2
\$\begingroup\$

This is a two-part question I haven't been able to get off my mind:

Given a resistor in a vacuum chamber (to ideally eliminate heat loss due to air), assume you apply a constant voltage across it. The temperature of the resistor will rise - now, assuming heat lost to radiation and conduction to the wires is negligible, will the temperature of the resistor rise indefinitely? Or will the temperature max out at a certain value (and is there a way to calculate this value assuming you know the value of every property of the resistor and the voltage source)?

Finally, if there is a value the temperature will max out at, how consistent a temperature would this be if, for example, it maxes out at about 330 K?

\$\endgroup\$
1
  • 1
    \$\begingroup\$ It is an error to assume that heat loss due to radiation is negligible. It isn't even negligible in air -- which is why heat sinks and often engine fins are painted black. It certainly isn't negligible in a vacuum. \$\endgroup\$
    – TimWescott
    Jun 3, 2020 at 18:11

2 Answers 2

Reset to default
2
\$\begingroup\$

If there is no heat loss (which is obviously not the case), the resistor will increase in temperature indefinitely, presumably until it melts.

In reality the temperature will increase until the sum of the heat loss through convection, conduction and radiation equals the input power.

\$\endgroup\$
4
  • \$\begingroup\$ To a first approximation, for a given setup the resistor temperature will rise a certain amount above ambient (even if "ambient" is defined as the temperature of the walls of the vacuum chamber). \$\endgroup\$
    – TimWescott
    Jun 3, 2020 at 18:10
  • \$\begingroup\$ @TimWescott You could measure the temperature of the resistor and heat the walls of the chamber to match, thermally bootstrapping it so that radiative cooling was negligible. \$\endgroup\$
    – Spehro Pefhany
    Jun 3, 2020 at 18:27
  • \$\begingroup\$ That would work. I was thinking of highly polished Dewar flasks -- that's kind of the opposite. \$\endgroup\$
    – TimWescott
    Jun 3, 2020 at 20:17
  • 1
    \$\begingroup\$ Well, opposite way to get to the same end. \$\endgroup\$
    – TimWescott
    Jun 3, 2020 at 20:18
0
\$\begingroup\$

The thermal capacity of clay/ceramic/silicon is 1.6 picoJoules per cubic micron per degree K.

A 1 millimeter cube resistor has 1,000 micron^3 volume, thus has heat capacity of 1.6 milliJoules per degree K.

Shining the photons of a 1.6 milliJoule IR laser onto the resistor/ceramic block will raise the temperature 1 degree per second.

So when does the system turn to plasma vapor?

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.