# How fast do you have to heat air to create a shock wave?

## Main Question or Discussion Point

Suppose you have a 1cm diameter heat source suspended 2m above the ground. How many watts of heat energy does it need to expend to create a shock wave?

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cmb
It'd depend on the function of thermal transfer between the heat source and the air. I suspect this becomes very non-linear if you get to the rate of heat transfer you're talking about.

I'd also suspect that in the real world, any known material 1cm would not withstand the thermal resistance in that scenario, and, itself, ablate and explode.

Therefore, as it is a result likely only possible by experimentation, and as there are no materials to perform that experiment, I'd hazard to say the question is moot.

Ok, well since we're not imaginative enough to conceive of an object or material that can release that amount of heat without exploding then lets just consider explosions:

Do all explosions create shockwaves?

cmb
Yes. Why would you imagine an explosion not causing a shock wave? Are you thinking of 'super-sonic' shock waves?

If an explosive causes a supersonic shockwave, it is called a 'high explosive'. If the shockwave is slower than local sound, it generates a wave more like a solition, though I am unclear the difference between a solition and a sub-sonic shock wave.

I did not know exactly what a shock wave is so I checked here:

http://en.wikipedia.org/wiki/Shock_waves

I still don't know, but it's interesting reading.....

it says: "...A shock wave ....is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium (solid, liquid, gas or plasma) or in some cases in the absence of a material medium, through a field such as the electromagnetic field.

Shock waves are characterized by an abrupt, nearly discontinuous change in the characteristics of the medium.[1] Across a shock there is always an extremely rapid rise in pressure, temperature and density of the flow..."

These two statements, for example, appear inconsistent.

Can we get a shock wave in outer space...in a vacuum...sure, see the latter part of the article......how does that fit (or not) in these descriptions??