Resistance - Let's go

(1) A force that acts on an object from a fluid when the object moves through it, and that acts to hinder the object's movement. A boundary layer is created near the surface of an object due to the viscosity of the fluid, and in that area, a force acting perpendicular to the surface of the object (pressure) and a force acting parallel to the surface (frictional stress) occur. When the pressure is integrated over the entire surface of an object, and pressure remains in the opposite direction to the object's movement, it is said that pressure resistance exists. In the case of a perfect fluid, pressure resistance does not exist. Examples of resistance due to viscosity = frictional resistance are familiar in everyday life, such as when moving underwater or when an iron ball sinks into starch syrup. In the case of an airplane wing, a vortex is generated at the tip of the wing and drags it backward. This vortex induces a downward air flow near the wing, generating a downward force on the airplane as it tries to rise. This is called induced resistance.

(2) The strength J of the current flowing through a uniform conductor is proportional to the potential difference V applied to both ends of the conductor (Ohm's law), and the proportionality coefficient R is called the electrical resistance. Electrical resistance does not depend on the strength J of the current or the potential difference V , but is determined by the type, thickness, length, and temperature of the conductor. In a metal conductor, current flows when free electrons are subjected to an electric field E caused by the potential difference. At this time, the flow of free electrons is hindered by collisions with metal atoms or impurity atoms that are in thermal motion, which causes electrical resistance. As the temperature of the metal is lowered, the thermal motion of the atoms becomes smaller, and so the resistance decreases. In certain special metals (mercury, lead, etc.), electrical resistance becomes zero at a certain finite temperature. This phenomenon is called superconductivity.

[Ryo Ikeuchi]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

(1)流体中を物体が動くときに、流体から物体に作用する力のうち、物体の運動を妨げようと働く力のこと。物体表面付近では流体の粘性のために境界層が生じ、その部分では、物体表面に垂直に働く力（圧力）と平行に働く力（摩擦応力）が生ずる。物体表面全体にわたって圧力を積分して、物体の運動方向の逆向きの圧力が残っている場合、圧力抵抗が存在するという。完全流体の場合、圧力抵抗は働かない。粘性による抵抗＝摩擦抵抗の例は、水中での運動のときや、鉄球が水飴(みずあめ)に沈んでいくときなど、日常においてなじみ深い。飛行機の翼の場合、翼の端から渦が発生し後ろへ引きずっている。この渦のため翼の近くで下向きの空気の流れが誘導され、上昇しようとする飛行機に対し下向きの力が生ずる。これを誘導抵抗という。

(2)一様な導線を流れる電流の強さJは、導線の両端にかけられた電位差Vに比例し（オームの法則）、その比例係数Rを電気抵抗という。電気抵抗は、電流の強さJや電位差Vにはよらず、導線の種類・太さ・長さ・温度などで決まる。金属導体では、電流は、自由電子が電位差に伴う電場Eを受けて流れるために生ずる。このとき、熱運動をしている金属原子や不純物原子と衝突して、自由電子の流れが妨げられることが電気抵抗の原因である。金属の温度を下げていくと、原子の熱運動が小さくなるため抵抗は小さくなる。ある特殊な金属（水銀・鉛など）では、ある有限の温度で電気抵抗がゼロになる。この現象を超伝導という。

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