electric current

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Electric current

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.

Current measurement

The ampere (amp) is used to measure the amount of current flow. The unit was named for French mathematician and physicist André Marie Ampère (17751836), 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.

Characteristics of an electric current

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 (). The unit was named in honor of German physicist Georg Simon Ohm (17891854), 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).

Electric circuits

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.

Current flow and electron flow

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.

Direct and alternating current

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.

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Electric Current

Current and the transfer of electric charge

The speed of an electric current

Electric current and energy

Electric current and magnetism

Direct current

Alternating current

Current flow vs. electron flow

Resources

Electric current is the result of the relative motion of net electric charge. In metals, the charges in motion are electrons. The magnitude of an electric current depends upon the quantity of charge that passes a chosen reference point during a specified time interval. Electric current is measured in amperes, with one ampere equal to a charge-flow of one coulomb per second.

A current as small as a picoampere (one-trillionth of an ampere) can be significant. Likewise, artificial currents in the millions of amperes can be created for special purposes. Currents between a few milliamperes to a few amperes are common in radio and television circuits. An automobile starter motor may require several hundred amperes.

Current and the transfer of electric charge

The total charge transferred by an unvarying electrical current equals the product of current in amperes and the time in seconds that the current flows. If one ampere flows for one second, one coulomb will have moved in the conductor. If a changing current is graphed against time, the area between the graphs curve and the time axis will be proportional to the total charge transferred.

The speed of an electric current

Electrical currents move through wires at a speed only slightly less than the speed of light. The electrons, however, move from atom to atom more slowly. Their motion is more aptly described as a drift. Extra electrons added at one end of a wire will cause extra electrons to appear at the other end of the wire almost instantly. Individual electrons will not have moved along the length of the wire but the electric field that pushes the charge against charge along the conductor will be felt at the distant end almost immediately. To visualize this, imagine a cardboard mailing tube filled with ping-pong balls. When you insert an extra ball in one end of the tube, an identical ball will emerge from the distant end almost immediately. The original ball will not have traveled the length of the tube, but since all the balls are identical it will seem as if this has happened. This mechanical analogy suggests the way that charge seems to travel through a wire very quickly.

Electric current and energy

Heat results when current flows through an ordinary electrical conductor. Common materials exhibit an electrical property called resistance. Electrical resistance is analogous to friction in a mechanical system. Resistance results from imperfections in the conductor. When the moving electrons collide with these imperfections, they transfer kinetic energy, resulting in heat. The quantity of heat energy produced increases as the square of the current passing through the conductor.

Electric current and magnetism

A magnetic field is created in space whenever a current flows through a conductor. This magnetic field will exert a force on the magnetic field of other nearby current-carrying conductors. This is the principle behind the design of an electric motor.

An electrical generator operates on a principle similar to an electric motor. In a generator, mechanical energy forces a conductor to move through a magnetic field. The magnetic field forces the electrons in the conductor to move, which causes an electric current.

Direct current

A current in one direction only is called a direct current, or DC. A steady current is called pure DC. If DC varies with time it is called pulsating DC.

Alternating current

If a current changes direction repeatedly it is called an alternating current, or AC. Commercial electrical power is transported using alternating current because AC makes it possible to change the ratio of voltage to current with transformers. Using a higher voltage to transport electrical power across country means that the same power can be transferred using less current. For example, if transformers step up the voltage by a factor of 100, the current will be lower by a factor of 1/100. The higher voltage in this example would reduce the energy loss caused by the resistance

Key Terms

Conventional current Current assuming positive charge in motion.

Coulomb The standard unit of electric charge, defined as the amount of charge flowing past a point in a wire in one second, when the current in the wire is one ampere.

Frequency Number of times per unit of time an event repeats.

Hertz A unit of measurement for frequency, abbreviated Hz. One hertz is one cycle per second.

Picoampere One trillionth of an ampere or 1012amperes.

Speed of light Speed of electromagnetic radiation, usually specified in a vacuum. Approximately 6.7× 108 miles per hour (3× 108 meters per second).

of the wires to 0.01% of what it would be without the use of AC and transformers.

