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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Coulomb's Law will tell you how much electric force is exerted on each object, but it won't tell you what direction that force is in. To know this, you still have to look at each object's type of charge. If the two objects have opposite charges, one positive and one negative, they will exert attractive forces on each other. That is shown in the equation by the arrows that point towards each object when one is positive and the other is negative. If the objects have the same charge, both positive or both negative, they will push each other away. The equation also shows this by the arrows that point away from each other.
To understand what this equation is telling you, think about what would happen if one or both of the objects became more charged. That would mean that q1 and/or q2 were larger, so the force between the two objects would increase. Also, think about what would happen if the two objects got farther away from each other. This would mean that r got larger, and since you divide by r, if r gets larger, the force gets smaller. In science, we say that this means that the electric force is directly proportional to the amount of charge (q1 and q2) and inversely proportional to the distance between the charges r. Both of these things influence the electric force between any two charged objects.
An electric force is exerted between any two charged objects. Objects with the same charge, both positive and both negative, will repel each other, and objects with opposite charges, one positive and one negative, will attract each other. The strength of the force depends on both the amount of charge that each object contains and the distance between the two charges. Coulomb's Law is a mathematical equation. It shows that as the amount of charge on the objects increases, the force between them increases, and as the distance between the charged objects increases, the force between them decreases. Amount of charge and distance both play a part in the electric force between two objects.
Electric Force-Related Key Terms & Examples
Example of electric force
Force: interaction between two objects
Electric Force: force exerted by two charged objects
Positive/Negative Charges: two unlike charges will exert a force that attracts each other while two like charges will exert repelling forces on each other
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