Please provide text references that support the above statement.

I believe that, like JimB's statement, this is your incorrect interpretation of ohm's law. However, in case I am wrong, I will look through my text books to see what they say and provide my own references.

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RadioRon

 

Ohm's law states that the ratio of the potential difference between two points of a certain conductor and the current flowing is constant. According to this law the resistance is an intrinsic property of a conductor. Actually no materials obey that law. Some materials approximate the ideal behaviour very closely, such as resistors and wires.

 

My understanding of Ohms Law is that the current is proportional to the voltage. So if we double the voltage, the current will double as a result.
The mathematical relationship between voltage and current (according to Mrs Ohms' little lad) can be represented by the linear relationship
y = m*x
Where y is the voltage, x is the current and m is a constant.
Comparing this with V = R * I, R the resistance is a constant.
Diodes and transistors do not exhibit constant resistance, even when the temperature remains constant, and so do not obey Ohms Law.
Bits of wire and resistors do obey Ohms law if the temperature remains constant.
Lamps do not maintain a constant temperature and so the resistance varies. If the filament temperature were to remain constant, a lamp would obey Ohms law.

JimB

__________________
Experience is directly proportional to the value of the equipment ruined.

 

Please provide text references that support your claim.

__________________
RadioRon

 

You first.

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L.Chung

 

Uh...yeah...but at at the same time...no...that is the definition of Ohm's law. You can't say that [insert definition of Ohm's law] is not Ohm's law, but is instead [insert definition of Ohm's law].

Dude, it's called Ohm's law, Ohm's law, Ohm's law. However, what should be kept in mind is that R is a function of temperature. Just because something's resistance varies with temperature does not mean it does not follow ohm's law. I think what you mean to say is that Ohm's law does not take into account the fact that passing current may increase the temperature of the material, thereby increasing (usually) the resistance. It's still Ohm's law nonetheless. Ohm's law assumes that the current is what it is, and R has been chosen to represent the material at a particular temperature. This will result in one of a few things: V=IR represents an instant in time if the current does cause the material to heat up, or you work on the assumption that the current does not heat up, or you have chosen R to appropriately represent the operating temperature of the material. It's still Ohm's law. If you want to pull non-idealities and the dynamic of heating up, go ahead, but it's still Ohm's law, don't say it's not- say it's a simplification.

What you are saying is kind of like saying F=ma is not a law of Newtonian motion because it is dF= (dm/dt)(dv/dt).

 

Let's agree on the following things first. Any reference (rather than opinion) is better in this case.

1. Does Ohm's Law allows changing resistance with current?

2. Does Ohm's Law allows dropping voltage across a material with increasing current?

__________________
L.Chung

 

There is nothing saying that R in Ohm's law (V=IR) is a constant variable. R can be modified to be R(I,T) to account for ambient temperature and current heating effects. Usually it's not though because it's just easier to guess the operating temperature. Just stop thinking of R as a constant and voila, problems solved.

So the answer would be yes to 1 and 2.

(For 2, you would need R to be the resistance equation of one of the few materials whose resistance decreases with increasing temperature to cause the reducing voltage + increasing current).

As soon as you make R a function though, you are asking for a parametric ass kicking since you gotta know everything about the resistors being used (cross sectonal area, length, volume , resistance-temperature characteristics, specific heat capacity, heat transfer characteristics with ambient blah blah blah).

Refer to University Physics, 11th edition, Young and Freedman, pg. 949-954

 

Ok, here's my first attempt:

-------------------------------------------------------------
“I shall now define, by the following equation, the quantity called the resistance, R, of the conductor BC:

R=V/I eq. 40.3

For some materials (for many different kinds of metallic wires, for example) and under certain conditions (at constant temperature for example) the resistance defined this way is a constant, independent of I. For other kinds of conductors (vacuum tubes, for example) the R defined in this way is not independent of I. In all cases the resistance defined by equation 40.3 is measured in ohms. Obviously, “volts divided by amperes” is equivalent to ohms. Equation 40.3 is known as Ohm’s law after Georg Simon Ohm, a German physicist (1787-1854).”

