The flow of electricity is formally known as electric current. Electricity is a phenomenon associated with the presence and motion of electric charge. When this electric charge moves in an organized manner through a conductor, such as a metal wire, it constitutes an electric current. This movement allows electrical energy to be transferred and utilized to power devices from simple light bulbs to complex computers.
Electric Current and the Movement of Charge
Electric current is defined as the net rate of flow of electric charge past a specific point in a circuit. In solid conductors, like copper wiring, the charge carriers are typically electrons, which are negatively charged particles. When a circuit is closed, these free electrons drift from atom to atom, creating the overall flow of charge.
The actual physical movement of electrons, known as electron flow, is from the negative terminal of a power source to the positive terminal. However, conventional current is used in most electrical analysis, based on a historical convention established before the discovery of the electron. Conventional current is defined as the direction a positive charge would flow, moving from the positive terminal to the negative terminal, opposite to the electron flow.
While the actual electrons move slowly, the electrical energy that drives them propagates at nearly the speed of light. This is similar to water pressure in a pipe: the pressure is felt instantly at the other end, even though the individual water molecules have not traveled the entire distance. The electric field that pushes the electrons is established almost immediately throughout the circuit.
The Forces That Drive and Resist Flow
For an electric current to flow, two conditions must be present: a driving force and a conductive path. The driving force is known as voltage (V), which represents the potential difference in electric energy between two points in a circuit. Voltage is often compared to water pressure; it is the “push” that motivates the electric charge to move.
The conductive path presents an opposition to this flow called resistance (R). Resistance is a material’s inherent property that impedes the movement of charge carriers, converting some electrical energy into heat. This is analogous to friction or a constriction that slows the flow in a water pipe.
Resistance is measured in Ohms (\(\Omega\)) and depends on the material’s composition, its length, and its cross-sectional area. A longer wire offers more resistance than a shorter wire of the same material. Voltage, measured in Volts (V), must be present to overcome this resistance and sustain the continuous movement of electric current.
Quantifying the Flow Rate
The practical, measurable quantity of electric current is quantified using the unit called the Ampere (A), often shortened to “amp”. The Ampere is the international standard unit for measuring electric current, relating directly to the quantity of electric charge moving past a point over time.
One Ampere is defined as the flow of one Coulomb of electric charge per second. A Coulomb is a very large quantity of charge, representing approximately \(6.24 \times 10^{18}\) electrons. When a wire carries a current of one Ampere, that number of electrons is passing a fixed point every second.
The current is the practical output of the interplay between the driving voltage and the opposing resistance. If the voltage is increased while the resistance remains unchanged, the flow rate, or the number of Amperes, will increase. The Ampere provides the essential measurement of the flow of electricity used by engineers to design and operate electrical systems.
Directional Patterns of Current
Electric current can be categorized into two types based on its directional pattern: Direct Current (DC) and Alternating Current (AC). Direct Current (DC) is characterized by a unidirectional flow, meaning the electric charge moves in only one direction through the circuit. Sources like batteries, solar cells, and power adapters produce DC.
Alternating Current (AC) is an electric current that periodically reverses its direction of flow. The voltage also reverses polarity along with the current, creating a wave-like pattern, typically a sine wave. The standard power delivered to homes and businesses from wall outlets is AC because it can be easily converted to higher or lower voltages using transformers.
The ability of AC to have its voltage easily modified makes it far more efficient for long-distance power transmission than DC. However, most modern electronic devices require the steady, one-directional flow of DC power to operate. Devices that plug into the wall use a power supply or adapter to convert the AC from the outlet into the DC required by the internal circuitry.