AP Physics C: Mechanics : Electricity

Example Questions

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Example Question #1 : Electricity And Magnetism Exam

A hollow metal sphere with a diameter of 10cm has a net charge of  distributed uniformly across its surface. What is the magnitude of the field a distance 2.0m from the center of the sphere?

Explanation:

Relevant equations:

(electric field of point charge)

Anywhere outside the metal sphere, the electric field is the same as it would be for a point charge of the same magnitude, located at the center of the sphere. So, calculate the electric field of a point charge given:

Plugging in gives:

Example Question #1 : Electricity And Magnetism Exam

Two infinite parallel conducting sheets each have positive charge density . What is the magnitude and direction of the electric field to the right of the right sheet?

, to the right

, to the left

, to the right

, to the right

, to the right

Explanation:

Relevant equations:

(field due to single infinite plane)

Electric field is additive; in other words, the total electric field from the two planes is the sum of their individual fields:

The direction of the electric field is away from positive source charges. Thus, to the right of these positively charged planes, the field points away to the right.

Example Question #1 : Electricity And Magnetism Exam

Four particles, each of charge , make up the four corners of a square with equal side lengths of . For the charge in the top left corner of the square, in what direction is the net force that it experiences due to its interactions with the other three particles?

downwards of right

upwards of left

Directly to the left

Directly to the right

upwards of left

Explanation:

The correct answer is 45 degrees upwards of left. Since all particles have charge , all forces will be repulsive (there will be no attracting forces). The particle in the top-right corner creates a repulsive force directly to the left, and the particle in the bottom-left corner creates a repulsive force directly upwards. These are equal in magnitude, since they are both at distance  from the top left corner. The bottom-right corner also creates a repulsive force, but acts along the same direction as the vector sum of a leftwards and upwards force.

Example Question #1 : Electricity And Magnetism Exam

Consider a spherical capacitor made of two nested spheres. The smaller sphere has a radius of  and a charge of , and lies within a larger sphere with radius  and a charge of

Which of the following equations accurately describes the capacitance of this spherical capacitor?

Due to symmetry, this scenario would not produce capacitance

Explanation:

To solve this problem, we will need to derive an equation.

We know that:

We can use Gauss's law to derive the electric field between the two circles yielding:

Doing our integration with respect to  from  to , we get:

We can plug this back into our equation for capacitance to get:

Example Question #1 : Electricity And Magnetism Exam

We have a point charge of . Determine the electric field at a distance of  away from that charge.

Explanation:

Coulomb's law for the electric field from point charges is , where we know the values of the following variables.

Using these values, we can solve for the electric field.

Example Question #2 : Electricity And Magnetism Exam

Two positive point charges of  and  are place at a distance  away from each other, as shown below. If a positive test charge, , is placed in between, at what distance away from  will this test charge experience zero net force?

Explanation:

To find the location at which the test charge experience zero net force, write the net force equation as , where  is the force on the test charge from , and  is the force on the same test charge from . Using Coulomb's law, we can rewrite the force equation and set it equal to zero.

In this equation, the distance, , is how far away the test charge is from , while  represents how far away the test charge is from .  Now, we simplify and solve for .

Cross-multiply.

We can cancel and . We do not need to know these values in order to solve the question.

Now that we have isolated , we can plug in the values given in the question and solve.

Example Question #2 : Electricity And Magnetism Exam

You are standing on top of a very large positively charged metal plate with a surface charge of .

Assuming that the plate is infinitely large and your mass is , how much charge does your body need to have in order for you to float?

Explanation:

Consider the forces that are acting on you. There is the downward (negative direction) force of gravity, . In order for you to float, there has to be an upward (positive direction) force, and that upward force is coming from the metal plate, . To show that you would float, the net force equation is written as , where  is the charge on you.

For plates that are charged, know that .

Knowing this, the force equation becomes .

Solve for .

Now we can plug in our given values, and solve for the charge.

Example Question #1 : Electricity

A point charge of  exerts a force of  on another charge with . How far apart are the two charges?

Explanation:

To find the distance between the two charges, use Coulomb's Law.

Since we want to find distance, , we solve for .

We know the values of the force and the two charges.

We can plug in these values and solve for the distance.

Example Question #1 : Electricity

What is the electric force between two charges, and , located apart?

Explanation:

The equation for finding the electric force between two charges is , where . Using this, we can rewrite the force equation.

Now, we can use the values given in the question to solve for the electric force between the two particles.

Example Question #1 : Using Coulomb's Law

What is the magnitude of the electric field at a field point  from a point charge of ?