Electric resistance - denki teikou (English spelling)

The property of a material to hinder electric current. It is often simply called resistance. Early experiments discovered that when electric current flows through a material, for many materials, the strength of the current is proportional to the strength of the applied voltage. This can be expressed by the formula I = (1/ R ) V.

Here, I is the current, V is the voltage, and 1/ R is the proportionality constant. This formula is called Ohm's law after its discoverer, and R is the (electrical) resistance. The magnitude of resistance varies depending on the type and dimensions of the material. Therefore, in order to express the inherent resistance of a material, the resistivity (or specific resistance) is defined as the resistance value determined by the strength of the current that flows when a voltage is applied in the direction of the length of a material with a cross-sectional area of ​​1 square meter and a length of 1 meter. In practice, values ​​measured using small samples are converted to meet the above conditions. However, in semiconductors and ionized gases, the current is not proportional to the voltage and does not follow Ohm's law. A circuit element (electrical device) that provides resistance to an electric circuit is called a resistor.

In electrical engineering, the term "resistance" is used to describe the electrical properties of a material, as well as the amount of resistance that resists the flow of current in an electrical circuit. In DC circuits, it is the amount that resists the flow of current when a voltage is applied to the circuit, and the current is inversely proportional to the resistance, which is equal to the reciprocal of the conductance. In AC circuits, it is the real part of the impedance. When an AC voltage is applied to a circuit, the current that flows is impeded by both resistance and reactance. For this reason, the phase of the current is generally different from the phase of the applied voltage. If this out-of-phase current is divided into a component that is in phase with the applied voltage and a component that is out-of-phase with the voltage, resistance represents the resistance of the current component that is in phase with the voltage, and this current component is inversely proportional to resistance. Furthermore, when the objects that make up the circuit do not exist, such as when radio waves are emitted from an antenna, the real part of the impedance expressed in the equivalent circuit can be treated as resistance. In this case, it is called radiation resistance. The resistance through which 1 ampere of current flows when a voltage of 1 volt is applied is defined as 1 ohm.

[Madashi Fuse and Masazumi Yoshizawa]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

物質が電流を妨げる性質。単に抵抗という場合が多い。物質中を電流が流れるとき、多くの物質について、その強さは加えた電圧の強さに比例することが初期の実験で発見された。これを式で表すと、I＝(1/R)Vとなる。

　ここではIは電流、Vは電圧、1/Rは比例定数を表す。この式は発見者の名にちなんでオームの法則とよばれ、Rを（電気）抵抗としている。抵抗の大きさは物質の種類と寸法によって異なる。そこで物質固有の抵抗を表すために、断面積1平方メートル、長さ1メートルの物質に長さの方向に電圧を加えたとき、流れる電流の強さから決まる抵抗の値を抵抗率（あるいは固有抵抗）と定めてある。実際には小さい試料を用いて測定した値を、前述の条件にあうように換算する。ただし半導体や電離した気体などでは、電流は電圧に比例せずオームの法則に従わない。電気回路に抵抗を与えるための回路素子（電気器具）を抵抗器という。

　なお、電気工学において「抵抗」という術語は、物質の電気的性質を表すとともに、電気回路における電流の流れにくさを表す量としても用いられる。直流回路においては、回路に電圧を加えたときの電流の流れにくさを表す量で、電流は抵抗に反比例し、抵抗はコンダクタンスの逆数に等しい。交流回路においては、インピーダンスの実数部をいう。回路に交流電圧を加えたとき流れる電流は、抵抗とリアクタンスの両方によって妨げられる。このため電流の位相は、一般に加えた電圧の位相とは異なる。この位相のずれた電流を、加えた電圧と同位相の成分とπ(パイ)ラジアンずれた成分に分けて考えると、抵抗は電圧と同位相の電流成分の流れにくさを表しており、この電流成分は抵抗に反比例する。さらに回路を構成する物体が実在しない場合、たとえばアンテナから電波が放射されるときなどにも、等価回路で表されるインピーダンスの実数部は抵抗として扱うことができる。この場合は放射抵抗とよんでいる。1ボルトの電圧を加えたとき1アンペアの電流が流れる抵抗を1オームと定めてある。

［布施　正・吉澤昌純］

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