Electric Force: Definition & Equation

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  • 0:00 What is Electric Force?
  • 0:40 Electric Force & Types…
  • 1:10 Electric Force &…
  • 3:09 Calculating Electric Force
  • 5:29 Lesson Summary
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Lesson Transcript
Instructor: Betsy Chesnutt

Betsy teaches college physics, biology, and engineering and has a Ph.D. in Biomedical Engineering

The electric force is one of the most powerful, fundamental forces in the universe. In this lesson, we will learn how charged objects exert electric forces on each other and how to calculate the strength of that force.

What is Electric Force?

Force is a word that is used in everyday language to mean many different things, but in physics, it has a very specific meaning. In physics, a force is an interaction between two objects that has the ability to change the motion of one or both of the objects. One of the fundamental forces of the universe is the electric force. This is the force that exists between all charged particles. The electric force is responsible for such diverse phenomena as making your hair stand up on a cold dry day, creating chemical bonds, and allowing you to see when you turn on a lamp on a dark night.

Electric Force and Types of Charges

There are two types of charge, positive and negative, and they interact with each other in predictable ways. Unlike charges exert attractive forces on each other, while like charges exert repelling forces on each other. This means that if two objects that are both positively charged come close to each other, they will repel, or push each other away. If a positively charged object comes close to a negatively charged object, the two objects will attract each other and try to come together.

Electric Force and Charged Particles

So it has been stated that electric force occurs between all charged particles. These very small particles are found inside atoms, and they are called protons and electrons. Each proton has a positive charge, and each electron has a negative charge. Protons and electrons are the smallest charged particles that exist. All other objects, which are made up of atoms, become charged because of an imbalance in the number of protons and electrons inside those atoms.

Protons are very tightly held in the nucleus of each atom, so they are not able to move around at all. In contrast, electrons are away from the nucleus and are free to move around within the atom. It is also relatively easy for electrons to move from one atom to another, creating an imbalance in the number of positively charged protons and negatively charged electrons and causing the atom to become charged. When this happens to many atoms in an object, the entire object becomes charged.

With regard to electric force, this imbalance of protons and electrons is what causes your hair to stand up after you brush it on a cold, dry day. As you brush your hair, electrons from your hair are transferred to the brush or comb. This means that each strand of hair is now positively charged. Because two objects, hair strands in this example, with the same charge repel each other each strand of hair pushes away from all the others, causing your hair to stand up. This phenomenon is more likely to happen when the air is very cold and dry. It can happen even on a hot, humid day. However, when there is a lot of water in the air, your hair picks up charges from the air more easily and loses its charge quickly.

We see this in other situations as well. When a child slides down a plastic slide, each strand of her hair becomes positively charged. They all repel each other, causing her hair to stand up. The transfer of electrons is to the slide, just like the brush or comb.

Calculating Electric Force Using Coulomb's Law

The strength of the electric force between any two charged objects depends on the amount of charge that each object contains and on the distance between the two charges. As the amount of charge gets bigger, the force gets bigger, and as the distance between the two charges gets larger, the force gets smaller. This is known as Coulomb's Law and can be written as mathematical equation.

In this equation, the symbol q1 represents the amount of charge on object 1, q2 represents the amount of charge on object 2, r represents the distance between the two objects, and k is a constant known as the electrostatic constant. Fsub12 simply means, the force of object 1 on object 2, and Fsub21 means the force of object 2 on object 1. The objects exert equal forces on each other, Fsub12 = Fsub21.

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