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Translations of Encyclopedia about Physics


Electrical Voltage and Resistance

According to Ohmís law, an electrical circuit which would not offer any resistance would have the capacity to hold a current of unlimited size. But nothing of the sort happens when we join a positive and negative electrical circuit with a piece of wire. Instead, a short circuit occurs which can cause great harm. Therefore, resistance is always necessary for regulating the size of an electrical current passing through an electrical circuit.

It is also necessary to differentiate between resistance as an electrical effect (referred to also as electrical conductivity) and resistance in terms of construction (such as when joining something to a plate).

The electrical conductivity of a particular substance depends on how fast electrons may pass through it. If a substance has a small amount of loose electrons or if the speed of those electrons slow down significantly on an atomic level, the material is considered a poor conductor because it causes electrical energy to be transformed into heat.

For electrical circuits, conductors need to be made out of materials which offer the least amount of resistance possible and which do not result in a decrease of current within the circuit. However, it happens many times that resistance is preferable, such as for example in a light bulb, where the thin filament within in converts electrical energy to light and heat, as such lighting up the light bulb. This is caused by the high resistance of the filament within the light bulb.

The resistance of a conductor does not depend solely on the electrical conductivity of the material it is made from but also on how long the conductor or cable is and what is the size of its cross-section (diameter).

The resistance offered by a component within an electrical circuit has the ability to limit the current passing through it. The size of the resistance within the circuit determines how much current can pass through under a certain amount of voltage and is measured in terms of ohms or kiloohms . The value of strong resistors is constant and such resistors are marked in special, coloured code. There are also resistors with variable values, called potentiometers, which are used for example in regulating the sound volume of stereo equipment.

Within a circuit, resistors can be aligned in parallel or in series. Resistors are most often used in voltage dividers, where the resistors are aligned in series and, based on the strength of the individual resistors, an exact amount of voltage can be drawn from.

Layered carbon is a common material used for making resistors. A carbon layer is a worse conductor than material commonly used to make conductors and the strength of the resistor depends on the thickness of the carbon layer.

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