# Resistance and Resistivity

**Resistance**it the**opposition**to the**passage of current**within a component. The Resistance of a component decides how much**voltage**will be**dropped**across it for a**particular current**.Resistance is measured in

**Ohms**(**Ω**). According to**Ohm’s Law**,**voltage**is the**product**of**current**and**resistance**. Therefore Ohms can be expressed in base units as \(\mathrm{Kg}\mathrm{m}^2\mathrm{s}^{-3}\mathrm{A}^{-2}\).

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**Everything has Resistance**, because everything has some opposition to the flow of Electric Charge.**Components**whose sole purpose is to provide a**Resistance**of a**certain value**are called**Resistors**.

- When Resistors are
**connected**in**Series**, the**total Resistance**across them will be**equal**to the**sum**of**each Resistor value**. The**total voltage**will be equal to the**sum**of the**voltages****across each Resistor**. This rule will also apply for other components.

- When Resistors are
**connected**in**Parallel**, the**reciprocal**of the**total resistance**will be equal the**sum**of the**reciprocals**of**each Resistor Resistance**. The t**otal voltage dropped**will be the same as the**voltages dropped**across**all the individual Resistors**.

## Resistivity

Resistance is a

**Sample Constant**, so is**specific**to**individual**components. However, there is a**Material Constant**that can be used to find the**Resistance**of**any**component of a**specific material**. This is**Resistivity**. Together with the**length**and**cross-sectional area**of a sample, it can calculate its**resistance**.Resistivity is given the symbol

**ρ**and is measured in**Ohm Meters**(**Ωm**, or Kgm^{3}s^{-3}A^{-2}in base units).*For example, copper has a Resistivity of 1.68 ×10*^{-8}Ωm, and Germanium 4.6 ×10^{-1}Ωm.The Resistance of a material of

**Resistivity ρ**,**length l**and**cross-sectional area A**is calculated by the formula: