Resistance (electricity)
Electrical resistance is the inherent opposition a material provides to the flow of electric current. This flow, analogous to water moving through pipes, results from the movement of free electrons within a circuit. Various materials exhibit differing levels of resistance; conductors like copper and silver allow electric current to pass easily, while insulators such as rubber and glass greatly hinder it. Resistance is quantified in ohms, a unit named after the physicist Georg Ohm, who formulated Ohm's Law, expressing the relationship between voltage, current, and resistance.
The characteristics of a material, including its length and cross-sectional area, significantly impact its resistance. For instance, longer wires encounter more collisions with atoms, increasing resistance, while wider wires provide less resistance. Additionally, the atomic structure of a material influences its ability to conduct electricity, with metals generally serving as better conductors due to their availability of free electrons. Electrical resistance plays a crucial role in various applications, including heating elements in appliances where controlled resistance generates heat. Understanding resistance is essential for designing efficient electrical circuits and devices.
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Resistance (electricity)
Electrical resistance is the natural opposition a material offers to the path of an electrical current through that material. Similar to how water flows through pipes, an electrical current is the movement of electrons in one direction. The path the current takes is known as an electrical circuit. Just as friction with the pipes slows the flow of water, resistance within the material slows the electrical charge in a circuit. Conductors, such as copper and silver, offer very little resistance to electrical current. Meanwhile, insulators, such as rubber and glass, offer such high resistance that they restrict the flow of electricity. Resistance can also be affected by the length and width of the material through which the charge is flowing. Electrical resistance is measured in units called ohms, named after nineteenth-century German physicist Georg Ohm.
Background
Atoms are tiny particles that are the fundamental building blocks of all matter. Individual atoms are themselves made up of subatomic particles called protons, neutrons, and electrons. Protons have a positive electrical change, neutrons have no electrical charge, and electrons have a negative charge. Normally, the number of protons and electrons in an atom are the same, balancing the positive and negative electrical force. Electrons orbiting closer to the nucleus are usually more stable, while those in the outer shells can sometimes be pushed from their orbits by an outside force. These “lost” electrons move freely as electricity.
When an atom loses an electron, the atom has one more proton and becomes positively charged. The free electrons are drawn toward the next positively charged atom and jump from atom to atom along a path. As each electron leaves an atom, it is replaced with another electron, creating a flow of electricity in one direction. The path along which an electrical current flows in called a circuit. As the name suggests, a closed circuit is a closed loop where the electricity can flow continuously; in an open circuit, the flow of electricity is interrupted and the current is broken.
Overview
As a free electron moves along a circuit, it is constantly colliding with other atoms as it travels in a zigzag pattern. Resistance is the natural interference the electron encounters as it attempts to travel along the circuit. The higher the resistance, the harder it is for the electrical current to flow freely through the circuit.
The concept of electrical resistance was first studied in the eighteenth century by British scientist Henry Cavendish who intentionally subjected himself to electric shocks to determine what he called “opposition to current.” In the early nineteenth century, Georg Ohm identified electrical resistance as part of a formula that became known as Ohm’s Law. In the formula, Ohm found that the voltage is equal to the electrical current times the resistance (V=IR). In scientific terms, voltage is the difference in potential energy between the two points in a circuit. Similar to how pressure pushes water through a pipe, voltage is an electrical force that propels the electrical current through a circuit. Current—noted by Ohm with an “I” for intensity—is the rate at which the charge flows through the circuit. Resistance is measured in units called ohms.
A main factor that impacts the electrical resistance in a material is the substance from which the material is made. Substances that can conduct electricity more easily offer the least amount of electrical resistance. These substances typically have an atomic structure that allows for the creation of more free electrons to carry the electrical charge.
Atoms whose outer shells are full do not lose electrons very easily; if the outer shell of an atom is not full, it is more likely to lose electrons, creating more free electrons. Conductors are materials with a higher amount of free electrons, giving them a higher capacity to carry a current and a lower electrical resistance. In general, metals make good conductors because they tend to have electrons that can leave their atoms and travel more easily. Silver has one electron in an outer shell that can hold eighteen at most. As a result, it offers very low electrical resistance and is considered one of the best materials for electrical conducting. However, its high cost makes it impractical for everyday use. Copper, which also has one electron in an outer shell that can hold eighteen, is far less expensive and one of the most widely used conductors. In contrast, insulators tend to be made from atoms with fewer free electrons. For example, rubber is a carbon-based substance in which the electrons are tightly bonded to the atom and are more difficult to come free and carry a current.
Collisions between the electrons and other atoms also raise the resistance of electricity in a circuit. The more collisions the electrons encounter, the slower their velocity and the more kinetic energy they lose. The energy is lost in the form of heat. This electrical resistance can be harnessed in products such as toasters or space heaters, in which a limited amount of heat is desirable. The resistance can be modified with an electrical component called a resistor, which adjusts the flow of a current through a circuit.
The length of the medium though which the electrical current must pass also impacts its resistance. Although copper is a good conductor, a longer length of copper wire has more atoms for the electrical current to collide with as it travels through the circuit. Electrical resistance is directly proportional to the length of a material. For example, doubling the length of a wire also doubles its resistance. In addition, the width, or cross-sectional area, of a material also affects electrical resistance. The wider the cross-sectional area of a wire, the less resistance the electricity flowing through it encounters. In this case, doubling the diameter of a wire would decrease its resistance by half.
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