temperature rise Vs resistance

Cables whether only energized or carrying load current, are a source of heat.

This heat energy causes a temperature rise in the cable that must be kept within limits that have been established through years of experience. The various components of a cable can endure some maximum temperature on a sustained basis with no undue level of deterioration.

There are several sources of heat in a cable, such as losses caused by current flow in the conductor, dielectric loss in the insulation, current in the shielding, sheaths, and armor. Sources external to the cable include induced current in a surrounding conduit, adjacent cables, steam mains, etc.

The heat sources result in a temperature rise in the cable that must flow outward through the various materials that have varying resistance to the flow of that heat. These resistances include the cable insulation, sheaths, jackets, air, conduits, concrete, surrounding soil, and finally to ambient earth.

In order to avoid damage, the temperature rise must not exceed those maximum temperatures that the cable components have demonstrated that they can endure.

It is the careful balancing of temperature rise to the acceptable levels and the ability to dissipate that heat that determines the cable ampacity.

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When electricity flows through a wire the electrons of the wire actually undergo continual displacement in the direction of flow. However, as the temperature rises which it does because of the energy loses required to force the electron displacements, the electrons also begin to undergo much more random motions. These randomized motions introduce addition dissipation of the electromotive energy which shows up as resistance to electricity flow.

This is the most lucid explanation I can provide without going into electron dynamics in metals.

I hope that this is helpful

As I understand it there is another factor.

The lower the temperature in a conductor the closer packed the atoms are. Hence there is less resistance to the transfer of the electron from one atom to the next.

j.

Total Losses = I²R + Other Losses (Die electric, Eddy Cyrrent, Losses associated with Leakage etc)

Total Dissipation = Conduction+Covection+ Radiation from the conductor, which depends on the temp of conductor, its insulator, amient etc,

Temp Rise = (Loss-Dissipation) /Sp heat

At the temp at which the Loss = dissipation is when the system is in the thermal eauillibrium and the insulation is designed to withstand this temp under worst circumstances + factor of safety

If you want a simple explanation which can be used to give a novice a pictorial idea then here goes. All atoms in solids undergo continuous vibration due to thermal energy. Electrons flow in metals by passing between atoms and when a voltage is applied will travel in the direction of the electrical field. However some will interact with the atoms on the way and this is resistance. The higher the temperature the more vibration and the more chance of interactions therefore the resistance increases. At absolute zero these atomic vibrations will cease and resistance will be zero making a superconductor. Further to this when an electron interacts with an atom the atoms vibrations will increase so pushing electrons through metals creates heat which in turn increases resistance and so on. This is how light bulb filaments work in which the temperature escalates till the conductor glows. In semiconductors each interaction between electrons and atoms can actually release loosely bound electrons so that the overall number or flux of electrons increases. This is interpreted as a decrease in resistance. Transistors use this property to amplify small signals into larger current flows.

Think of a small boy walking through a crowd at a football match. He is the electron and the members of the crowd the atoms. His progress is good until a goal is scored and the crowd excitedly jump up and down. Now the boy finds it much harder to walk and gets jostled sometimes going backwards. The resistance has increased. If each person in the crowd also has a son (aloosely boud electron) then when the boy bumps into someone the son breaks free also walking off. This is like a semiconductor. Hope this isn`t too basic but solid high school explanation which will give a picture to focus on. By the way little girls also go to football matches (sorry to all you pc readers).

very good example .Thanks for sharing

Perhaps responders have gotten off track. Maybe the inquirer is only wanting an expaination of temperature coefficient, given certain materials, such as resistors.

The formula for finding the resistance of a conductor at a different temperature is as follows:

R1- resistance at temperature T1

R2 - resistance at temperature T2

a = temperature coefficient of resistance at T1 for a conductor with a resistance R1

T1 - initial of reference temperature

T2 - temperature at which the resistance R2 is desired

example T1 = 25C conductor resistance = 0.102 ohms per 1000 ft a = 0.00385 at 25C for copper. a = 0.00395 for AL

Desired resistance at 90C

R2 = R1(1 + a(T2 - T1))

R2 = 0.102(1 + 0.00385(90 - 25)) = 0.102 x 1.25 = 0.128 ohms

Most answers are not relevent .He has asked for an explanation of rise of resistance with rise in temp

Answers which criticize other answers without improvement are surely the least helpful of all. The question did lack a certain amount of detail and was thus open to interpretation.

Thanks all

What I really needed to know is what will exactly happens to the resistance of the conductor when there is a temperature rise in this conductor. discussion is still open

It need not be. answer

R2 = R1(1 + a(T2 - T1))

I believe that everybody has just about asserted the same view to you:

When the temperature rises, the resistance increases.