Why Ohm's Law is NOT V=IR - there's more to it! Resistance must be constant

[riyadh144]

I just found this video and it is amazing... It is nice to go back to basics once in a while..

[JohnnyMalaria]

Strictly, Ohm described his linear relationship for metal conductors, so applying it to semiconductors is inconsistent with his original work anyway. And Ohm never claimed V = IR.

[themadhippy]

The other often mis quoted law ohm came up with is P=IV,many forget to mention the add on,  only for DC,and get confused when the measured current on there  1HP  motor  doesn't appear to be playing by the rules

[WattsThat]

A proper title for the video would have been “why ohms law doesn’t apply to semiconductors”.

As is, it borders on dishonest and just trolling for view counts. This is one of the many things that makes YouTube less useful everyday.

[TimFox]

Yes, Ohm's law is an approximation that is reasonably accurate for many materials (such as the wire in wire-wound resistors), but not a fundamental law of electrodynamics.  For two-wire components listed as "resistors" in catalogs, the deviation from the proportional relationship between V and I is called the "voltage co-efficient of resistance", which is rarely specified on data sheets.  I have found substantial deviations in high-resistance, short-length components such as 0805-size 50 megohm parts at 10 V.  The high-resistivity materials needed to obtain that resistance, with the high voltage gradient (V/mm) along the path suffer from the non-linear behavior of the actual material.
In physics, in the linear limit of small voltage gradients, the law is stated as
J = (sigma) E
where the vector J is the current density (A/m²), the vector E is the electric field or voltage gradient (V/m), and (sigma) is the conductivity tensor which may be interesting for an anisotropic medium (such as crystalline graphite).

[riyadh144]

Well Ohms law isn't even applicable to real life resistors since they change due to temperature. Also inductors and capacitors are better described by the law then.

[SilverSolder]

Strictly speaking, Ohm's law still holds even if R is a function of some other quantity....   temperature, time, voltage, whatever....  you just have to do more work to solve the equation?

[TimFox]

For a complicated physical situation with only two terminals, you can write an equation for the current  as a function of the voltage, temperature, etc.  At a given current or voltage, etc., you can then express the resistance as either the ratio of the voltage divided by current, or the partial derivative of the voltage with respect to the current.  If reactance is important (not uncommon in actual components), you have to include its effects separately.  Again, the proportional law between voltage in current is often a very useful approximation, but not strictly exact.

[vwestlife]

I was taught E = I * R.  E for electromotive force. If you're going to change it to V for voltage to make it easier to remember, then why not also change I to A for amperage?

[TimFox]

Contemporary American physics textbooks use V for electric potential (scalar) and E for electric field (vector).  Since "EMF" is not a force, and does not have the dimensions of {mass} x {acceleration}, the preferred term is "electromotance" for the line integral of the electric field around a loop, but the abbreviation "EMF" or "emf" is often used instead.
Since engineers reserve i or I for electric current (scalar), they gave us j for the unit imaginary number.
The late professor U Fano at the University of Chicago was once giving a lecture on the quantum-mechanical calculation of the dielectric properties of matter.  He performed a perturbation calculation for the polarization, defining the Hamiltonian (energy) as a function of the applied field, and then decomposed the applied E field into an integral over frequency of E(w) e^iwt dw, in order to obtain the polarization as a function of frequency. (Writing "w" for omega.)  A theory-weenie in the front row objected, since the Hamiltonian was not Hermitian (i.e., the energy was not real-valued), so he fixed the equation by substituting "j" for "i" and declaring the Hamiltonian to now be Hermitian.