When alternating current flows in a circuit the charge drifts back and forth repeatedly. There is a transfer of energy with each current pulse. Simple electric motors deliver their mechanical energy in pulses related to the power line frequency.

Power lines in North America are based on AC having a frequency of 60 Hertz (Hz). In much of the rest of the world the power line frequency is 50 Hz. Alternating current generated aboard aircraft often has a frequency of 400 Hz because motors and generators can work efficiently with less iron, and therefore less weight, when this frequency is used.

Alternating current may also be the result of a combination of signals with many frequencies. The AC powering a loudspeaker playing music consists of a combination of many superimposed alternating currents with different frequencies and amplitudes.

Current flow vs. electron flow

We cannot directly observe the electricallycharged particles that produce current. It is usually not important to know whether the current results from the motion of positive or negative charges. Early scientists made an unfortunate choice when they assigned a positive polarity to the charge that moves through ordinary wires. It seemed logical that current was the result of positive charge in motion.

Later it was confirmed that it is the negatively-charged electron that moves within wires.

The action of some devices can be explained more easily when the motion of electrons is assumed. When it is simpler to describe an action in terms of the motion of electrons, the charge motion is called electron flow. Current flow, conventional current, or Franklin convention current are terms used when the moving charge is assumed to be positive.

Conventional current flow is used in science almost exclusively. In electronics, either conventional current or electron flow is used, depending on which flow is most convenient to explain the operation of a particular electronic component. The need for competing conduction models could have been avoided had the original charge-polarity assignment been reversed.

Resources

BOOKS

Bodanis, David. Electric Universe: The Shocking True Story of Electricity. New York: Crown, 2005.

Panofsky, Wolfgang K.H. and Melba Phillips. Classical Electricity and Magnetism. New York: Dover, 2005.

Donald Beaty

views updated

Electric current

Electric current is the result of the relative motion of net electric charge . In metals, the charges in motion are electrons. The magnitude of an electric current depends upon the quantity of charge that passes a chosen reference point during a specified time interval. Electric current is measured in amperes, with one ampere equal to a charge-flow of one coulomb per second.

A current as small as a picoampere (one-trillionth of an ampere) can be significant. Likewise, artificial currents in the millions of amperes can be created for special purposes. Currents between a few milliamperes to a few amperes are common in radio and television circuits. An automobile starter motor may require several hundred amperes.

Current and the transfer of electric charge

The total charge transferred by an unvarying electrical current equals the product of current in amperes and the time in seconds that the current flows. If one ampere flows for one second, one coulomb will have moved in the conductor. If a changing current is graphed against time, the area between the graph's curve and the time axis will be proportional to the total charge transferred.

The speed of an electric current

Electrical currents move through wires at a speed only slightly less than the speed of light . The electrons, however, move from atom to atom more slowly. Their motion is more aptly described as a drift. Extra electrons added at one end of a wire will cause extra electrons to appear at the other end of the wire almost instantly. Individual electrons will not have moved along the length of the wire but the electric field that pushes the charge against charge along the conductor will be felt at the distant end almost immediately. To visualize this, imagine a cardboard mailing tube filled with ping-pong balls. When you insert an extra ball in one end of the tube, an identical ball will emerge from the distant end almost immediately. The original ball will not have traveled the length of the tube, but since all the balls are identical it will seem as if this has happened. This mechanical analogy suggests the way that charge seems to travel through a wire very quickly.

Electric current and energy

Heat results when current flows through an ordinary electrical conductor. Common materials exhibit an electrical property called resistance. Electrical resistance is analogous to friction in a mechanical system. Resistance results from imperfections in the conductor. When the moving electrons collide with these imperfections, they transfer kinetic energy , resulting in heat. The quantity of heat energy produced increases as the square of the current passing through the conductor.

Electric current and magnetism

A magnetic field is created in space whenever a current flows through a conductor. This magnetic field will exert a force on the magnetic field of other nearby current-carrying conductors. This is the principle behind the design of an electric motor .

