Resistance is a measure of a material’s tendency to resist the flow of an electric current and depends on material composition, thickness, length, and temperature. Ohm’s law states that electric current equals voltage divided by resistance. Resistance generates heat and is used for electric heating, cooking, and lighting. Resistors and fuses protect delicate components from damage. Lie detectors measure skin conductivity. Transformers increase transmission voltage and minimize energy losses during power transmission.
In physics, resistance is a measure of a material’s tendency to resist the flow of an electric current. It depends on the nature of the material, its thickness and length and the temperature. Resistance is low in substances, such as metals, which are good conductors, and high in materials, such as plastics and rubber, which are insulators. When an electric current meets resistance, some of its energy is converted into heat and sometimes light, reducing the current. This phenomenon can be a problem, but it also has many uses.
Factors affecting resistance
Resistance can be thought of as the inverse of conductivity, and the most important factor in conductivity is material composition. An electric current is a flow of electrons and resistance is encountered when they collide with atoms. Metals have many loosely held electrons which allow current to flow easily, whereas non-metals do not. Liquids that contain ions, such as a saline solution or molten salt, are also good conductors, because these mobile, electrically charged atoms and molecules allow current to flow.
In a wire or cable, thickness and length also play a role. Resistance increases with length, since there are more atoms to collide with, but decreases with thickness, since there are more electrons available in a thicker wire to carry the current. It also increases with increasing temperature. The lower the conductivity of a material, the higher the voltage, or electromotive force, required to drive current through it.
Ohm’s law
The relationship between resistance, current, and voltage is known as Ohm’s law, named after the German physicist Georg Ohm (1789-1854), who is credited with discovering the effect of a material’s composition, length, and thickness on the quantity current flowing through it at a given voltage. The ohm unit is also named after him. The law, in its usual form, states that electric current equals voltage divided by resistance. Physics equations typically use letters and symbols to express relationships; Ohm’s law is normally written as
I = V / R
where I = current in amperes, V = electromotive force in volts and R = resistance in
ohm
. This means that if you know the values of two of these factors, you can find the value of the other. The equation can be written as
R = V / I
V = IS
depending on which values are known and which one needs to be calculated. For example, if the electromotive force is six volts and the current flowing is two amperes, the resistance is 3 ohms: R = V/I.
it is used
The fact that electrical resistance generates heat is used to provide electric heating for homes and for cooking. Electric hobs, ovens, grills and toasters all depend on this phenomenon. Similarly, the filament bulb uses a very thin wire to generate light when a current passes through it.
Devices called resistors are used to draw down the amperes within certain circuits to protect delicate components from damage, and fuses are used to protect electrical equipment from current surges. These consist of a wire whose composition, thickness and length are adjusted to produce a level of resistance which causes them to melt by the heat generated when the current exceeds a certain value. This breaks the circuit and prevents the current from causing damage. They are commonly used in plugs and come in various types, such as 3 amp, 5 amp, and 13 amp.
Lie detectors are based on the fact that the conductivity of human skin is significantly increased by sweat, which contains ionic compounds, such as salt. The subject is connected to a device that passes a small current through the skin and measures its value. The idea is that lying down increases the amount of sweating, which increases the conductivity of the skin and results in a higher current flow.
Power transmission
In order for electricity to be transmitted from power plants to homes, it needs to travel long distances through power lines. This would make it impractical at the voltages at which electricity is initially produced, as too much energy would be lost through the resistance of the wires. For this reason, transformers are used to greatly increase the transmission voltage while minimizing energy losses. The voltage is reduced again by transformers close to the houses to be supplied.
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