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Physics LibreTexts

1. Electric Charge

Almost all of the main ideas we explore with electric fields are generalized and presented in the previous section about all fields. If you have not read that section, return to it and read it now. This section will help you apply the ideas that were somewhat familiar in a gravitational context to a new arena. By the end of this section, you will understand the ideas of electric charge, electric force, electric field, electric potential energy, and electric potential.

Electric Charge is Analogous to Mass

Previously, we said that the Earth’s gravity is stronger than the Moon’s gravity. We were able to quantify this statement by determining that, at its surface, Earth has a greater gravitational field than the Moon.  This meant that a given mass felt a greater gravitational force on the surface of the Earth than on the surface of the Moon. We determined the strength of the gravitational field by considering a spherical mass and the radius of the relevant body; numerically it can be shown that the Earth’s gravitational field is stronger.

To make an analogous argument about electric fields, we must determine the electrical analog to mass. It will probably come as no surprise to you that this analog is electrical charge. The greater an object’s charge, the greater the electric field it produces can be felt. Nearly all our ideas about mass and gravity are analagous to our discussion of charge, but with one clear difference:  unlike mass, charge comes in two types, positive and negative.

Units of Electrical Charge

Electric charges are measured in Coulombs, abbreviated C. The electron has a charge of \(−1.602 \times 10^{−19} \text{ C}\) (the proton’s charge has the same magnitude but it's positive).

All atoms, molecules, or charged macroscopic objects get their charge from either an excess or deficit of electrons compared to the number of protons they have; a charge of \(1.602 \times 10^{−19} \text{ C}\) is thus considered fundamental, as all observed particles or objects with charge have some integer multiple of this value.