WebAssign Jan 30th
Which contributes more to an atom's size?
electrons
Which contributes more to an atom's mass; electrons or protons?
protons
A droplet of ink in an industrial ink-jet printer carries a charge of 16 ✕ 10-13 C and is deflected onto paper by a force of 2.4 ✕ 10-7 N. Find the strength of the electric field to produce this force.
1.50e+05 N/C
If you put in 10 joules of work to push 1 coulomb of charge against an electric field, what will be its voltage with respect to its starting position? When released, what will be its kinetic energy if it flies past its starting position?
10 joules per coulomb is 10 volts. When released, its 10 joules of potential energy will become 10 joules of kinetic energy as it passes its starting point.
What is the voltage at the location of a 0.0002 C charge that has an electric potential energy of 0.4 J (both measured relative to the same reference point)?
2000 V
An energy of 0.5 J is stored in the metal ball on top of a Van De Graaff machine. A spark carrying 17 microcoulomb(s) (17 ✕ 10-6 C) discharges the ball. What was the ball's potential relative to ground?
29400 V
Two point charges are separated by 6 cm. The attractive force between them is 12 N. Find the force between them when they are separated by 12 cm.
3 N
If a large enough electric field is applied, even an insulator will conduct an electric current, as is evident in lightning discharges through the air. Explain how this happens, taking into account the opposite charges in an atom and how ionization occurs.
A neutral atom in an electric field is electrically distorted (see Figure 21.11 on page 381). If the field is strong enough, the distortion results in ionization, where the charges are torn from each other. The ions then provide a conducting path for an electric current.
Measurements show that there is an electric field surrounding the Earth. Its magnitude is about 100 N/C at the Earth's surface and points inward toward the Earth's center. From this information, can you state whether the Earth is negative or positive?
Planet Earth is negatively charged.
When a container of gas is heated, what happens to the average speed of its molecules?
The average speed increases.
By what specific means do the bits of fine threads line up in the electric fields shown in Figure 21.19?
The bits of thread become polarized in the electric field, one end positive and the other negative, and become the electric counterparts of the north and south poles of the magnetic compass. Opposite forces on the end of the fibers (or compass needle) produce torques that orient the fibers along the field direction (look ahead to Figure 23.3 on page 420).
(a) Find the voltage increase when 12 J of work is done to push a 0.0008 C charge into an electric field. (b) Find the voltage increase when 24 J of work is done to push a 0.0016 C charge into the electric field.
15000,15,000
The potential difference between a storm cloud and the ground is 100 million volts. If a charge of 5 C flashes in a bolt from cloud to Earth, what is the change of potential energy of the charge?
5.00e+08 J
Two point charges are separated by 12 cm. The attractive force between them is 24 N. If the two charges attracting each other have equal magnitude, what is the magnitude of each charge?
6.20e-06 C
Suppose that the strength of the electric field about an isolated point charge has a certain value at a distance of 1 m. How will the electric field strength compare at a distance of 2 m from the point charge?
At twice the distance the field strength will be 1/4. the inverse-square law
Suppose that the strength of the electric field about an isolated point charge has a certain value at a distance of 1 m. How will the electric field strength compare at a distance of 2 m from the point charge? What law guides your answer?
At twice the distance the field strength will be 1/4. the inverse-square law
What safety is offered by staying inside an automobile during a thunderstorm? Defend your answer.
Because charges of like sign mutually repel one another, when lightning hits a conductor such as an automobile, the charges will spread out over the outer conducting surface, and the electric field inside cancels to zero. Strictly speaking, it is only static charge that occupies only the outer surface of a conductor and produces a zero field within (see the answer to the next exercise), but the rule is obeyed approximately for currents that are not too large.
A gravitational field vector points toward the Earth; an electric field vector points toward an electron. Why do electric field vectors point away from protons?
By convention only, the direction of an electric field at any point is the direction of the force acting on a positive test charge placed at that point. A positive charge placed in the vicinity of a proton is pushed away from the proton, hence, the direction of the electric field vector is away from the proton.
How can you charge an object negatively with only the help of a positively charged object?
By induction: Bring the positively charged object near the object to be charged and the far side of the uncharged object will become positively charged. If you then touch the far side, you will in effect remove this charge because electrons will flow from your body to the positive charge. Remove your finger and the object then has a negative charge. (Interestingly enough, touching any side will produce the same result.)
With respect to forces, how are electric charge and mass alike? How are they different?
Charge and mass are alike in that both determine the strength of a force between objects. Both appear in an inverse-square law of force. They differ in that charge can be positive or negative while mass is always positive. They differ also in the strength of force.
When one material is rubbed against another, electrons jump readily from one to the other but protons do not. Why is this? (Think in atomic terms.)
Electrons are easily dislodged from the outer regions of atoms, but protons are held tightly within the nucleus.
It is relatively easy to strip the outer electrons from a heavy atom like that of uranium (which then becomes a uranium ion) but very difficult to remove the inner electrons. Why do you suppose this is so?
