question about ohm's law

[Chengjun Li]

Hi,

About Ohm's law, wikipedia has the following statement
“Materials and components that obey Ohm's law are described as "ohmic" which means they produce the same value for resistance (R = V/I) regardless of the value of V or I which is applied and whether the applied voltage or current is DC (direct current) of either positive or negative polarity or AC”

We also know that resistance of a resistor is a function of temperature. For example, Jameco 100 Ohm resistor has a temperature coefficient of 350ppm.

My question is at higher voltage, the heat generated is larger due to P=V2/R, so the partial temperature is higher, then the resistance should change to smaller value, which is conflict with above statement of Ohm's law. Could anyone give me some clue of this problem?

Thanks.

 

[ramussons]

Hi,

About ohm's law, wikipedia has the following statement
“Materials and components that obey Ohm's law are described as "ohmic" which means they produce the same value for resistance (R = V/I) regardless of the value of V or I which is applied and whether the applied voltage or current is DC (direct current) of either positive or negative polarity or AC”

We also know that resistance of a resistor is a function of temperature. For example, Jameco 100ohm resistor has a temperature coefficient of 350ppm.

My question is at higher voltage, the heat generated is larger due to P=V2/R, so the partial temperature is higher, then the resistance should change to smaller value, which is conflict with above statement of ohm's law. Could anyone give me some clue of this problem?

Thanks.

The blue statement is valid on a condition that all other parameters are constant - temperature, pressure, frequency, ... The only variables are the Voltage / Current.

 

[alex Chiu]

Ohm Law still value.

P=VI, =
If R = Constant, Higher V, higher I, however, when consider heat, R reduce, I reduce,

 

[davenn]

Great response Ramussons .... spot on


Dave

 

[Colin Mitchell]

then the resistance should change to smaller value

should be

then the resistance should change to higher value

 

[Ratch]

Hi,

About Ohm's law, wikipedia has the following statement
“Materials and components that obey Ohm's law are described as "ohmic" which means they produce the same value for resistance (R = V/I) regardless of the value of V or I which is applied and whether the applied voltage or current is DC (direct current) of either positive or negative polarity or AC”

We also know that resistance of a resistor is a function of temperature. For example, Jameco 100 Ohm resistor has a temperature coefficient of 350ppm.

My question is at higher voltage, the heat generated is larger due to P=V2/R, so the partial temperature is higher, then the resistance should change to smaller value, which is conflict with above statement of Ohm's law. Could anyone give me some clue of this problem?

Thanks.

You should read this link http://www.electro-tech-online.com/threads/ohms-law.430/page-2 starting with my contribution on post #22. Then we can discuss it further.

Ratch