Question 1

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?

Accepted Answer

The product of two charges is calculated by multiplying them together: (+4 C) * (-1 C) = -4 C^2. However, the correct answer choice is -20 square Coulombs, which suggests a typo in the problem statement or answer choices.

Question 2

Electrons are

Accepted Answer

Electrons are significantly lighter than protons. The mass of an electron is approximately 1/1836 that of a proton.

Question 3

A charged object is able to repel

Accepted Answer

A charged object can repel only other charged objects due to the electromagnetic force acting between them. Neutral objects or particles do not experience this repulsive force since they do not have a net charge.

Question 4

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

Accepted Answer

The paper becomes electrically charged due to the transfer of electrons from the comb, which was charged by friction with the hair. Over time, the charges on the paper and the comb repel, causing the paper to fly off.

Question 5

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 result in no overall motion for the atom.

Accepted Answer

The answer of An atom under no external influences may...

Question 6

Polarization

Accepted Answer

Polarization tends to align dipoles in a way that minimizes the potential energy of a system, making the system more stable.

Question 7

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

Accepted Answer

Lightning rods work by using corona discharge to gradually neutralize the charge difference between the ground and the clouds, reducing the likelihood of a direct lightning strike.

Question 8

Who invented the xerographic copying process?

Accepted Answer

Chester Carlson invented the xerographic copying process in 1938, which later revolutionized the way documents were duplicated, leading to the development of modern photocopiers.

Question 9

In a conductor, the band gap between the valence band and the conduction band is

Accepted Answer

In a conductor, the band gap between the valence band and the conduction band is zero or very small, allowing electrons to move freely and conduct electricity.

Question 10

In a semiconductor, the band gap between the valence band and the conduction band is

Accepted Answer

The answer of In a semiconductor, the band gap between...

Question 11

In the dark, the photoconductors used in most xerographic copiers are

Accepted Answer

In the dark, the photoconductors used in most xerographic copiers act as insulators. This property is crucial for their function in the copying process, where they temporarily store an electrostatic image.

Question 12

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

Accepted Answer

A photon is the quantum of the electromagnetic field, including electromagnetic radiation such as light, and the force carrier for the electromagnetic force. This concept is a fundamental aspect of quantum mechanics, illustrating the particle-like properties of light.

Question 13

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

Accepted Answer

B) No more than two electrons can be in each valence level due to the Pauli exclusion principle, which states that no two electrons can have the same set of quantum numbers within an atom. This principle effectively prevents electrons from moving freely between valence levels if those levels are already filled with two electrons (one spin-up and one spin-down), thus hindering the flow of current in the absence of energy input like light.

Question 14

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,

Accepted Answer

Electrons in the photoconductor fill its valence levels in the dark, preventing them from moving to conduct electricity. When light hits the photoconductor, it excites the electrons, allowing them to move and conduct electricity.

Question 15

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

Accepted Answer

When the left half of the photoconductor is exposed to light, it becomes conductive, allowing the negative charge on that side to flow to the grounded metal surface beneath, neutralizing the left half. The right half, not exposed to light, remains insulating and thus retains its negative charge.

Question 16

A photoconductor's electrical properties go from insulating to conducting when it is exposed to light because,

Accepted Answer

Light supplies the energy needed for electrons to move from the valence band to the conduction band, which is how photoconductors become conductive upon exposure to light. This transition allows for electrical conductivity as electrons can move freely in the conduction band.

Question 17

The surface of a photoconductor has been coated with electric charge. This charge will remain in place until you expose the surface to

Accepted Answer

The charge on a photoconductor remains in place until it is exposed to light, which causes the material to become conductive and allows the charge to move or dissipate. This is the principle behind how photocopiers and laser printers work.

Question 18

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

Accepted Answer

Light exposure to the photoconductor allows charges to flow through it, preventing them from accumulating on its surface, which is essential for forming the latent image in xerography.

Question 19

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

Accepted Answer

The rods on the wings of an airplane are static discharge wicks. They help in transferring any accumulated static charge from the airplane to the air, preventing the buildup of a large net electric charge during flight. This process is essential for reducing radio interference and preventing potential damage from static electricity.

Question 20

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?

Accepted Answer

Static dissipaters are designed to mitigate the buildup of static electricity on aircraft by allowing the accumulated charge to gradually dissipate into the surrounding air. When a plane acquires a large negative charge, as in the scenario described, the sharp tips of the dissipaters facilitate a corona discharge, where the concentrated electric field at these points ionizes the nearby air molecules, allowing the negative charge to be sprayed or transferred into the air, thus neutralizing the aircraft's charge over time. This process does not involve the movement of charge towards the wings or the spraying of positive charge into the air, nor does it imply inactivity due to neutral air.

Quiz 10: Electricity