Thermal physics

1 The greater density of solids compared to liquids and gases means that convection cannot occur.
2 Convection currents are reliant on changes in density, which occur as molecules move from hotter to cooler regions.
3 Conduction relies upon the collision and interaction of electrons between neighbouring molecules, in order to drive transfer of energy.

Which of the following statements regarding thermal physics is/are correct?

A Only 1
B Only 2
C Only 3
D 1 and 2
E 2 and 3

Hi the answer is B only 2, but could someone explain why statement 1 is incorrect?

Convection (options 1 & 2) is transfer of heat from hotter particles that decrease in density to cooler and denser particles nearby.

Conduction (option 3) is electrons moving from one atom to another and not collisions. That’s my understanding, anyway.


Hey but then how can convection occur in solids?

Convection can’t happen in solids because the particles can’t move in solids.

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“1 The greater density of solids compared to liquids and gases means that convection cannot occur.”

Shouldn’t this one be correct then? I want to know what makes it incorrect since the answer is only 2

I think option 1 should be correct too.

Maybe its the wording? I agree with you, but maybe its because the reason it states? The density of the solid is irrelevant, more the fact that it is a solid.

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Maybe option 1 is incorrect because it’s not the difference in density between solids and liquids/gases that prevent convection in solids–it’s the inability of particles to move in solids.

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Ahh ok that makes more sense now. Thank you all, you guys!

The conventional definition of convection is movement of heat in fluids or gasses. In solids, it is called conduction, because molecules/atoms in solids cannot move freely either due to ionic bonds or strong intramolecular covalent bonds. They move, but their movement is called vibrations. Vibration is not a free movement like in liquids or gasses. It is like a spring on a table surface (Stokes’ Law), having a limited oscillatory dislocation.

The density factor plays a role in solids conduction, but has nothing to do with convection, as convection does not happen in solids. :blush: