Resistanssi

Resistanssi, in physics and electronics, refers to electrical resistance—the property of a material or component that opposes the flow of electric current. It is defined as the ratio of the voltage across the component to the current through it (R = V/I) for a material that behaves approximately linearly (ohmic). The SI unit is the ohm (Ω), and the symbol commonly used is R.

The resistance of a conductor depends on the material, temperature, length, and cross-sectional area. It can be expressed as R = ρL/A, where ρ is the resistivity of the material, L is the length, and A is the cross-sectional area. Longer conductors and thinner cross-sections increase resistance, while materials with lower resistivity have smaller resistance.

Temperature and frequency influence resistanssi. For many metals, resistance increases with temperature due to enhanced lattice scattering of electrons. In semiconductors and some other materials, the dependence can be more complex and non-linear, and at high frequencies the effective resistance can differ because of reactance.

In circuits, resistors can be arranged in series or parallel. In series, resistances add (R_total = R1 + R2 + …). In parallel, the reciprocal of the total resistance is the sum of the reciprocals (1/R_total = 1/R1 + 1/R2 + …). The inverse quantity, conductance G = 1/R, is sometimes used.

Resistanssi is measured with instruments such as multimeters, and for very low or very high values specialized methods (e.g., four-terminal sensing) are used to reduce measurement errors. Applications include voltage dividers, current limiting, heating elements, and temperature sensing (thermistors). In Finnish, resistanssi denotes the electrical resistance, the same concept as in other languages.