College Physics : Electromagnetics

Study concepts, example questions & explanations for College Physics

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Example Questions

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Example Question #1 : Magnetic Force

Suppose that a magnetic field is oriented such that it is pointing directly to the left, as in the picture shown below. If a positively charged particle were to begin traveling through this magnetic field to the right, in which direction would the particle's trajectory begin to curve?

Magnetic field

Possible Answers:

The particle would continue to move to the right unaffected

The particle would move down the page

The particle would move out of the page

The particle would move into the page

Correct answer:

The particle would continue to move to the right unaffected

Explanation:

In order to answer this question, it's important to understand the factors that determine the magnetic force experienced by a charge. We can begin by writing out the equation for magnetic force.

As shown in the above equation, the magnetic force is directly proportional to the particle's charge, its velocity, and the strength of the magnetic field itself. But, for the purposes of the this question, the most important factor is the angle of the particle's velocity with respect to the magnetic field.

Notice that if theta is equal to zero, then the sine of theta will be equal to zero as well. This, in turn, will cause the magnetic force to also be zero. This is also true if we were to define theta as .

Since the particle is moving in a direction that is parallel to the magnetic field lines but in the opposite direction, we have a situation in which theta is equal to . This means that the magnetic force on the particle is zero. As a result, the particle will continue to move through the magnetic field without changing its direction.

Example Question #1 : Faraday's Law Of Induction

A single string of wire has a resistance of . If the wire is connected to a  power source, what is the strength of the magnetic field  away from the wire?

Possible Answers:

Correct answer:

Explanation:

So this is all about the magnetic field strength around a current carrying wire.

The equation for this is:

But you must use Ohm's Law  in order to find the current in the wire.

Since the wire has  of resistance and the voltage through the wire is , that means the current in the wire is .

Being sure to change  into , plug everything in and get the answer, which is

Example Question #1 : Electromagnetics

A charged particle traveling along the +x axis enters an electric field directed vertically upward along the +y-axis. If the charged particle experiences a force downward because of this field, what is the sign of the charge on this particle?

Possible Answers:

None of these

It is positive

It is neutral

It is negative

Correct answer:

It is negative

Explanation:

Positive charges in an electric field will experience an electric force that is in the same direction as the electric field. If the charge is negative, the force will be in the opposite direction of the electric field. Since the charged particle experiences a force which is opposite to the electric field, the sign of the charge must be negative.

Example Question #1 : Coulomb's Law

When the magnitude of two interacting charges is increased by a factor of 2, the electrical forces between these charges is __________.

Possible Answers:

quadrupled

reduced by a factor of 3

doubled

reduced by a factor of 

reduced by a factor of 4

Correct answer:

quadrupled

Explanation:

In Coloumb's law, an increase in both interacting charges will cause an increase in the magnitude of the electrical force between them. Specifically if the magnitude of both interacting charges is doubled, this will quadruple the electrical force.

Example Question #1 : Coulomb's Law

What is the main difference between electrical and gravitational forces?

Possible Answers:

Electrical forces attract and gravitational forces repel

Gravitational forces are always attractive but electrical forces can be attractive or repulsive

Electrical forces obey the inverse square law and gravitational forces do not

Gravitational forces obey the inverse square law and electrical forces do not

Correct answer:

Gravitational forces are always attractive but electrical forces can be attractive or repulsive

Explanation:

Electric forces can be attractive or repulsive because charges may be positive or negative. In the case for gravitational forces, there are only attractive forces because mass is always positive.

Example Question #1 : Coulomb's Law

Figure 1

Three equal charges are at three of the corners of a square of side d. A fourth charge of equal magnitude is at the center of the square as shown in Figure above. Which of the arrows shown represents the net force acting on the charge at the center of the square?

Possible Answers:

C

A

D

B

Correct answer:

B

Explanation:

Because of the principles of superposition, each electric force that acts from the charges at the corners on to the charge at the center can be broken into components. Since all the charges are positive, all the forces will be repulsive. The forces acting from the top left and bottom right corners will cancel, leaving only the repulsive force coming from the bottom left corner. 

Example Question #1 : Coulomb's Law

An electron traveling along the +x-axis enters an electric field that is directed vertically down, i.e., along the negative y-axis. What will be the direction of the electric force acting on the electron after entering the electric field?

Possible Answers:

Upward

To the right

Into the page

Out of the page

Correct answer:

Upward

Explanation:

Positive charges in an electric field will experience an electric force that is in the same direction as the electric field. If the charge is negative, the force will be in the opposite direction of the electric field. Since we are talking about an electron moving in an electric field that points in the negative y-direction, the electron will feel a force that points in the positive y-direction, or upwards. 

Example Question #1 : Electromagnetics

A charged rod carrying a negative charge is brought near two spheres that are in contact with each other but insulated from the ground. If the two spheres are then separated, what kind of charge will be on the spheres?

Possible Answers:

None of these

The sphere near the charged rod becomes positive and the other becomes negative

The spheres do not get any charge

The sphere near the charged rod becomes negative and the other becomes positive

Correct answer:

The sphere near the charged rod becomes positive and the other becomes negative

Explanation:

When the negatively charged rod is brought near one of the two spheres, the presents of the negative charge will induce a flow of charge in the spheres such that regions farthest away from the charged rod will become most negative and regions near the rod will become most positive. This is called charge by induction.

Example Question #1 : Electrostatics

By what method will a positively charged rod produce a negative charge on a conducting sphere that is placed on an insulating surface?

Possible Answers:

None of these

Charge by conduction

Charge by convection

Charge by induction

Correct answer:

Charge by induction

Explanation:

Charge by induction happens when a charged object is brought in the vicinity of a neutral object. The presents of the charged object will cause the free charges in the neutral object to shift such that the neutral object becomes polarized. When the charged object is positive, this will induce a negative charge on a neutral object. 

Example Question #91 : College Physics

A conductor is placed in an electric field under electrostatic conditions. Which of the following statements is correct for this situation?

Possible Answers:

All of these

The electric field on the surface of the conductor is perpendicular to the surface

All valence electrons go to the surface of the conductor

The electric field is zero inside the conductor

Correct answer:

All of these

Explanation:

A conductor is defined as a object free to move charges. In particular, valence electrons, which are the outer most electron in each atom and the most free to move, travel inside the conductor until the net electric field inside the conductor is zero. These electrons will move until this condition has been met. Because of the presents of charged particles at the surface and the condition that they are no longer moving, any electric field at the surface must be perpendicular to that surface.

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