An electrical generator operates on a principle similar to an electric motor. In a generator, mechanical energy forces a conductor to move through a magnetic field. The magnetic field forces the electrons in the conductor to move, which causes an electric current.

Direct current

A current in one direction only is called a direct current, or DC. A steady current is called pure DC. If DC varies with time it is called pulsating DC.

Alternating current

If a current changes direction repeatedly it is called an alternating current, or AC. Commercial electrical power is transported using alternating current because AC makes it possible to change the ratio of voltage to current with transformers. Using a higher voltage to transport electrical power across country means that the same power can be transferred using less current. For example, if transformers step up the voltage by a factor of 100, the current will be lower by a factor of 1/100. The higher voltage in this example would reduce the energy loss caused by the resistance of the wires to 0.01% of what it would be without the use of AC and transformers.

When alternating current flows in a circuit the charge drifts back and forth repeatedly. There is a transfer of energy with each current pulse. Simple electric motors deliver their mechanical energy in pulses related to the power line frequency .

Power lines in North America are based on AC having a frequency of 60 Hertz (Hz). In much of the rest of the world the power line frequency is 50 Hz. Alternating current generated aboard aircraft often has a frequency of 400 Hz because motors and generators can work efficiently with less iron , and therefore less weight, when this frequency is used.

Alternating current may also be the result of a combination of signals with many frequencies. The AC powering a loudspeaker playing music consists of a combination of many superimposed alternating currents with different frequencies and amplitudes.

Current flow vs. electron flow

We cannot directly observe the electrically-charged particles that produce current. It is usually not important to know whether the current results from the motion of positive or negative charges. Early scientists made an unfortunate choice when they assigned a positive polarity to the charge that moves through ordinary wires. It seemed logical that current was the result of positive charge in motion. Later it was confirmed that it is the negatively-charged electron that moves within wires.

The action of some devices can be explained more easily when the motion of electrons is assumed. When it is simpler to describe an action in terms of the motion of electrons, the charge motion is called electron flow. Current flow, conventional current, or Franklin convention current are terms used when the moving charge is assumed to be positive.

Conventional current flow is used in science almost exclusively. In electronics , either conventional current or electron flow is used, depending on which flow is most convenient to explain the operation of a particular electronic component. The need for competing conduction models could have been avoided had the original charge-polarity assignment been reversed.

Resources

books

Hewitt, Paul. Conceptual Physics. Englewood Cliffs, NJ: Prentice Hall, 2001.

Hobson, Art. Physics: Concepts and Connections. Upper Saddle River, NJ: Prentice Hall, 1994.

Ostdiek, Vern J., and Donald J. Bord. Inquiry Into Physics. 3rd ed. St. Paul, MN: West Publishing Co., College & Schl. Div., 1995.

Donald Beaty

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conventional current

—Current assuming positive charge in motion.

Coulomb

—The standard unit of electric charge, defined as the amount of charge flowing past a point in a wire in one second, when the current in the wire is one ampere.

Frequency

—Number of times per unit of time an event repeats.

Hertz

—A unit of measurement for frequency, abbreviated Hz. One hertz is one cycle per second.

Picoampere

—One trillionth of an ampere or 10–12 amperes.

Speed of light

—Speed of electromagnetic radiation, usually specified in a vacuum. Approximately 6.7 × 108 miles per hour (3 × 108 meters per second).

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electric current Movement of electric charges, usually the flow of electrons along a conductor or the movement of ions through an electrolyte. This is caused by freely moving particles usually charged by a mains supply or battery. Current (symbol I) flows from a positive to a negative terminal, although electrons actually flow along a wire in the opposite direction. It is measured in amperes. Direct current (DC) flows continuously in one direction, whereas alternating current (AC) regularly reverses direction. The frequency of AC current is measured in hertz (Hz). See also electricity; particle physics

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electric current, net movement or flow of electric charge from one point to another or across some boundary. See alternating current; direct current; electricity.

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current, electric See electric current

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electric current: see electricity.