Electric Circuit

Electric circuit is the path followed by an electric current. Electricity must flow in a circuit to do useful work.

An electric circuit has three basic parts: (1) a source of electric energy, such as a battery or generator; (2) an output device, such as a motor or lamp; and (3) a connection between the source and the output device, such as a wire or cable.

The source converts some type of nonelectric energy into electric energy. For example, an electric generator changes mechanical energy into electric energy. The electric source creates an electromotive force (emf) that causes an electric current to flow in the circuit. Emf is measured in units called volts, and the current it produces is measured in units called amperes. Electric outlets in homes in the United States and Canada supply electric energy at from 110 to 120 volts. But the electric outlet itself is not a source. Electric transmission lines connect the outlet to a generator at an electric power plant, which is the source.

An output device uses the electric energy from the source to do something useful. For example, a lamp provides light and an electric motor produces mechanical motion to operate a vacuum cleaner. The source and output device must be connected so that electric current can flow from the source to the device and back again. The return path is necessary so that an electric charge does not collect at any point in the circuit. A collected charge would oppose the flow of current and keep the circuit from functioning.

Various devices may be added to a circuit to control the current flowing in it. For example, the circuit of a lamp may include a switch to turn the lamp on and off easily. When the switch is off, a gap separates the connecting wires so that the current cannot complete its path. A circuit with this kind of gap is called an open circuit. A closed circuit has no gaps in the path of the current.

Some circuits, including those used in homes, are equipped with a fuse or a circuit breaker. Either of these devices acts as an automatic switch that opens the circuit if too much current flows through it. Excessive current may overheat the wires and start a fire or damage output devices.

An electric circuit may be simple or complex. A simple circuit, used in such equipment as flashlights and lamps, may consist of only the three basic circuit parts. A complex circuit, such as that used in computers and television sets, has hundreds or even thousands of circuit parts. No matter how many circuit parts a circuit has, all circuits except the simplest can be classified as one of three types: (1) series, (2) parallel, and (3) complex. Almost all electric circuits are complex circuits, which consist of both series and parallel types.

Series circuits use a single path to connect the electric source or sources to the output device or devices. If a series circuit is drawn on paper, a line starting at any circuit part will pass through all the other circuit parts only once before returning to the starting point. For example, the circuit in a two-battery flashlight connects the positive terminal of the first battery to the negative terminal of the second battery. The positive terminal of the second battery touches the center terminal of the flashlight bulb. If the switch is closed, the outer terminal of the bulb touches the negative terminal of the first battery, completing the circuit and lighting the bulb.

Series circuits may be found chiefly in flashlights, some Christmas tree lights, and other simple equipment. These circuits have limited uses because any change in one circuit part affects all the circuit parts. If one light bulb in a series circuit burns out, all the other bulbs also go out because the burned-out bulb has opened the circuit.

The voltage provided by a group of electric sources connected in series is the sum of their individual voltages. But the same amount of current flows through each source and output device. For example, each battery in a two-battery flashlight supplies 1 1/2 volts, and the two together supply 3 volts. The same amount of current flows through each battery and the bulb. Electric sources are connected in series to provide more voltage than one source alone can produce.

Parallel circuits provide more than one path for current. After current leaves a source, it follows two or more paths before returning to the source. If two identical flashlight bulbs are connected in parallel, current flows from a battery through each lamp individually and then back to the battery. Either bulb may be removed from the circuit without breaking the circuit for the other bulb. When both bulbs are on, each receives half the total current from the battery.

Parallel circuits provide the same voltage for every source and output device in the circuit. For example, two 1 1/2-volt flashlight batteries connected in parallel provide an emf of 1 1/2 volts. Electrical sources are connected in parallel to provide more current than one source can produce. But only sources with the same voltage can be connected in parallel. Otherwise, excess current would flow from one source into the other and be wasted.

All household lights and appliances are connected in parallel because a parallel circuit allows all devices to operate on the same voltage. The voltage does not change if a piece of equipment is added or removed. However, the total current passing through the fuse or circuit breaker may increase or decrease. The total current is the sum of the currents flowing through each piece of equipment.

Circuit mathematics. Electricians and engineers use several mathematical formulas to calculate the current and voltage in each part of a circuit. The most important of these formulas are Ohm's law and Kirchhoff's laws. They were discovered by two German physicists, Georg S. Ohm and Gustav R. Kirchhoff.

Ohm's law relates the voltage and current in a circuit to the resistance of the circuit. Resistance opposes the flow of electricity and consumes power from the circuit by changing electric energy into heat. Electricians measure resistance in units called ohms. Ohm's law is expressed in the equation E = IR. This law states that the voltage (E) equals the current (I) multiplied by the resistance (R), through which the current flows. For example, if a current of 3 amperes passes through a resistance of 2 ohms, the voltage is 3 amperes X 2 ohms = 6 volts.

In a series circuit, the total resistance equals the sum of the resistances of each device in the circuit. The addition of devices to a series circuit increases the resistance and thus decreases the total current. But in a parallel circuit, adding devices provides additional paths for the current and decreases the total resistance.

Kirchhoff's first law states that the sum of the currents entering any point in a circuit equals the sum of the currents leaving that point. This law is based on the fact that an electric charge cannot accumulate at any point in a closed circuit. Kirchhoff's second law states that the sum of the changes in voltage around any circuit is zero. In other words, the voltage increases through the sources by the same amount that it decreases through the output devices. For example, starting at the base end of a two-battery flashlight, the emf increases through each battery. It increases by 11/2 volts in each, for a total increase of 3 volts. The emf decreases 3 volts going through the bulb.

Contributor: Phillip W. Alley, Ph.D., Prof. of Physics, State Univ. of New York at Geneseo.

Electric Circuit

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