AP Physics 2 : Electrostatics

Study concepts, example questions & explanations for AP Physics 2

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Example Questions

Example Question #2 : Charge Distribution

You have a neutral balloon. If you were to add 21,000 electrons to it, what would its net charge be?

 = charge of one electron

Possible Answers:

None of the other answers is correct

Correct answer:

Explanation:

The elemental charge is the magnitude of charge, in Coulombs, that each electron or proton has. Because electrons have a negative charge, don't forget to add a negative sign into the equation.

When you convert the answer to microcoulombs, the answer is :

Example Question #101 : Electrostatics

Which of the following best represents the charge of an electron?

Possible Answers:

Correct answer:

Explanation:

The equation for the quantity of charge is:

where  is the charge quantity,  represents the number of electrons, and  is the charge of an electron, also known as the elementary charge.

Rewrite the equation.

One coulomb, , consists of electrons,

Substitute these two values into the formula.

This number represents the electron's fundamental charge. 

Example Question #1 : Charge Distribution

Imagine you have a neutral balloon. If you remove 16,000 electrons from it, what is the net charge on the balloon?

Possible Answers:

Correct answer:

Explanation:

Because this is a neutral balloon, the net charge is equal to the charge the was removed, but opposite in sign. There were 16,000 electrons removed, each of which has a charge of . Therefore, the total charge that was                       removed is:

 

To answer the question, we must remember that if that much charge was removed from the balloon, the balloon will now be negative.

Example Question #1 : Charge Distribution

A hollow metal sphere of radius has a charge of distributed evenly on the entirety of the surface. Find the surface charge density.

Possible Answers:

Correct answer:

Explanation:

Surface area of sphere:

Plug in values:

Example Question #1 : Other Electrostatic Concepts

You place a box with square faces of side length 4m in a uniform electric field of strength . There is no charge in the box. What is the total electric flux going through the box?

Possible Answers:

Correct answer:

Explanation:

A way to think of flux is to count the number of electric field lines exiting a shape and subtract from it the number of field lines entering the shape. The only way for there to be more lines exiting the shape than entering the shape (and the only way to have any flux) is when the shape is enclosing charge.

In our problem, the box has no charge in it. Therefore, it has the same amount of field lines leaving it as it does entering it. This means that there is 0 net flux through the box. 

Example Question #162 : Electricity And Magnetism

A point charge  can be found at the center of a metal box that is isolated, ungrounded, and uncharged. Which of the following statements is true?

Possible Answers:

The electric field inside the box is 0.

The net charge on the outside surface of the box is .

The electric field everywhere outside the box can be calculated as if it was a point charge.

The electric field outside the box is 0.

The electric field inside the box is a uniform strength.

Correct answer:

The net charge on the outside surface of the box is .

Explanation:

Because the box is a conductor, the electrons are able to shift when charge is applied. Because the box is isolated, there is nothing else exerting influence over the box.

The electrons on the shell are able to move from the electric field made by , so they group together on the inner surface of the box to make a net charge of . Because  worth of electrons moved from the outer surface, the outer surface necessarily has a net charge of  now, because it was neutral before.

Example Question #1 : Other Electrostatic Concepts

Which of the following will cause a superconductor to have 0 resistance?

Possible Answers:

Cooling it to below its critical temperature

Heating it to above its critical temperature

Increasing the applied pressure to above the critical pressure.

Stretching the wire to a sufficiently small diameter

Increasing the applied voltage to beyond the critical voltage

Correct answer:

Cooling it to below its critical temperature

Explanation:

A superconductor is a material that has 0 electrical resistance when cooled to below a certain temperature.

In general, materials have a decreasing resistance as they are cooled. With a superconductor, once the critical temperature is reached, the resistance abruptly goes to 0. Superconductivity is a quantum mechanical phenomenon.

Example Question #3 : Other Electrostatic Concepts

Suppose there is an electric field oriented so that its positive terminal points vertically down towards the ground. If a negatively charged particle of mass  and charge  were placed in the electric field, which of the following expressions gives the electric field strength that would allow the charge to exist in static equilibrium despite the force of gravity?

Possible Answers:

Correct answer:

Explanation:

We are told that there is an electric field pointing vertically down and that the positive end is near the top. If a charged particle were placed in this field, it will experience an upward electric force, while its mass would cause it to experience a downward gravitational force. We are looking for a situation in which these two forces are equal in order for the particle to be in static equilibrium, thus we need to set the gravitational force equal to the electric force.

Example Question #4 : Other Electrostatic Concepts

If a system has 1.4 million electrons, find the net charge of the system. 

Possible Answers:

Correct answer:

Explanation:

To calculate the net charge of the system, it is necessary to know the charge of an electron.  Write the charge of an electron.

 

Multiply this number by the existing number of electrons in the systems.

Example Question #5 : Other Electrostatic Concepts

Suppose that a charge of  is moved a distance of  from point A to point B while within an electric field. In doing so,  of work is done. What is the voltage difference between points A and B?

Possible Answers:

Correct answer:

Explanation:

For this question, we need to figure out the voltage difference between two points. We're provided with the charge of the particle, the amount of energy put into the process, and the distance traversed by the particle.

First, let's write an equation for voltage.

Where  is electrical potential energy, and  is the charge of the particle.

This equation describes the change in potential energy that occurs when a given quantity of charge undergoes a displacement while within an electric field. Since we are putting energy into this process to make it occur, and the charge is positive, we know that the voltage change will also be positive; that is, the positively charged particle will move towards the positive terminal of a voltage source and away from the negative terminal.

Plugging in the values given to us, we obtain:

Notice that we did not need to know the distance that the particle traveled in this case; that information is extraneous.

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