Ref: “The New College Physics, A Spiral Approach” Albert V Baez, Freeman and Company, San Francisco, 1967, pp512.
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“A German physicist named George Simon Ohm was the first to discover the relationship between current, electromotive force and resistance. The discovery is called Ohm’s law and simply expressed is - that for any circuit or part of a circuit under consideration the current in amperes is equal to the electromotive force in volts divided by the resistance in ohms. This law, mathematically expressed, is as follows:
Current = Electromotive Force/Resistance
If in the above expression we substitute the proper symbols for current, electromotive force and resistance we have the following equation:

I=V/R

This is the equation for Ohm’s Law.”

RadioRon Note: there is no mention of this Law only applying to cases where R is constant.

Ref: “Principles of Electricity applied to Telephone and Telegraph Work” A training course text, American Telephone and Telegraph Company, January, 1953 (revised from the 1938 edition), page 7.

----------------------------------------
“One ohm is defined as the amount of resistance that allows one ampere of current to flow between two points that have a potential difference of one volt. Thus, we get Ohm’s Law, which is:

R=E/I
Where R=resistance in ohms
E= potential or EMF in volts and
I= current in amperes.”

RadioRon Note: no mention that Ohm’s Law is not obeyed when R is variable.

Ref: “The ARRL Handbook” The American Radio Relay League, Newington, CT, 2000, pp 5.2.

----------------------------------------------------------------
“The relationship between electromotive force (voltage), the flow of current (amperes), and the resistance which impedes the flow of current (ohms), is very clearly expressed in a simple but highly valuable law known as Ohm’s Law. This law states that the current in amperes is equal to the voltage in volts divided by the resistance in ohms. Expressed as an equation:
I=E/R”

RadioRon Note: no mention in this text that this law does not hold when R is a function of I

Ref: “Radio Handbook” 22nd Edition, William I. Orr, Howard W Sams Company, Indianapolis, 1981, pp 2.6-2.7

---------------------------------------------------------------------

Now, this reference explains things in more detail and I believe explains why we are arguing.


“Over a hundred years ago, Georg Simon Ohm discovered that every time he closed a switch in a circuit such as that of Figure 4-1, the current became the same constant value. He also discovered that, providing the temperature of the conductor did not change, doubling the emf doubled the current and tripling the emf tripled the current. In other words:
*For a given circuit, the ratio of the applied emf to current is a constant*
This became known as Ohm’s Law of constant proportionality. We can express Ohm’s Law in equation form as

E/I=k eq. 4-1

Where E is the applied emf in volts, I is the resulting current in amperes, and k is a numerical constant.

Carrying this discovery a step further, Ohm found that changing the conductors or the load to some other size resulted in a different value for the numerical constant. If the current is one ampere when the emf is 10 volts, the constant becomes 10/1, or simply 10; and if the current is 0.5 ampere when the emf is 10 volts, the constant becomes 10/0.5, or 20. From these results, Ohm concluded that this constant E/I ratio for given circuit is, therefore, a property of that circuit. And since, for a given emf, the value of the constant increases as the current decreases, this constant can be thought of as representing the opposition of the circuit to the flow of current. This property then should be given a name that suggests opposition, such as resistance. Therefore:
*Resistance is the opposition of an electric circuit to the flow of current through that circuit.
The Letter symbol for resistance is R*

We can substitute the symbol R for the constant k in Equation 4-1. Since resistance is an electrical property, we require a unit of measurement for it. Although it would be quite proper to express resistance in terms of volts per ampere, it was decided to honor Ohm’s original discovery by stating that:

**the ohm is the basic unit of electric resistance. The unit symbol for ohm is the Greek letter omega**

Since Equation 4-1 has now become

R=E/I or R=V/I eq. 4-2

Where R is resistance in ohms, E is the applied emf in volts, V is the voltage drop across the resistance in volts, and I is the resulting current through the resistance in amperes, the size of the ohm is automatically established.
** An electric circuit has a resistance of one ohm when an applied emf of one volt causes a current to flow at the rate of one ampere**

Few people nowadays recall the original wording of Ohm’s law of constant proportionality which allowed us to define resistance and to establish a unit of resistance. As a result, Ohm’s law is usually stated simply by the equation
R=E/I”

Ref: “Introduction to Electric Circuits” Herbert W. Jackson, Prentice-Hall Inc, Englewood Cliffs, New Jersey, 1970, pp 70 to 71.

