an electric current when you walk?

No. An electric current is produced when a net charge moves. If your body is electrically neutral, no current is produced when you walk.

Suppose you charge a comb by rubbing it through your hair. Do you produce a current when you walk across the room carrying the comb?

No. By rubbing the comb through your hair you have transferred charge from your hair to your comb, but the net charge of you and the comb together is still zero. No net charge is moving so the current is zero.

Why aren't permanent magnets really permanent?

Over time, domains are knocked out of alignment and become randomly aligned. This happens especially if the magnetics are exposed to high temperatures, the added thermal energy helps to move the domains.

When an automobile moves over a wide, closed loop of wire embedded in a road surface, is the magnetic field of the Earth within the loop altered? Is a pulse of current produced? Can you cite a practical application for this at a traffic intersection?

Part of the Earth's magnetic field is enclosed in the wide loop of the wire imbedded in the road. If this enclosed field is somehow changed, then in Faraday's Law of Induction predicts that a current pulse will be produced in the loop. Such a change is produced when the iron pars of a car pass over it, momentarily increasing the strength of the field. A practical application is triggering automobile lights. (Cyclists like me with carbon bikes do not cause this change and do not trigger the traffic lights to change)

Why is it safe to remain inside a car during a lightning storm?

The car is acting like an electrical shield (faraday cage). Since there is no electrical field in interior of the car there is no harm to the occupants - the lightning (electrical discharge) won't be attracted to your body. Do not touch any conductive parts of the car

A magician places an aluminum ring on a table. Underneath is a hidden electromagnet or coil of wire. When the magician says "abracadabra" (and pushes a switch that starts current flowing through the coil under the table), the ring jumps into the air. Explain his "trick."

The changing magnetic field produced when the current starts to flow in the electromagnet under the table induces a current in the aluminum ring. This current, in turn generates a magnetic field that opposes the field produced by the magnetic under the table. Both magnetic fields repel each other, the aluminum ring becomes momentarily a magnet that is repelled by the hidden magnet under the table.

Explain why a comb that has been rubbed through your hair attracts small bits of paper.

The charged comb causes the paper to become polarized, with the side nearest the comb acquiring a charge opposite to the charge of the comb. The result is an attractive interaction between the comb and the paper

Electrons move at a very slow velocity through a wire, at the drift velocity of 10-4 m/s. Why do the lights come on immediately when you turn them on?

The electrons do not flow directly from the power source through the wire. The electrons nearest the lights move into the bulb first and are replaced by other electrons. The phenomenon is analogous to a bowling ball hitting one end of a line of balls: the effect of the colliding ball travels through the line and the one at the end of the line is pushed to your hand almost immediately. Each ball has very little movement.

Why does the filament of a lightbulb glow while the connecting wires do not?

The filament has higher resistance and therefore dissipates more energy than the low resistance wire (P=I2R)

An electroscope is a simple device consisting of a metal ball that is attached by a conductor to two thin leaves of metal foil protected from air disturbances in a jar, as shown. When the ball is touched by a charged body, the leaves that normally hang straight down spread apart. Why?

The leaves receive the charge from the charged object and repel each other since they have the same sign charge.

One way to make a compass is to stick a magnetized needle into a piece of cork and float it in a glass bowl full of water. The needle will align itself with the horizontal component of the Earth's magnetic field. Since the north pole of this compass is attracted northward, will the needle float toward the north side of the bowl? Defend your answer.

The needle is not pulled toward the north side of the bowl because the south pole of the magnet is equally attracted southward. The net force on the needle is zero.

Why are objects with vast amounts of electrons normally not electrically charged?

The objects aren't charged because of their equal number of protons

Why are you more likely to get an electric shock opening up a door in the winter than the summer?

There is generally more humidity or moisture in the air in the summertime. Moisture is electrically conductive and the water molecules "bleed" static charge away from objects. When there is less static charge, the electric field strength is not large enough for a shock to occur.

Some bicycles are equipped with an electric generator to power a light. Will a cyclist coast farther if the lamp connected to the generator on his bicycle is turned off? Explain.

Think about conservation of energy. A cyclist will coast farther if the lamp is disconnected from the generator. The energy that goes into lighting the lamp is taken from the bike's kinetic energy, so the bike slows down.

. Why don't birds get shocked on electrocuted when they are sitting on a high voltage power wire?

When a bird is perched on a single wire, its two feet are at the same electrical potential, so the electrons in the wires have no motivation to travel through the bird's body. No moving electrons means no electric current. Our bird is safe, for the moment anyway... If that bird stretches out a wing or a leg and touches a second wire, especially one with a different electrical potential, it will open a path for the electrons—right through the bird's body.

Why does an iron core increase the magnetic induction of a coil of wire?

When a changing current is applied to the coil of wire, Faraday's law of induction shows that there will be a magnetic field created. The iron becomes magnetized when its domains are aligned by this magnetic field. The magnetic field of the iron adds to the magnetic field from the wire.

Suppose you charge a comb by rubbing it through the fur on your dog's back. Do you produce a current when walk across the room carrying the comb?

Yes. There is a net charge on the comb, and by moving it from one place to another you have created an electric current.

Why is charge usually transferred by electrons rather than by protons?

electrons are loosely bound outside of the nucleus, whereas protons are tightly bound within