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# 3.8 Describing Electric Force

Daniella Garcia-Loos

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## Describing Electric Force

### CollegeBoard Objectives:

Electric force results from the interaction of one object that has an electric charge with another object that has an electric charge.
a. Normal force, friction, and tension are forces on a macroscopic scale you have heard of but truly stem from microscopic electric forces.
b. Electric forces can attract or repel, depending on the charge of the objects.

Comparing the gravitational force and the electric force:

• Both forces are fundamental forces of nature that arise between objects
• Both forces can be either attractive or repulsive, depending on the properties of the objects involved
• The strength of both forces is determined by the mass (for gravitational force) or charge (for electric force) of the objects and the distance between them
• Both forces follow an inverse-square law, meaning that the strength of the force decreases with the square of the distance between the objects
• The gravitational force is much weaker than the electric force, so it is only noticeable at large scales (e.g. planets and stars) or for objects with very large mass
• The gravitational force only arises between objects with mass, while the electric force can arise between objects with either mass or charge
• The gravitational force is responsible for the behavior of objects under the influence of gravity, while the electric force is responsible for the behavior of charged particles and the interaction between charged objects

### Electric Force Basics

Name: Electric Force
Units: Newtons
Math Relation: Inverse Square Law
Fundamental Property: Charge
Pioneer: Charles Coulomb
Force Type: Attractive or Repulsive
Now let's look at the forces between charged objects and a charged object in an electric field.
• The electric force is a fundamental force of nature that arises between charged particles. It is an important force in the study of electromagnetism, which is the branch of physics that deals with the interactions between electricity, magnetism, and light.
• The electric force can be either attractive or repulsive, depending on the type of charges involved. Opposite charges, such as positive and negative, experience a force of attraction, while like charges, such as positive and positive or negative and negative, experience a force of repulsion.
• The strength of the electric force is determined by the magnitude of the charges and the distance between them. According to Coulomb's law, the electric force between two charged particles is given by the equation:
F = k * q1 * q2 / r^2
where F is the electric force, k is a constant known as the Coulomb's constant, q1 and q2 are the magnitudes of the charges, and r is the distance between the charges.
• The electric force is one of the four fundamental forces of nature, along with the gravitational force, the weak nuclear force, and the strong nuclear force. It is responsible for the behavior of charged particles and the interaction between charged objects, such as the attraction between a charged balloon and a piece of paper.

### Force Interactions with Charges

 Object 1 Object 2 Force Type Positive Positive Repulsive Negative Negative Repulsive Positive Negative Attractive Negative Positive Attractive

### Force Interactions with an Electric Field

 Direction of the E Field Charge Direction of Force Left Positive Left Right Negative Left Up Neutral No Force Down Positive Down Into the page Negative Out of the page Out of the page Neutral No Force
Example Problem:
Two point charges are placed a certain distance apart in a vacuum. One charge has a positive charge of 3 Coulombs, while the other has a negative charge of 4 Coulombs. Qualitatively, what is the expected direction of the electric force between the charges? Quantitatively, what is the magnitude of the electric force between the charges, according to Coulomb's law?
Solution:
To answer this question qualitatively, you would need to consider the fact that opposite charges experience a force of attraction, while like charges experience a force of repulsion. Based on this information, you could conclude that the electric force between the two charges in this example would be attractive.
To answer the question quantitatively, you would need to use Coulomb's law to calculate the electric force between the charges. Using the equation provided in the previous answer, you would plug in the values for the charges (q1 = 3 Coulombs, q2 = -4 Coulombs) and the distance between them (r) to determine the magnitude of the electric force.
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