In Physics, what is Resistance? (with photos)

Power lines and transformer.

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 plastic and rubber, which are insulators. When an electrical current encounters 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 that affect endurance

A fuse works due to resistance.

Resistance can be considered the inverse of conductivity, and the most important factor in conductivity is the composition of the material. An electric current consists of a flow of electrons, and resistance is encountered when these collide with atoms. Metals have many loosely held electrons that allow a current to flow easily, while non-metals do not. Liquids that contain ions – for example, a salt solution or molten salt – are also good conductors, as these mobile, electrically charged atoms and molecules allow a current to flow.

Resistors add a “fixed” amount of resistance to a circuit.

In a wire or cable, thickness and length also play a role. Resistance increases with length, as there are more atoms to collide with, but decreases with thickness, as in a thicker wire there are more electrons available to carry the current. It also increases with increasing temperature. The lower the conductivity of a material, the greater the voltage, or electromotive force, needed to make current flow through it.

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Ohm’s Law

German physicist Georg Ohm discovered how the composition, length and thickness of a material influence how much current will flow through it at a given voltage.

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 amount of current flowing through it. will flow through it at a given voltage. The ohm unit is also named after him. The law, in its usual form, states that electric current is equal to voltage divided by resistance. Physics equations often use letters and symbols to express relationships; Ohm’s law is usually written as

I = V / R

where I = current in amperes, V = electromotive force in volts and R = resistance in


. This means that if the values ​​of two of these factors are known, the value of the other can be found. The equation can be written as

R = V / I


V = IR

depending on which values ​​are known, and which must be calculated. For example, if the electromotive force is six volts and the current flowing is two amps, the resistance is 3 ohms: R = V/I.


The fact that electrical resistance generates heat is used to provide electrical heating for homes and for cooking. Electric stoves, ovens, grills and toasters all depend on this phenomenon. Likewise, the filament bulb uses a very thin wire to generate light when a current flows through it.

Devices called resistors are used to reduce amperage within certain circuits to protect delicate components from damage, and fuses are used to protect electrical equipment from current surges. They consist of a wire whose composition, thickness and length are adjusted to produce a level of resistance that causes them to melt with the heat generated when the current passes 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.

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Lie detectors rely on the fact that the conductivity of human skin is greatly 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 increases the amount of sweat, which increases the skin’s conductivity, and results in greater current flow.

power transmission

In order for electricity to be transmitted from generating stations into homes, it is necessary for it to travel long distances through power lines. This would make it impractical at the voltages at which the electricity is initially produced, as too much energy would be lost through the resistance of the cables. For this reason, transformers are used to greatly increase the voltage for transmission, minimizing loss of energy. The voltage is reduced again by transformers close to the homes to be supplied.

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