An electric current is usually thought of as a flow of electrons. When two ends of a battery are connected to each other by means of a metal wire, electrons flow out of one end (electrode or pole) of the battery, through the wire, and into the opposite end of the battery.
An electric current can also be thought of as a flow of positive "holes." A "hole" in this sense is a region of space where an electron might normally be found but does not exist. The absence of the electron's negative charge can be thought of as creating a positively charged hole.
In some cases, an electric current can also consist of a flow of positively charge particles known as cations. A cation is simply an atom or group of atoms carrying a positive charge.
The ampere (amp) is used to measure the amount of current flow. The unit was named for French mathematician and physicist André Marie Ampère (1775–1836), who founded the modern study of electric currents. The ampere is defined in terms of the number of electrons that pass any given point in some unit of time. Since electric charge is measured in coulombs, an exact definition for the ampere is the number of coulombs that pass a given point each second.
Potential difference. In order for an electric current to flow, a number of conditions must be met. First, a potential difference must exist between two points. The term potential difference (or voltage) means that the force created by a group of electrons in one place is greater than the force of electrons in some other place. The greater force pushes electrons away from the first place and toward the second place.
Potential differences usually do not occur in nature. In most cases, the distribution of electrons in the world around us is fairly even. Scientists have invented certain kinds of devices, however, in which electrons can be accumulated, producing a potential difference. A battery, for example, is nothing other than a device for producing large masses of electrons at one electrode (a point from which electric current is sent or received) and a deficiency of electrons at the other electrode. This difference accounts for the battery's ability to generate a potential difference, or voltage.
Electrical resistance. A second condition needed in order for a current to flow is a path along which electrons can travel. Some materials are able to provide such a path, and others are not. Materials that permit a flow of electric current are said to be conductors. Those that block the flow of electric current are called nonconductors or insulators. The metal wire connecting the two battery poles in the example cited earlier provides a path for the movement of electrons from one pole of the battery to the other.
The conductivity of materials is an intrinsic (or natural) property based on their resistance to the movement of electrons. The electrons in some materials are tied up in chemical bonds and are not available to conduct an electric current. In other materials, large numbers of electrons are free to move, and they transmit a flow of electrons easily.
Electrical resistance (or resistivity) is measured in a unit known as the ohm (Ω). The unit was named in honor of German physicist Georg Simon Ohm (1789–1854), the first person to express the laws of electrical conductivity. The opposite of resistance is conductance, a property that is measured in a unit called the mho (ohm spelled backwards).
The resistance of a piece of wire used in an electric circuit depends on three factors: the length of the wire, its cross-sectional area, and the resistivity of the material of which the wire is made. To understand the effects of electrical resistance, think of water flowing through a hose.
The amount of water that flows through the hose is similar to the current in the wire. Just as more water can pass through a fat fire hose than a skinny garden hose, a fat metal wire can carry more current than a skinny metal wire. For the wire, the larger the cross-sectional area, the lower its resistance; the smaller the cross-sectional area, the greater its resistance.
A similar comparison can be made with regard to length. It is harder for water to flow through a long hose simply because it has to travel farther. Similarly, it is harder for current to travel through a long wire than through a short wire.
Resistivity is a property of the material of which the wire itself is made and differs from material to material. Imagine filling a fire hose with molasses rather than water. The molasses will flow more slowly simply because of its viscosity (stickiness or resistance to flow). Similarly, electric current flows through some metals (such as lead) with more difficulty than it does through other metals (such as silver).
In most cases, the path followed by an electric current is known as an electric circuit. At a minimum, a circuit consists of (1) a source of electrons (such as a battery) that will provide a potential difference and (2) a pathway on which the electrons can travel (such as a metal wire). Recall that potential difference (or voltage) refers to a greater force of electrons in one place than in another; that greater force propels electrons toward the place with the lower force.
For any practical (or useful) application, a current also requires (3) an appliance whose operation depends on a flow of electric current. Such appliances include electric clocks, toasters, radios, television sets, and various types of electric motors. In many cases, electric circuits also contain (4) some kind of meter that shows the amount of electric current or potential difference in a circuit. Finally, a circuit is likely to include (5) various devices to control the flow of electric current, such as rectifiers, transformers, condensers, and circuit breakers.
Appliances may be placed into an electric circuit in one of two ways. In a series circuit, current flows through the appliances one after the other. In a parallel circuit, an incoming current is divided up and sent through each separate circuit independently.
An important advantage of parallel circuits is their resistance to damage. Suppose that any one of the appliances in a series circuit is damaged so that current cannot flow through it. This breakdown prevents current from flowing in any of the appliances. Such a problem does not arise with a parallel circuit. If any one of the appliances in a parallel circuit fails, current still continues to flow through the other appliances in the circuit.
The principle mathematical relationship governing the flow of electric current in a circuit was discovered by Ohm in 1827. Ohm's law states that the amount of current (i) in a circuit is directly related to the potential difference (V) and inversely related to the resistance (r) in the circuit. In other words, i = V/r. What Ohm's law says is that an increase in potential
difference or a decrease in resistance produces an increase in current flow. Conversely, a decrease in potential difference or an increase in resistance produces a decrease in current flow. The more complicated an electric circuit becomes, the more difficult it becomes to apply Ohm's law.
The field of electrical engineering is burdened with a strange problem that developed more than 200 years ago. When scientists first studied the flow of electric current from one place to another, they believed that the flow was produced by the motion of tiny particles. Since the electron had not yet been discovered, they assumed that those particles carried a positive charge.
Today we know otherwise. Electric current is a flow of negatively charged particles: electrons. But the custom of showing electric current as positive has been around for a long time, and it is still widely used. For that reason, it is not uncommon to see electric current represented as a flow of positive charges, even though we have known better for a long time.
The type of electric current described thus far is direct current (DC current). Direct current always involves the movement of electrons from a region of high negative charge to one of lower negative charge. The electric current produced by batteries is direct current.
Interestingly enough, the vast majority of electric current used for practical purposes is alternating current (AC current). Alternating current is current that changes the direction in which it flows very quickly. In North America, for example, commercial electrical power lines operate at a frequency of 60 hertz. (Hertz is the unit of frequency.) In a 60 hertz line, the current changes its direction 60 times every second.
Other types of alternating current also are used widely. Outside of North America, a 50 hertz power line is more common. And in airplanes, alternating current is usually rated at 400 hertz.