AP Physics 1 : Electrostatics

Study concepts, example questions & explanations for AP Physics 1

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Example Questions

Example Question #11 : Electric Force Between Point Charges

Two electric charges are placed  apart, where  and .

What is the magnitude of force between them? Is it replusive or attractive?

Possible Answers:

Correct answer:

Explanation:

The force between the two charged particles is proportional to the product of their charges, according to Coulomb's Law. Whether the force is attractive or repulsive depends on the signs of the charges. Like signs will repel while opposite signs will attract.

Using Coulomb's Law to find the magnitude of the charge:

Therefore, the magnitude of the force has been discovered. Finally, since the signs are opposite ( and ), the force is attractive. Therefore the answer is:

Example Question #12 : Electric Force Between Point Charges

Which of the following pairs of charges would exhibit the most electrostatic repulsive force?

Possible Answers:

 charge and  charge  apart

 charge and  charge  apart

 charge and  charge  apart

 charge and  charge  apart

 charge and  charge  apart

Correct answer:

 charge and  charge  apart

Explanation:

The correct answer is the  charge and  charge  apart.

This results in the largest repulsive force according to the following equation:

 is the repulsive force,  is the magnitude of the charge,  is the distance between the charges and  is Coulomb's constant.

 and  charges placed  apart provides the greatest repulsive force.

Example Question #1 : Coulomb's Law

Two protons are on either side of an electron as shown below:

Diagram

The electron is 30 µm away from the proton on its left and 10 µm away from the proton on its right. What is the magnitude and direction of the net electric force acting on the electron?

A proton has a charge of 

Possible Answers:

 to the right

 to the left

 to the right

 to the left

 to the right

Correct answer:

 to the right

Explanation:

The net force on the electron is the sum of the forces between the electron and each of the protons:

These forces are given by Coulomb's law:

Using the numbers given, we get:

Because opposite charges attract,  points left (the negative direction) and  points right (the positive direction).

Therefore, the net force is

Because this value is positive, the direction is rightward.

Example Question #2 : Coulomb's Law

Charges A and B are placed a distance of  from one another. The charge of particle A is  whereas the charge of particle B is . Charge B experiences an electrostatic force of  from charge A. Similarly, charge A experiences an electrostatic force of  from charge B.

What is the ratio of  to ?

Possible Answers:

Correct answer:

Explanation:

This question is very simple if you realize that the force experienced by both charges is equal.

The definition of the two electrostatic forces are given by Coulomb's law:

In this question, we can rewrite this equation in terms of our given system.

It doesn’t matter if the charges of the two particles are different; both particles experience the same force because the charges of both particles are accounted for in the electrostatic force equation (Coulomb's law). This conclusion can also be made by considering Newton's third law: the force of the first particle on the second will be equal and opposite the force of the second particle on the first.

Since the forces are equal, their ratio will be .

Example Question #1 : Coulomb's Law

An excess charge of  is put on an ideal neutral conducting sphere with radius . What is the Coulomb force this excess charge exerts on a point charge of  that is  from the surface of the sphere?

Possible Answers:

Correct answer:

Explanation:

Two principal realizations help with solving this problem, both derived from Gauss’ law for electricity:

1) The excess charge on an ideal conducting sphere is uniformly distributed over its surface

2) A uniform shell of charge acts, in terms of electric force, as if all the charge were contained in a point charge at the sphere’s center

With these realizations, an application of Coulomb’s law answers the question. If  is the point charge outside the sphere, then the force  on  is:

In this equation, is Coulomb’s constant, is the excess charge on the spherical conductor, and is total distance in meters of  from the center of the conducting sphere.

Using the given values in this equation, we can calculate the generated force:

Example Question #1 : Electric Fields

If the distance between two charged particles is doubled, the strength of the electric force between them will __________.

Possible Answers:

quadruple

double

remain unchanged

be halved

be quartered

Correct answer:

be quartered

Explanation:

Coulomb's law gives the relationship between the force of an electric field and the distance between two charges:

The strength of the force will be inversely proportional to the square of the distance between the charges.

When the distance between the charges is doubled, the total force will be divided by four (quartered).

Example Question #1 : Coulomb's Law

If we have 2 charges,  and , that are  apart, what is the magnitude of the force exerted on  by if we know that  has a charge of  and  has a charge of ?

Possible Answers:

Correct answer:

Explanation:

Use Coulomb's Law

Plug in known values and solve.

A negative value for electric force indicates an attractive force. This makes sense since our two charges have opposite signs. Since we're asked for magnitude, all answer choices are positive.

Example Question #4 : Coulomb's Law

If we have 2 charges,  and , that are  apart, what is the force exerted on  by  if we know that  has a charge of  and  has a charge of ?

Possible Answers:

Correct answer:

Explanation:

Use Coulomb's law.

Plug in known values and solve.

Note that this force is positive, which means it's repulsive.

Example Question #5 : Coulomb's Law

If we have 2 charges,  and , that are  apart, what is the force exerted on  by  if we know that  has a charge of  and  has a charge of ?

Possible Answers:

Correct answer:

Explanation:

Use Coulomb's law.

Plug in known values and solve.

Note that this force is positive, which means that it's repulsive.

Example Question #5 : Coulomb's Law

If we have 2 charges,  and , that are  apart, what is the force exerted on  by  if we know that  has a charge of  and  has a charge of ?

Possible Answers:

Correct answer:

Explanation:

Use Coulomb's law.

Plug in known values and solve.

Note that the force between two charges of the same sign (both positive or both negative) is positive. This indicates the force is repulsive, which makes sense since both charges are positive.

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