Electrical Definitions What is charge? Matter is made up of atoms that consist of protons and neutrons in the nucleus and electrons outside the nucleus. Some kinds of atoms have electrons that are relatively free to leave the atom. Atoms that are deficient in one or more electrons attract electrons and electrons attract electron-deficient atoms. We say that this attractive force is due to the fact that an electron has a negative charge and the electron-deficient atom has a positive charge. We measure charge in units of coulombs, after Coulomb who did pioneering work describing the forces that exist between charges. What is current? Current is the flow of charged particles, such as electrons. It is the rate at which charges pass a point. Current is measured in coulombs/second or amperes, after Ampere who did pioneering work in the forces that exist between wires in which current is flowing. What is voltage? Strictly speaking, voltage is the work it takes to move a unit charge from one point to another in an electric field. This is analogous to the work it takes to move a mass from one point to another in a gravitational field. For our purposes, we can consider voltage to be an electrical force or a pressure that causes current to flow. A voltage results whenever positive and negative charges are separated from one another such that in one place there are more positive charges than negative charges and in another place there are more negative charges than positive charges. We say there is a voltage difference between the two places or points. Voltage is measured in volts, after Volta who invented the battery. What is resistance? The electrons in some substances move more easily than in other materials. A substance in which electrons move easily has less resistance than a substance in which the electrons do not move easily. Resistance is a measure of the difficulty with which electrons move through a substance. Substances that have low resistance are good conductors of electric current. Substances that have high resistance are insulators. Substances that have intermediate resistance are semiconductors. Resistance is measured in ohms, after Ohm, who established the relationship of voltage, current, and resistance to be V=RI. What is work? Work is something that gets done without regard to how long it takes. If I lift a brick or if a battery is charged, then a certain amount of work is done. Work is measured in joules, after Joule who did pioneering work in work and energy. Mechanical work is expressed as force times distance: Work=Fd. In a gravitational field, g, F=Mg, so work in the direction of a gravitational field: Work=Mgd joules. In an electric field, E, F=QE where Q is the charge in coulombs. Work =QEd joules. What is the relationship between work and voltage? Electric potential, measured in volts, is the work to move a unit charge from one point to another in an electric field: V=Work/Q = Ed volts. So Work=QV joules. What is energy? Whenever work is done, energy is produced. Pumping water up to a tank on a hill requires work but results in potential or stored energy which can be released when the water flows downhill. When a battery is charged, charges are stored for subsequent release and the voltage between the terminals of the battery increases. A battery stores electrical energy. Because of the equivalence of work and energy, energy is also measured in joules. What is power? Power is the rate at which work is done or energy is used: P=Work/time joules/second where one joule/second is one watt. Power is measured in joules/second or watts, after Watt, a mechanical engineer who did pioneering work in power and made dramatic improvements to the steam engine. If I lift a brick one meter in one second or if I lift it in two seconds, I have done the same amount of work; but the power is greater the faster I lift the brick. In a gravitational field, P=Mgd/t=Mgv, where v is the velocity of the mass, M. Electrical power is also measured in joules/second or watts. P=Work/time=QV/seconds. But Q/t is rate of charge flow, which is current. So P=current times voltage or P=IV. If a resistor is connected to a battery, current flows through the resistor and the energy that was stored in the battery is used. What is Ohm’s law? The voltage across a resistor equals the magnitude of the resistance times the magnitude of the current that flows through the resistor. V=RI What is Kirchhoff’s law for voltages around a closed loop of a circuit? The sum of the voltage rises equals the sum of the voltage drops. A voltage rise is a voltage produced by an energy source, such as a battery or a generator.