Physics Chapter 10
You and a friend are spending a weekend at a carnival. You get bored and begin to make electrostatics measurements. It turns out that you have a charge of +3 Coulombs and your friend a charge of -5 Coulombs. What is the net electric charge?
-2 Coulombs.
You and a friend are walking together along a beautiful shoreline. You get bored and begin to make electrostatics measurements. It turns out that you have a charge of +4 Coulombs and your friend a charge of -1 Coulombs. What is the product of your two charges?
-20 square Coulombs
You and a friend are spending the weekend making up missed lab experiments. You are on to second - semester material and are studying the electrostatic interaction of two charges. You notice that two charges are exerting a certain force on one another. If the distance between two electric charges doubles, then the force they exert on each other changes by how much?
1/4 as large.
You and a friend are spending the weekend making up missed lab experiments. You are on to second - semester material and are studying the electrostatic interaction of two charges. You notice that two charges are exerting a certain force on one another. If the distance between two electric charges becomes 1/3 of what it originally was, then the force they exert on each other changes by how much?
9 times as big.
Most commercial airliners have static dissipaters on their wingtips. These sharp metal spikes extend from the end of each metal wing and point toward the rear of the plane. Suppose a plane had just flown through a negatively charged cloud and acquired a large negative charge. It is now flying through neutral air. Which of the following should you expect to happen near the static dissipaters?
A corona discharge at the dissipater tips will spray negative charge into the air.
Which of the following can cause a charged particle to accelerate?
An electric field, and a gravitational field.
Who originally named electric charges as positive and negative?
Benjamin Franklin
An atom under no external influences may be thought of as a positively charged nucleus surrounded by a negatively charged electron cloud. Now suppose the atom is placed in a uniform external electric field (like you'd find between two parallel charged plates). Please explain why this would stretch the atom out, elongating it.
By definition, the electric field points in the direction a positive charge experiences a force. Hence, the nucleus is pulled downfield while the electrons are pulled upfield. For the system to minimize its potential energy, the atom is elongated as much as possible before the forces between the electrons and nucleus are equal to the forces exerted by the external electric field. The external electric field polarizes the atom.
Who invented the xerographic copying process?
Chester Carlson
In further studying physics, you learn that light and particles have a dual nature, with light behaving like particle and vice versa. A particle (quantum) of light is called
a photon.
In a semi conductor the band gap is
a region of energies which the electrons cannot have.
Suppose you have two identical socks that exert a force of 10 N on each other. Now you move them so they are three times as far apart as they were originally. a) Calculate the new force between the objects. b) Will this force be attractive or repulsive? Please explain.
a.) Since the separation is being tripled the force would go to 1/9 its original value if the charges remained constant because Coulomb's Law is an inverse square law. So, the new force is 10/9 N. b.) Since the force stays the same and it was repulsive to begin with, it will remain repulsive. One could also deduce this given that the socks are identical.
Suppose you have two identical socks that repel each other with a certain force. Now you move them so they are three times as far apart as they were originally, but the force between them stays the same. a) What is one way in which the charges might change so the force could remain constant? b) In this case, let the charges remain the same as the socks separate so that the force decreases as they are separated. In moving the socks apart did the potential energy increase or decrease? Please explain.
a.) Since the separation is being tripled the force would go to 1/9 its original value if the charges remained constant. Therefore the product of the charges must increase by a factor of 9 to make up for the distance change. This could be done in many ways. One charge could increase by 9 times, both charges could increase by 3 times, one charge could reduce to 1/3 its original value and the other could increase to 27 times its original value, etc. Although credit was given for all correct answers here, the only one that satisfies the socks' remaining identical is for both charges to increase by a factor of 3. b.) The potential energy of the system decreased. One way to think about this is that the separation required an inward force exerted through an oppositely directed distance (displacement) resulting in negative work, which lowers the system's energy. Another way to look at this is that were you to let the balloons go, they would not move so rapidly when they are farther apart, which means that less potential energy was available to become kinetic energy.
When a xerographic copier first applies electric charges to the surface of its photoconductor, it does so in the dark. The reason for applying these charges in the dark is that light exposure would
allow the charges to flow through the photoconductor so that they wouldn't accumulate on its surface.
A positive charge and a negative charge will
attract each other
Suppose you have a semiconductor with many electrons that have been able to jump the band gap. This object really is a (an)
conductor
You have just pulled your clothes from the dryer and find that a sock is clinging to your jeans with static electricity. You hold the jeans in one hand and the sock in the other and pull the two apart. As jeans and sock move apart, the forces between them become weaker because the
distance between the garments increase.
Semiconductors
do not obey Ohm's law.
After running a plastic comb through you hair several times you hold it above a small scrap of paper. The paper jumps off the table and sticks to the comb. But after some time the object flies off the comb. It leaves the comb because the paper becomes
electrically charged.
After running a plastic comb through you hair several times you hold it above a small scrap of paper. The paper jumps off the table and sticks to the comb because the paper becomes
electrically polarized.
A xerographic copier uses a special photoconductor surface that allows light from an original document to control the placement of black powder on white paper. The photoconductor only conducts electricity when it's exposed to light because, in the dark,
electrons in the photoconductor completely fill its valence levels and can't shift from one level to another in order to transport charge through the material.
In an insulator, the band gap between the valence band and the conduction band is
extremely large.
Suppose you have a semiconductor and are not able to bias it such that any electrons leave the valence band. This object really then is a (an)
insulator
In the dark, the photoconductors used in most xerographic copiers are
insulators.
A photoconductor's electrical properties go from insulating to conducting when it is exposed to light because,
light supplies the energy needed to move electrons from the valence band to the conduction band.
The surface of a photoconductor has been coated with electric charge. This charge will remain in place until you expose the surface to
light.
In a semiconductor, the band gap between the valence band and the conduction band is
moderately large, but not huge.
Electrons are
much lighter than protons
A charged object is able to attract
neutral objects but not neutral particles
A photoconductor cannot carry an electric current in the dark because all of its valence levels contain two electrons and moving a valence-level electron into one of the empty conduction levels requires too much energy. While the photoconductor would be able to carry current if its electrons could move from one valence level to another, such movement is impossible because
no more than two electrons can be in each valence level: one spin-up and one spin-down.
A charged object is able to repel
only charged objects
You stick two pieces of adhesive tape on a glass window and then pull them off suddenly. If you now hold the tape pieces near each other, they will be
repelled because they have like charges.
You are an engineer and are building metal objects for a science lab that are supposed to hold charge for a long time. The object should not be
sharp and pointed.
If an electron and a proton both experience the same electric force,
the electron will have greater acceleration.
You have covered a grounded metal surface with a layer of photoconductor. Working in the dark, you sprinkle negative charge onto this surface. If you now expose only the left half of the photoconductor to light, you will find that
the left half becomes neutral while the right half remains negatively charged.
You have just built a house out in the country and it comes to your attention that you need to install a lightning rod. Being naturally curious you ask a hardware salesperson how lightning rods work and they correctly tell you that
they use corona discharge to bleed off local charge accumulations.
A typical commercial jet has a sharpened metal rod projecting backward from the tip of each wing. These two rods prevent the airplane from accumulating a large net electric charge during flight by
transferring any charge on the plane to the air behind the plane.
n a conductor, the band gap between the valence band and the conduction band is
zero or very small.