----------------------------------------------------------
Discussion:

I find the last reference to suggest that some of you may have been taught Ohm’s Law of constant proportionality in its original form, which is now effectively obsoleted by the more modern form R=E/I. It is not appropriate to believe that the original form is the most correct, because at the time Ohm was not aware of conductors whose resistance was dependent on current such as we see in semiconductors. In addition, Jackson teaches that Ohm himself realized after further study that his so-called constant of proportionality could indeed vary depending on the materials used. This lead to the definition of resistance, not the definition of constant resistance, as one of the three variables (a variable is something that can vary) that constitutes what we now call Ohm’s Law.

I also thought it might be instructive to approach this from a logical argument point of view by asking "if a diode does not obey Ohm's Law, what law then describes the resistance of the diode when a specific emf is applied and a resulting specific value of current flows?"


Your turn.

__________________
RadioRon

 

As a counter to RadioRon I offer the following from The Art of Electronics by Horowitz and Hill, Second edition, page 44:

"Before jumpimg into some circuits with diodes, we should point out two things:
(a) A diode doesn't actually have a resistance (it doesn't obey Ohms law).
(b) If you put some diodes in a circuit, it wont have a Thevenin equivalent."

JimB

__________________
Experience is directly proportional to the value of the equipment ruined.

 

That's impressive work towards finding the definition of Ohm's Law.

But, the cream of the cake, which is yet to be presented, is something that is published which clearly stated that Ohm's Law also applies to cases where the resistance is not constant and changes with current. Also it will be a nail in the coffin for all the above argument if you can find any literature which state that diode and active junction obey ohm's law.

JimB has given one on diode and junction from AOE that stated otherwise.

Also, the equation I=V/Z in the case of AC was taught to me as "equivalent of ohm's law" and not ohm's law. One must be careful to note under what situation a law is being discovered and stated. If you extends the scope of the variables, I think it will cease to be called the same law.

__________________
L.Chung

 

It is my belief that Ohm's law applies only to phisical resistances (resistors, wires..). Instead the resistance of a diode (assuming that it is biased in the active region and you're applying a signal) is an incremental (or small-signal) quantity that has the dimensions of resistance.

 

I disagree with (a) ...
If I have a diode and resistor in series across a 10v power supply ...
and if I adjust the resistor to allow the power supply to supply exactly 1mA (minimising any heating effect) ...
if I measure 0.7v dropped across the diode - - - will there not be 9.3v across the resistor ?

If the resistor is dropping 9.3v then its resistance will be R=V/I = 9.3/0.001 = 9300 ohms --- simple Ohm's law

If the diode is dropping 0.7v then its resistance will be R=V/I = 0.7/0.001 = 700 ohms ### IF Ohm's law is correct ###

If the whole series circuit (diode and resistor) has a resistance of 9300+700 = 10000 ohms and passes 1mA then Ohm's law gives ...
V=I*R = 0.001 * 10000 = 10v (the power supply's output)

So why does Ohm's law not apply ?

I agree that this is at one instant in time with controlled conditions (I stated heating - which can usually be ignored in small power circuits) but Ohm's law IS an instantaneous equation - there is no time term in it !

Books like The Art of Electronics always seem to wade far too deep to be real to me - especially as this started out as a simple enquiry about the application of a simple formula.

__________________
I need a memory upgrade ...
My head is full !

 

OK
So you have 1mA flowing through the diode.
What happens when the current is increased to 2mA?
The volt drop across the diode is 0.7volts (plus a little bit), certainly not 1.4volts as would be predicted by Ohms law.

Just to turn this debate around a bit, how many data sheets have you seen which show the "resistance" if a diode and how it varies with current (and temperature)?

JimB

__________________
Experience is directly proportional to the value of the equipment ruined.

 

It's a non-linear equation, so it's easier just to use a graph an I-V graph and specify the forward voltage drop since it changes so little with current. I still wouldn't exactly call it resistance though, but nothing says the R term in V=IR does not have to be a constant. My post was referring specifically to the argument that light bulb filaments do not follow Ohm's law (not diodes) since it heats up a lot and therefore resistance changes a lot during operation (I am arguing that it does and is meant for instaneous points in time before any more heating increases the temperature and changes the resistance of the material).