Any radio waves in spectrum of heat radiation? [duplicate]

Question

I'll like to know if heat radiation may also involve radio wave frequencies. If a battery is connected to a resistor, does the radiation from the resistor also have frequencies in the radio wave ranges?

Answer 1

Checking out this link to Wikipedia, you see that (if you approximate) the resistor as a black body, then "Black-body radiation has a characteristic, continuous frequency spectrum that depends only on the body's temperature, called the Planck spectrum or Planck's law" given as: $$B_f(T) = \frac{Cf^3 }{\exp(Df/T) - 1}$$ where $C, D$ are constants and $B_f(T)$ is (roughly speaking) "the amount of radiation emitted by the body at the frequency $f$ when its (absolute) temperature is $T$". The shape of this distributions is shown in a figure in the link, with the curve called the "classical result" is an approximation and what was thought to be correct before Planck derived the correct law). The $x-$axis is labelled "wavelength" and the relationship between wavelength $\lambda$ and frequency $f$ is $\lambda f = c$, where $c$ is the speed of light. From the figure, you can see a continuous distribution over the whole electromagnetic spectrum. The peak value of emission is given by Wien's Displacement law (also mentioned in the article) and for the resistor would be probably be in the (far)infra-red region of the spectrum, far from the frequency range of radio waves.

Answer 2

does the radiation from the resistor also have frequencies in the radio wave range?

When you remember how a radio wave is produced you can recognize wether it is possible or not to produce radio waves with a resistor. For the sake of simplicity I’ll talk about flat resistors (not coils). Connect one pole of your electric source to one end of the resistor and the second pole to earth and switch your source on and of. Pushing by this way electrons forth and back the resistor you get a periodical lost of energy with the frequency of your switch (or the alternating current frequency). The efficiency of our radio generator and the radiated power depends from the length of your resistor, the resistance and the frequency of the source.

If a battery is connected to a resistor

you get a half radio wave and nothing more. What you get - if you connect both ends of a resistor to a battery - is electromagnetic radiation. The flow of electrons through the resistor is accompanied by permanent collisions and by this with permanent accelerations. And as you perhaps know any acceleration of electrons is accompanied with the emission of photons respectively EM radiation. This EM radiation is of very different wavelengths (EM radiation from thermic sources) in contrast to radio waves which are produced by a radio generator.