Electric Charges, Voltage, and Current

Fundamentals of Electricity

811

Electric Charges

All matter is made up of atoms that contain both positively and negatively charged particles (protons and electrons). Surrounding every charged particle is an electric field that can exert force on other charged particles. A positive field surrounds a proton, and a negative field surrounds an electron. Field strength is the same for every electron and proton, with a magnitude of one “fundamental unit” of 1.602×10191.602×10^{-19} Coulombs. A coulomb is a measure of charge derived (in a somewhat circular fashion) from a measurement of electric current—one coulomb of charge is transferred by one ampere of current in one second (to get a matter of scale, one coulomb of charge flows through a 120W light bulb in one second). If one coulomb of protons could be isolated and held one meter apart from one coulomb of electrons, an attractive and equally repellent force (given by Coulombs law) of 8.988×1098.988×10^9 Newtons, equivalent to almost one million tons at the earth’s surface, would exist between them. It is this large intra-particle force that is harnessed to do work in electric circuits.

Figure 1. Coulomb's Law
Figure 1. Coulomb’s Law

Voltage

A positive electric field surrounding a group of one or more protons will exert a repelling force on other groups of protons, and an attracting force on groups of electrons (example: magnets). Since an electric field can cause charged particles to move, it can do some amount of work, and so it is said to have potential energy. The amount of energy an electric field can impart to unit charge is measured in joules per coulomb, more commonly known as voltage. For our purposes, voltage may be thought of as the “electro-motive force” that can cause charged particles to move. A power supply is a local, contained imbalance of electrons, with material on one side (the negative side) containing an abundance of electrons, and material on the other (positive) side containing a relative absence of electrons. The potential electrical energy available in the power supply, measured in volts, is determined by the number of electrons it can store, the separation distance between negative and positive materials, the properties of the barrier between the materials, and other factors. Some power supplies (like small batteries) output less than a volt, while others (like power generation stations) can output tens of thousands of volts. In general, power supplies of up to 9V – 12V are considered “safe” for humans to interact with, but some people can have adverse (and potentially fatal) interactions with even low-voltage supplies. In our work, we will not encounter any supplies above 5V.

Current

Electrons carry the smallest possible amount of negative charge, and billions of them are present in even the tiniest piece of matter. In most materials, electrons are held firmly in place by heavier protons, these materials are called insulators. By contrast, in other materials (like metals) electrons can move more easily from atom to atom, these materials are called conductors. The movement of electrons in a conductor is called electric current, measured in amperes or amps. If a power supply is used to impress a voltage across a conductor, electrons will move from the negative side of the supply through the conductor towards the positive side. All materials, even conductors, exhibit some amount of resistance to the flow of electric current. The amount of resistance determines how much current can flow—the higher the resistance, the less current can flow. By definition, a conductor has very low resistance, so a conductor by itself would never be placed across a power supply because far too much current would flow, damaging either the supply or the conductor itself. An electronic component called a resistor would be used in series with the conductor to limit current flow (more on resistors later).

Important Ideas

  • All matter contains atoms that have positive and negative charges.
  • If one coulomb of protons could be isolated and held one meter apart from one coulomb of electrons, an attractive and equally repellent force of 8.988×1098.988×10^9 Newtons, equivalent to almost one milli on tons at the earth’s surface, would exist between them. This concept is known as Coulomb’s Law.
  • An energy field is commonly measured in voltage. This energy passes through materials that either help it move (conductors) or slow it down (insulators).
  • Current and voltage are closely related in any electrical circuit. Remember that we are interested in moving charges around to perform some useful work. Voltage provides an energy level difference between two points which causes charges to move around. The resulting rate of charge motion is the current.