For the outer electrons, the attractive force of the nucleus is largely canceled by the repulsive force of the inner electrons, leaving a force on the outer electrons little different from the force on the single electron in a hydrogen atom. For the inner electrons, on the other hand, all of the electrons farther from the nucleus exert no net force (it is similar to the situation within the earth, where only the earth below, not the earth above, exerts a gravitational force on a deeply buried piece of matter). So the inner electrons feel the full force of the nucleus, and a large amount of energy is required to remove them. Stripping all of the electrons from a heavy atom is especially difficult. Only in recent years have researchers at the University of California, Berkeley succeeded in removing all of the electrons from the atoms of heavy elements like uranium.
Why are the tires for trucks carrying gasoline and other flammable fluids manufactured to conduct electricity?
More than a decade ago, before truck tires were made electrically conducting, chains or wires were commonly dragged along the road surface from the bodies of trucks. Their purpose was to discharge any charge that would otherwise build up because of friction with the air and the road. Electrically-conducting tires now in use prevent the buildup of static charge that could produce a spark--especially dangerous for trucks carrying flammable cargoes.
Is it necessary for a charged body to actually touch the ball of the electroscope for the leaves to diverge?
No. It is not necessary for the charged body to touch the ball of an electroscope. If a negative charge is simply brought near, some electrons in the ball are repelled and driven to the gold leaves, leaving the ball positively charged. Or if a positive charge is brought near the ball, some electrons will be attracted and move up to the ball to make it negative and leave the leaves positively charged. This is charge separation due to induction. (If by small chance you are attempting an answer to this question without having witnessed this, pity, pity, pity! Better that your time is spent studying the physics of familiar things.)
How do the number of protons in an atomic nucleus dictate the chemical properties of the element?
The chemical properties of an element depend on the electrons in the atomic shells. But the number of electrons, in turn, are dictated by the number of protons in the nucleus. So in this indirect way, the number of protons in the atomic nucleus dictate the chemical properties of the element.
When combing your hair, you scuff electrons from your hair onto the comb. Is your hair then positively or negatively charged? How about the comb?
The comb will have a negative charge.
Carbon, with a half-full outer shell of electrons -- four in a shell that can hold eight -- readily shares its electrons with other atoms and forms a vast number of molecules, many of which are the organic molecules that form the backbone of living matter. Looking at the periodic table, what other element might play a role like carbon in life forms on some other planet?
The element below carbon in the periodic table, silicon, has similar properties and could conceivably be the basis of organic molecules elsewhere in the universe.
If you place a free electron and a free proton in the same electric field, how will the forces acting on them compare? Their accelerations? Their directions of travel?
The forces on the electron and proton will be equal in magnitude, but opposite in direction. Because of the greater mass of the proton, its acceleration will be less than that of the electron, and be in the direction of the electric field. How much less? Since the mass of the proton is nearly 2000 times that of the electron, its acceleration will be about 1/2000 that of the electron. The greater acceleration of the electron will be in the direction opposite to the electric field. The electron and proton accelerate in opposite directions.
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? (Electroscopes are useful not only as charge detectors, but also for measuring the quantity of charge: the more charge transferred to the ball, the more the leaves diverge.)
The leaves, like the rest of the electroscope, acquire charge from the charged object and repel each other because they both have the same sign of charge. The weight of the conducting gold foil is so small that even tiny forces are clearly evident.
Strictly speaking, when an object acquires a positive charge by the transfer of electrons, what happens to its mass? (Think small.)
The mass decreases slightly.
What happens to its mass when it acquires a negative charge?
The mass increases slightly. When an object acquires a positive charge, it loses electrons and its mass decreases. How much? By an amount equal to the mass of the electrons that have left. When an object acquires a negative charge, it gains electrons, and the mass of the electrons as well. (The masses involved are incredibly tiny compared to the masses of the objects. For a balloon rubbed against your hair, for example, the extra electrons on the balloon comprise less than a billionth of a billionth of a billionth the mass of the balloon.)
When the chassis of a car is moved into a painting chamber, a mist of paint is sprayed around the chassis. When it is given a sudden electric charge and mist is arracted to it, presto - the car is quickly and uniformly painted. What does the phenomenon of polarization have to do with this?
The paint particles in the mist are polarized and are therefore attracted to the charged chassis.
The average speed of a perfume vapor molecule at room temperature may be about 300 m/s, but you'll find the speed at which the scent travels across the room is much less. Why?
The speed at which the scent of a fragrance travels is much less than the speed of the individual molecules that make it up because of the many collisions among molecules. Although the molecular speed between collisions is great, the rate of migration in a particular direction through obstructing molecules is very much less.
If you are caught outdoors in a thunderstorm, why should you not stand under a tree? Can you think of a reason why you should not stand with your legs far apart? Or why lying down can be dangerous? (Hint: Consider electric potential difference.)
The tree is likely to be hit because it provides a path of less resistance between the cloud overhead and the ground. The tree and the ground near it are then raised to a high potential relative to the ground farther away. If you stand with your legs far apart, one leg on a higher-potential part of the ground than the other, or if you lie down with a significant potential difference between your head and your feet, you may find yourself a conducting path. That, you want to avoid!
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.
When combing your hair, you scuff electrons from your hair onto the comb. Is your hair then positively or negatively charged?
Your hair will have a positive charge.
