Electricity

C

Voltage

When the two terminals of a battery are connected by a conductor, an electric current flows through the conductor. One terminal continuously sends electrons into the conductor, while the other continuously receives electrons from it. The current flow is caused by the voltage, or potential difference, between the terminals. The more willing the terminals are to give up and receive electrons, the higher the voltage. Voltage is measured in units called volts. Another name for a voltage produced by a source of electric current is electromotive force.

D

Resistance

A conductor allows an electric current to flow through it, but it does not permit the current to flow with perfect freedom. Collisions between the electrons and the atoms of the conductor interfere with the flow of electrons. This phenomenon is known as resistance. Resistance is measured in units called ohms. The symbol for ohms is the Greek letter omega, Ω.

A good conductor is one that has low resistance. A good insulator has a very high resistance. At commonly encountered temperatures, silver is the best conductor and copper is the second best. Electric wires are usually made of copper, which is less expensive than silver.

The resistance of a piece of wire depends on its length, and its cross-sectional area, or thickness. The longer the wire is, the greater its resistance. If one wire is twice as long as a wire of identical diameter and material, the longer wire offers twice as much resistance as the shorter one. A thicker wire, however, has less resistance, because a thick wire offers more room for an electric current to pass through than a thin wire does. A wire whose cross-sectional area is twice that of another wire of equal length and similar material has only half the resistance of the thinner wire. Scientists describe this relationship between resistance, length, and area by saying that resistance is proportional to length and inversely proportional to cross-sectional area.

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Usually, the higher the temperature of a wire, the greater its resistance. The resistance of some materials drops to zero at very low temperatures. This phenomenon is known as superconductivity.

E

Ohm’s Law

The relationship between current, voltage, and resistance is given by Ohm’s law. This law states that the amount of current passing through a conductor is directly proportional to the voltage across the conductor and inversely proportional to the resistance of the conductor. Ohm’s law can be expressed as an equation, V = IR, where V is the difference in volts between two locations (called the potential difference), I is the amount of current in amperes that is flowing between these two points, and R is the resistance in ohms of the conductor between the two locations of interest. V = IR can also be written R = V/I and I = V/R. If any two of the quantities are known, the third can be calculated. For example, if a potential difference of 110 volts sends a 10-amp current through a conductor, then the resistance of the conductor is R = V/I = 110/10 = 11 ohms. If V = 110 and R = 11, then I = V/R = 110/11 = 10 amp.

Under normal conditions, resistance is constant in conductors made of metal. If the voltage is raised to 220 in the example above, then R is still 11. The current I will be doubled, however, since I = V/R = 220/11 = 20 amp.

F

Heat and Power

A conductor’s resistance to electric current produces heat. The greater the current passing through the conductor, the greater the heat. Also, the greater the resistance, the greater the heat. A current of I amp passing through a resistance of R ohms for t seconds generates an amount of heat equal to I²Rt joules (a joule is a unit of energy equal to 0.239 calorie).

Energy is required to drive an electric current through a resistance. This energy is supplied by the source of the current, such as a battery or an electric generator. The rate at which energy is supplied to a device is called power, and it is often measured in units called watts. The power P supplied by a current of I amp passing through a resistance of R ohms is given by P = I²R.

V

How Electric Current Is Conducted

All electric currents consist of charges in motion. However, electric current is conducted differently in solids, gases, and liquids. When an electric current flows in a solid conductor, the flow is in one direction only, because the current is carried entirely by electrons. In liquids and gases, however, a two-directional flow is made possible by the process of ionization (see Electrochemistry).

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