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source of electric current, I2R, good conductors, electromotive force, small light bulb
An electric current is a movement of charge. When two objects with different charges touch and redistribute their charges, an electric current flows from one object to the other until the charge is distributed according to the capacitances of the objects. If two objects are connected by a material that lets charge flow easily, such as a copper wire, then an electric current flows from one object to the other through the wire. Electric current can be demonstrated by connecting a small light bulb to an electric battery by two copper wires. When the connections are properly made, current flows through the wires and the bulb, causing the bulb to glow.
Current that flows in one direction only, such as the current in a battery-powered flashlight, is called direct current. Current that flows back and forth, reversing direction again and again, is called alternating current. Direct current, which is used in most battery-powered devices, is easier to understand than alternating current. Most of the following discussion focuses on direct current. Alternating current, which is used in most devices that are “plugged in” to electrical outlets in buildings, will be discussed in the Alternating Current section of this article.
Other properties that are used to quantify and compare electric currents are the voltage (also called electromotive force) driving the current and the resistance of the conductor to the passage of the current. The amount of current, voltage, and resistance in any circuit are all related through an equation called Ohm’s law.
Conductors and Insulators
Conductors are materials that allow an electric current to flow through them easily. Most metals are good conductors.
Substances that do not allow electric current to flow through them are called insulators, nonconductors, or dielectrics. Rubber, glass, and air are common insulators. Electricians wear rubber gloves so that electric current will not pass from electrical equipment to their bodies. However, if an object contains a sufficient amount of charge, the charge can arc, or jump, through an insulator to another object. For example, if you shuffle across a wool rug and then hold your finger very close to, but not in contact with, a metal doorknob or radiator, current will arc through the air from your finger to the doorknob or radiator, even though air is an insulator. In the dark, the passage of the current through the air is visible as a tiny spark.
Measuring Electric Current
Electric current is measured in units called amperes (amp). If 1 coulomb of charge flows past each point of a wire every second, the wire is carrying a current of 1 amp. If 2 coulombs flow past each point in a second, the current is 2 amp.
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.
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.
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