Basically, it only applies in metals, not in semiconductors, or more appropriate, devices that current and voltage are non-linear. But it is ABSOLUTELY Ohm's Law. Does it apply in every instance, with every material? No. But it is still Ohm's Law, and is ABSOLUTELY correct to refer to that relationship: V=IR, as Ohm's Law.To the Ineffable All,
There seems to be some confusion about what Ohm's law is. The formula V=IR or V=IZ is NOT Ohm's law. It is the resistance or impedance formula. Ohm's law is a property of a material, not a method of calculating current,impedance, or voltage. Read what the physics books say about this.
"We stress that the relationship V=IR is not a statement of Ohm's law. A conductor obeys Ohm's law only if its V--I curve is linear, that is, if R is independent of V and I. The relationship R = V/I remains as the general definition of the resistance of a conductor whether or not the conductor obeys Ohm's law. ..... Ohm's law is a specific property of certain materials and is not a general law of electromagnetism, for example like Gauss's law."
The above snippet is from Physics, by Prof David Halliday, University of Pittsburgh & Prof Robert Resnick,Rensselaer Polytechnic Institute, 1967 , page 780.
And the following.
"Ohm's law states that for many materials (including most metals), the ratio of the current density and electric field is a constant, which is independent of the electric field producing the current.
Materials that obey Ohm's law, and hence demonstrate this linear behavior are said to be ohmic. The electrical behavior of most materials is quite linear for very small changes in the current. Experimentally, one finds that not all materials have this property. Materials that do not obey Ohm's law are said to be nonohmic. Ohm's law is not a fundamental law of nature, but an emperical relationship valid only for certain materials."
The above is from Physics for Scientists and Engineers, Raymond A Serway, James Madison University, Third edition, 1990, page 745.
There you have it. Ohm's law should not be confused with the always correct resistance or impedance formula. It is a property of a material, not a method of calculation. Materials like semiconductors with their bent V--I curves do not obey Ohm's law. Ratch
Basically, it only applies in metals, not in semiconductors, or more appropriate, devices that current and voltage are non-linear. But it is ABSOLUTELY Ohm's Law. Does it apply in every instance, with every material? No. But it is still Ohm's Law, and is ABSOLUTELY correct to refer to that relationship: V=IR, as Ohm's Law.
Are you debating actual engineering concepts, or semantics?
Most any university engineering (electrical) professor (Ph.D) will tell you, and explain that it is in fact Ohm's law, fundamentally, when applied to metals. Once you get into semiconductors, and other materials, with a non-linear relationship, you will in fact start to see that it does not apply, but it is still HIS LAW. It is applied through observation.
"The greater the voltage, the greater the resulting current. For a large class of conductors, the current increases in direct proportion to the voltage. Physical experimentation leads to the following equation: i = v/R, or, v=Ri, which is know as Ohm's Law. "
Source: Foundations of Electrical Engineering J.R. Cogdell
So, is the law wrong when concerning metals, or as it has been applied here in this thread?
Or, are you just debating semantics?
Seriously, no one in this thread is wrong in stating that those equations are Ohm's Law..... An Ohm, is in fact, a Volt/Ampere.
Pardon?Chaerl said:The voltage across the resistor is directly proportional to its resistance. True. so you need a constant. In the case of ohm's law, V=IR, I is the constant. But in reality, is it constant? Even the ideal resistor has a slight non-linearity. That is why there is a tolerance. I hope this clears up things.
Ohm's law does not have anything to do with temperature though. You would use the resistance of the material at a specific temperature, but resistance is still a single discreet variable in the equation. The equation itself does not change, just the value of one of it's variables which is dependent on another external variable.Ohm's law cannot be directly applied to semiconductors due to the fact that the resistance of a semiconductor can be affected two different ways when temperature is applied to the semiconductor material. Intrinsic semiconductors have less resistance as the temperature increases, while extrinsic semiconductors have the opposite effect, and their resistance increases as the temperature increases.
Huh! It's a law. Breaking a law is against the law.hi sceadwian,
Semi-conductors are not conductors which do not conduct very well.
I know that I am slightly misquoting you. Sorry, just trying to stress the point.
Semiconductors have a totally different characteristic to a resistor.
Ohms law dosn't apply.
I admit that I haven't studied physics for a while, but it was my understanding that Hooke's law states that the extension of an object (such as a steel spring, a piece of rubber, etc.) was directly proportional to the force applied to it, and that materials for which Hooke's law applies are termed Hookean materials; rubber does not demonstrate a linear distance of extension when increasing force is applied. If you know better, then I bow before your knowledge Please shareSince when?