Ohm

The ohm (symbol: Ω, the uppercase Greek letter omega) is the unit of electrical resistance in the International System of Units (SI). It is named after German physicist Georg Ohm (1789–1854). Various empirically derived standard units for electrical resistance were developed in connection with early telegraphy practice, and the British Association for the Advancement of Science proposed a unit derived from existing units of mass, length and time, and of a convenient scale for practical work as early as 1861. Following the 2019 revision of the SI, in which the ampere and the kilogram were redefined in terms of fundamental constants, the ohm is now also defined as an exact value in terms of these constants.

The ohm (symbol: Ω, the uppercase Greek letter omega) is the unit of electrical resistance in the International System of Units (SI). It is named after German physicist Georg Ohm (1789–1854). Various empirically derived standard units for electrical resistance were developed in connection with early telegraphy practice, and the British Association for the Advancement of Science proposed a unit derived from existing units of mass, length and time, and of a convenient scale for practical work as early as 1861. Following the 2019 revision of the SI, in which the ampere and the kilogram were redefined in terms of fundamental constants, the ohm is now also defined as an exact value in terms of these constants. \Omega = \frac{\mathrm{V}}{\mathrm{A}} = \frac{1}{\mathrm{S}} = \frac{\mathrm{W}}{\mathrm{A^{2}}} = \frac{\mathrm{V}^{2}}{\mathrm{W}} = \frac{\mathrm{s}}{\mathrm{F}} = \frac{\mathrm{H}}{\mathrm{s}} = \frac{\mathrm{Wb}}{\mathrm{C}} = \frac{\mathrm{J{\cdot}s}}{\mathrm{C^{2}}} = \frac{\mathrm{J}}{\mathrm{s{\cdot}A^{2}}} = \frac{\mathrm{kg{\cdot}m^{2}}}{\mathrm{s{\cdot}C^{2}}} = \frac{\mathrm{kg{\cdot}m^{2}}}{\mathrm{s^{3}{\cdot}A^{2}}} In many cases the resistance of a conductor is approximately constant within a certain range of voltages, temperatures, and other parameters. These are called linear resistors. In other cases resistance varies, such as in the case of the thermistor, which exhibits a strong dependence of its resistance with temperature. In the US, consecutive vowels in the prefixed units "kiloohm" and "megaohm" are commonly reduced to one, producing "kilohm" and "megohm". s is the measurement of resistance in ohms. In alternating current circuits, electrical impedance is also measured in ohms. P=V I =\frac{V^2}{R} = I^2 R, where is the power, is the resistance, is the voltage across the resistor, and is the current through the resistor. A linear resistor has a constant resistance value over all applied voltages or currents; many practical resistors are linear over a useful range of currents. Non-linear resistors have a value that may vary depending on the applied voltage (or current). Where alternating current is applied to the circuit (or where the resistance value is a function of time), the relation above is true at any instant, but calculation of average power over an interval of time requires integration of "instantaneous" power over that interval. Since the ohm belongs to a coherent system of units, when each of these quantities has its corresponding SI unit (watt for , ohm for , volt for and ampere for , which are related as in ) this formula remains valid numerically when these units are used (and thought of as being cancelled or omitted).

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