AP Physics 2 : Other Electrostatic Concepts

Example Questions

Example Question #102 : Electrostatics

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?

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 #1 : Other Electrostatic Concepts

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?

The electric field inside the box is 0.

The electric field inside the box is a uniform strength.

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 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 #104 : Electrostatics

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

Heating it to above its critical temperature

Cooling it to below its critical temperature

Increasing the applied voltage to beyond the critical voltage

Stretching the wire to a sufficiently small diameter

Increasing the applied pressure to above the critical pressure.

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 #105 : Electrostatics

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?

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 #2 : Other Electrostatic Concepts

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

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 #107 : Electrostatics

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?

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.

Example Question #3 : Other Electrostatic Concepts

If the area of the plates of a parallel plate capacitor is doubled and the distance between the plates is halved, then the capacitance is __________.

halved

unchanged

doubled

quartered