# The Ohm - Intro # Rounding Out Ohm's Law # Resistivity - The Nervous Breakdown Whatever this is I'm sure it's best expressed as a minecraft enchantment - Bane of Physics Student 5. In short, the *resistivity* of an object refers to a constant 'resistance' per unit area. Now that we know this, let's express this constant in terms of the resistance: $R = \frac{\rho \ell}{A}$ **Where:** - $\ell$ is the *length* of the object - $A$ is the *surface area* of the object - $\rho$ is the *resistivity* From [[Current (ElectroMagnetism)|currents]] we've talked about a constant known as the *current density* - the current flowing through every little crack in the object. #