What exactly is a resistance?

[Eletronpower]

What does a resistance physically represent? Is it necessary to be a conductor only? Can it not be a semiconductor?
I surfed the internet and in many places found the definition of a resistance as something following ohms law. Does that mean that things that don't follow Ohms Law cannot be termed resistance? Don't we define resistance for diodes and transistors, but they don't follow the Ohms Law.
So, what exactly is a resistance, is it just something that to some extent opposes current flow or something that not only opposes current flow but also follows Ohm's Law?


Thank you in advance!

 

[audioguru]

A resistor does not "oppose" current flow, instead it reduces the amount of current flow. It is linear according to Ohm's Law: 10mA of current in 1oo ohms produces a voltage drop of 1.0V, 20mA produces 2.0V, 30mA produces 3.0V etc.
The resistance of a forward-biased semiconductor is very non-linear: 10mA produces a voltage drop of 0.62V, 20mA produces 0.67V, 30mA produces 0.70V, 100mA produces 0.8V, 1A produces 1.0V etc.

 

[Eletronpower]

Yes I understand. So what exactly is the definition of a resistance? Is it necessary for a resistance to follow Ohms Law?

 

[MikeMl]

Pretty good definition

In my mind, a "Resistor" obeys Ohm's Law
"Resistance" may just be dv/di at a local operating point on some curve...

 

[KeepItSimpleStupid]

The strict definition is the ratio of voltage to current, so there can be a resistance as a function of I or t or other variables.

Resistive is when that relationship is a constant.

A resistor follows the relationship of R=pL/A where p is rho or the bulk resistivity of a material; L is the length and A is the cross-sectional area.

p has units of ohms-length e.g. ohm-cm. R has units of Volts/Amp or Ohms.

Rds(on) has units of ohms and can be treated as a resistor sometimes, but it isn't.

Some parasitics are resitive. e.g. as a result of leakage currents.

So while it isn't a physical part, it allows the unerstanding of a circuit model.

 

[Eletronpower]

But the case of diodes and transistors, the resistance does not follow Ohms Law, but still it is called resistance. But here, the definition of resistance is not satisfied.

 

[DerStrom8]

But the case of diodes and transistors, the resistance does not follow Ohms Law, but still it is called resistance. But here, the definition of resistance is not satisfied.

You are wrong there. The resistance DOES follow Ohm's law. It's just in diodes and transistors the effect of the resistance is (usually) negligible, so it is ignored. There are some cases in which this resistance is critical, like operating MOSFETs in the linear region, in which case it still follows Ohm's law.

 

[crutschow]

Physically the resistance is determined by how freely the electrons can move in a material and how difficult it is for them to move between the atoms in the material. It varies from vary easy in a good conductor, such as copper, to virtually impossible in a good insulator, such as glass. A resistance does not have to obey Ohm's law which typically defines a linear resistance (resistance constant over a range of voltage). So a resistance can be non-linear, such as in semiconductors, and still be resistance, since it impedes current flow . The incremental resistance of a non-linear resistance can be defined as the first derivative (dv/di) of Ohm's law at a particular point on the voltage/current curve as Mike noted.

Note that the definition of resistance is properly applied to an impedance that is independent of frequency and dissipates power. (I make that distinction because certain impedances, such as a transmission line, can have an impedance independent of frequency but dissipate no theoretical power). Reactive elements such as inductors and capacitors can also reduce the flow of current, but that impedance is frequency dependent and is called reactance. In general any combination of resistance and reactance is called impedance.