High School Physics : Electric Force

Study concepts, example questions & explanations for High School Physics

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Example Questions

Example Question #1 : Electric Force

A large electroscope is made with “leaves” that are  long wires with negligible mass with tiny  spheres at the end.  When charged, nearly all the charge resides in the sphere.  If the wires each make a  angle with the vertical, what total charge  must have been applied to the electroscopes.

Possible Answers:

Correct answer:

Explanation:

Each of these charged spheres exert a force on each that is equal and opposite on one another in the x-direction.  The tension force on each of these charged spheres is equal to the force from the other sphere in the x-direction and the force of gravity in the y-direction.  We will be able to use this information to determine the charge on the electroscope.

 

First we must analyze the forces on the charges and determine the force on the charges.  We know that

 

Rearranging this we get

 

 

Next we can use Coulomb’s law to determine the magnitude of each charge.

 

In order to figure out the distance between the two spheres we can trigonometry with the length of the string.

Therefore the distance between the two charges is double this value.

 

 

 

 

 

 

 

Example Question #1 : Electric Force

Two charges -Q and -3Q are a distance l apart.  These two charges are free to move but do not because there is a third(fixed) charge nearby.  What must the magnitude of the third charge and its placement be in order for the first two to be in equilibrium.

Possible Answers:

The charge must be placed  away from the larger charge with a magnitude of 

The charge must be placed  away from the larger charge with a magnitude of .

The charge must be placed  away from the smaller charge with a magnitude of +.

The charge must be placed  away from the smaller charge with a magnitude of .

Correct answer:

The charge must be placed  away from the smaller charge with a magnitude of .

Explanation:

First let us determine the force of the two charges on each other.

 

 

 

 

The easiest way to analyze this is to assume all of these are in a straight line and we have a charged placed somewhere along the same x-axis between the charges that keeps them all from moving.  Let us assume the smaller charge is at the origin and the distance to the fixed charge is some distance .

 

 

 

Let us also assume that the larger charge is a distance l away from the origin (away from the smaller charge) and therefore a distanc l-r away from the fixed charge.

 

 

To be in equilibrium we know that the net force on the smaller charge must equal .

 

 

We also know that the net force on the larger charge must equal 

 

 

We can set these two equations equal to each other.

 

 

The force between the charges can be cancelled since it is on both sides.

 

 

The k and Q values can also be cancelled out on both sides.

 

 

Cross multiply on both sides

 

 

 

 

This is a quadratic that needs to be solved with a quadratic formula.

 

 

 

 

 

 

 

 

So the two options for the  value would be

 

 

 

Of these two, the one that makes the most sense based on our assumptions is the  as this would be in the middle of two charges.

 

We can now go back to our first equation with the smaller charges

 

 

We can plug in our value for the .

 

 

The  and  can be cancelled out

 

 

 

 

 

The charge must be placed  away from the smaller charge with a magnitude of .

 

 

 

 

 

 

 

 

Example Question #3 : Electric Force

Particles of charges , , and  are placed in a line.  The center charge is  and is  away from each of the others.  Calculate the net force on the center charge from the other two.

Possible Answers:

Correct answer:

Explanation:

We can calculate the force from each of the two charges on the center charge using Coulomb’s Law

 

The first charge on the middle charge

 

 

The last charge on the middle charge

 

 

To find the total force we need to add both of these forces together.

 

 

Example Question #1 : Electric Force

A point charge of +Q is placed at the center of a square.  When a second point charge of -Q is placed at one of the square’s corners it is observed that an electrostatic force of 4N acts on the positive charge at the square’s center.  Now, identical charges of -Q are placed at the other three corners of the square.  What is the magnitude of the net electrostatic force acting on the positive charge at the center of the square?

Possible Answers:

Correct answer:

Explanation:

Since the charges are opposite, the center charge +Q is going to be attracted to each of the charges in the corner of the square.  Since each charge is equidistant from the center charge, they will each exert 4N of force on the charge in the center.  However, since each corner charge is pulling the center charge equally and oppositely, the net force on the system is equal to 0 and the charge will not move.  All of the forces cancel out with one another.

Example Question #2 : Electric Force

When the distance 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 2–√

reduced by a factor of 3

reduced by a factor of 4

doubled

Correct answer:

reduced by a factor of 4

Explanation:

In Coulomb’s Law, an increase in the distance will cause a decrease in the magnitude of the electrical force between them.  Since this is an example of an inverse square law, doubling the distance will reduce the force by a factor of 4.

Example Question #6 : Electric Force

What is the main difference between electrical and gravitational forces?

Possible Answers:

Electrical forces attract and gravitational forces repel

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

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

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

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 #8 : Electric Force

What is the magnitude of force a   charge exerts on a  charge  away?

Possible Answers:

Correct answer:

Explanation:

We can calculate this using Coulomb’s Law

 

Example Question #1 : Electric Force

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

reduced by a factor of 4

doubled

reduced by a factor of 2–√

Correct answer:

quadrupled

Explanation:

In Coulomb'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 #2 : Electric Force

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At each corner of square of side  there are point charges of magnitude Q, 2Q, 3Q and 4Q.  Determine the magnitude and direction of the force on the charge 3Q.

Possible Answers:

 away from the other charges

 toward Q

 toward Q

  away from the other charges

Correct answer:

  away from the other charges

Explanation:

We will need to use Coulomb’s Law to analyze the force on the 3Q charge from all the other forces.  We will then summarize the net force in the  and  direction to determine the force on the 3Q charge.

 

In the x-direction

 

Force from 4Q on 3Q

 

Force from Q on 3Q in the x direction

 

Add these together in the x-direction 

 

 

 

In the y-direction

 

From from 2Q on 3Q

 

 

Force from Q on 3Q in the y direction

 

 

Add these together in the y-direction 

 

 

 

Now we can find the resultant of these sides using the Pythagorean theorem.

 

 

 

 

 

 

 

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