When a balloon is rubbed against a sweater, the balloon becomes charged. Because of this charge, the balloon can stick to walls, but when placed beside another balloon that has also been rubbed, the first balloon will fly in the opposite direction. This phenomenon is the result of a property of matter called electric charge. Electric charges produce electric fields: regions of space around electrically charged particles or objects in which other electrically charged particles or objects would feel force. An electric charge, which can be either positive or negative, is a property of matter that causes two objects to attract or repel. If the objects are oppositely charged (positive-negative), they will attract; if they are similarly charged (positive-positive or negative-negative), they will repel. The unit of electric charge is the Coulomb, which is defined as the amount of electricity that is conveyed by an electrical current of 1 Ampere in 1 second. Atoms, which are the basic units of matter, are made of three types of particles: electrons, neutrons, and protons. Electrons and protons themselves are electrically charged and have negative and positive charge, respectively. A neutron is not electrically charged. Many objects are electrically neutral and have a total net charge of 0. If there is an excess of either electrons or protons, thus yielding a net charge that is not zero, the objects are considered charged. One way to quantify electrical charge is by using the constant e = 1.602 *10-19 Coulombs. An electron, which is the smallest quantity of negative electrical charge, has a charge of -1.602 *10-19 Coulombs. A proton, which is the smallest quantity of positive electrical charge, has a charge of +1.602 *10-19 Coulombs. Thus, 10 electrons would have a charge of -10 e, and 10 protons would have a charge of +10 e. Electric charges attract or repel each other because they exert forces on each other. The force between two electric point charges—idealized charges that are concentrated at one point in space—is described by Coulomb’s law. Coulomb’s law states that the strength, or magnitude, of the force between two point charges is proportional to the magnitudes of the charges, and inversely proportional to the distance between the two charges.