Recitation 8-10

169. What are three greenhouse gases and what are their sources?

Carbon dioxide - burning of fossil fuels Water vapor - natural source from evaporation of water Methane - burning of natural gas

145. 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)

144. Why are thick wires rather than thin wires usually used to carry large currents?

Thicker wire have less resistance, which according to ohms law means lower current and therefore they dissipate less power ( P=I2R)

147. 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.

151. 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 charge. Also the spin of the electron on its axis also creates a magnetic field.

142. 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.

152. 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).

155. 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.

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

An electrically charged material is one with an excess or deficiency of electrons, compared with the number of protons in the atomic nucleus of the material.

128. 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?

As a car travels through the air friction "strips" electrons from the atmosphere and causes them to build up on the surface. The wires at toll-collecting stations are highly electrically conductive. When the electrons on the surface of the car come in contact with the wires they are easily transferred to wires rather staying on the car (this is called bleeding off charge). Therefore paying the toll is not a shocking experience for the driver

154. 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 magnetic 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.

153. 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.

148. 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.

125. 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 electrons from a garment of one type material are rubbed onto another type of material. One become positive and the other negative depending on where they are on the triboelectric series. Due to the opposite charge the clothes stick or cling onto one another. The clothes stay charged for a long time since they are insulators. If you fabric softener or a dryer sheet to the dryer static cling can be avoided. I you don't add these, after removing clothes from the dryer rub them with a metal wire (i.e. a metal hanger) to discharge the clothes (electrons will flow from clothes to the metal to your body and ground)

166. You can get a sunburn on a cloudy day, but you can't get a sunburn even on a sunny day if you are behind glass. Explain.

Clouds are transparent to ultraviolet light, which is way clouds offer no protection from sunburn. Glass, however, is opaque to ultraviolet light and will therefore shield you from sunburn.

150. 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!

140. Does more current flow out of a battery than into it? Does more current flow into a lightbulb than out of it? Explain.

Current flows through electrical devices, just as water flows through a plumbing circuit of pipes. If a water pump produces water pressure, water flows through both the pump and the pipes. The analogy for a circuit is the battery acts as the pump and the pipes are the wires. The water doesn't come from the pump just as the electron don't come from the battery. The current that flows in the lamp is the same electric current that flows through the wires and through the battery.

139. What is the direction of the electric current produced by an electron that falls towards the ground?

Electric current is defined as the direction that positive charges move. In this case it is the negative charge that is moving so the direction of the current is upward.

146. Why is the current in an incandescent bulb greater immediately after it is turned on than it is a few moments later?

Electrical energy is dissipated in the filament and its temperature increases. Most materials when they are heated increase their resistance since there is more likely to be collisions between electrons and the lattice. Higher resistance according to Ohm's law means less current and therefore less electrical energy dissipation (P=I2R) and therefore less light.

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

Electrically conducting tires now is use 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. Tires are made electrically conductive by adding carbon black to them. In the old days before tires were made electrically conductive all cars had a conductive strap attached to them that dragged along the roadway, providing a conductive path for static buildup

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

Electrons are loosely bound on the outside of atoms, whereas protons are very tightly bound within the atomic nucleus

131. 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.

132. What causes lightning?

Lightning is a very large electrical discharge (spark) between the clouds and the ground. Within a towering rain cloud there are ice particles of different sizes. When these rub against one another the large ice particles (hail) become positively charge and the smaller ice particles become negatively charged. The small particles gather or fall to the base of the cloud making it negatively charged. The ground or earth is very conductive and has lots of free electrons which are repelled by the cloud. Therefore the ground becomes positive and the base of the cloud is negative creating a large electric field which may cause a large electrical discharge know as lightning.

158. 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 strings for an electric guitar

126. Why does dust tend to accumulate on electronic components?

Many electronic components get hot and need to be cooled by a fan. The moving dust rubs against surfaces of the electronics and becomes charged and sticks to other surfaces (any insulating surface, like plastic)

138. 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.

135. 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.

136. 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.

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

No. Conservation of charge states that the total charge in the universe remains constant, charge cannot be created or destroyed.

156. 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.

161. 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)

168. What are the similarities and differences between radio waves and light?

Radio waves and light are both electromagnetic waves. Move at the speed of light and are created by the oscillating charges. They differ in their wavelength.

164. List the following in order of wavelengths-light waves, X-rays, or radio waves? Which has a higher speed in a vacuum?

Radio, light, x-rays. All electromagnetic waves travel at the same speed- 300,000 km/sec.vacuum?

143. Will a lamp with a thick filament draw more current or less current than a lamp with a thin filament?

Remember that resistance is R = ρL/A . Thicker wire has a larger cross-sectional area, A. So the thicker wire (lower AWG or gauge) has lower resistance.

133. 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

162. A magician places an aluminum ring on a table. Underneath is a hidden electromagnet. 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 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.

149. 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.

167. When astronomers observe a supernova explosion in a distant galaxy, they see a sudden, simultaneous rise in visible light and other forms of electromagnetic radiation. Is this evidence to support the idea that the speed of light is independent of frequency? Explain.

The fact that the different parts of the electromagnetic spectrum emitted in the explosion are received simultaneously is evidence for the frequency (and wavelength) independence of the speed of light. If wave speed depended on frequency, different frequencies would be received at different times

163. What is the fundamental source of electromagnetic radiation?

The fundamental source of electromagnetic radiation is oscillating electric charges that emit oscillating electric and magnetic fields.

129. 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, like the rest of the electroscope, acquire charge from the charged object, and repel each other since they both have the same sign of charge. The weight of the conducting gold foil is so small that even tiny forces are evident.

157. 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.

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

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

134. 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 wasf irst indicated by this observation.)

The reason the leaves of an electroscope collapse at lower altitude is that the excess electric charge bleeds off into the moisture (electrical conductor) of the atmosphere. At high levels of the atmosphere, cosmic rays from space are more likely to produce ions (they are more energetic due to less collisions as they travel to the lower levels of the atmosphere which has a higher density of particles). 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.

165. We hear people talk of "ultraviolet light" and "infrared light." Why are these terms misleading? Why are we less likely to hear people talk of "radio light" and "X-ray light"?

The terms are misleading in that they imply that ultraviolet and infrared are forms of visible light. More correctly they are forms of electromagnetic radiation. So in the sense used "light" is used to mean "electromagnetic radiation." The usage stems from the fact that the ultraviolet and infrared regions of the spectrum are adjacent to visible light. The terms "radio light" and "x-ray light" are rarely ever used.

121. We do not feel the gravitational forces between ourselves and the objects around us because these forces are extremely small. Electrical forces, in comparison, are extremely huge. Since we and the objects around us are composed of charged particles, why don't we usually feel electrical forces?

There are no positives and negatives in gravitation -the interactions between masses are only attractive, whereas electrical interactions may be attractive as well as repulsive. The mass of one particle cannot "cancel" the mass of another, whereas the charge of one particle can cancel the effect of the opposite charge of another particle.

127 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.

160. 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.

141. 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.

159. 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 create. 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.

137. 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.