Ohm’s law is one of the first equations taught to electronics students, showing the voltage-current relationship in a resistor.
It can be derived from Maxwell’s equations and holds true for linear homogenous materials. This typically means pure metals.
Any device designed to meet Ohm’s law is a called a resistor.
Really, Ohm’s law doesn’t need any basis in physics because it’s just a defined as a ratio of voltage to current.
\(R = V/I\)Nothing is 100% Ohmic because the equation is limited by the construction of devices and material properties.
This includes:
-materials heat up as they dissipate power, which changes their resistance
-at some temperature a material will melt or otherwise change, destroying the device
-at high enough voltages materials will break down and will spontaneously conduct
-dissimilar metals generate small voltages based on the thermoelectric effect
-chemical residues can generate voltages from galvanic action
-mechanical strain creates voltage in piezoelectric materials
-at extremely low voltage the intrinsic noise of a device will dominate the intended signal
-certain current mechanisms operate with fundamentally different physics, including ionic fluids or charged particle streams (electron beams, particle accelerators)
The ratio interpretation allows the intuition of Ohm’s law to be used as a conceptual tool with many more applications that just resistors.
All devices have an intrinsic voltage-current relationship which can be linear, non-linear or even be a multi-valued function.
At any point one can define a differential or small-signal resistance:
This idea is used with surprising frequency even if rarely discussed. Incandescent lightbulbs have such a strong temperature dependency in normal use that one needs to discuss “cold” and “hot” resistances.
Constant power loads (like switching DC-DC regulators) have a negative differential input resistance which is important to their application.
Perhaps most importantly, the majority of active devices are characterized for input and output resistance despite the behaviour being created by complex, non-linear device physics.
It is this non-linear that forces any component datasheet to specify in detail the operating point, including temperature and voltage ranges.
Understanding that a resistance is very often more than just Ohm’s law is a key point in understanding electronic devices.