conduction, convection currents, radiation

[thingsfallapart]

Hey - do you think anyone could help explaining the process of conduction and radiation, convection currents for me? Physics exam next Monday, and I have to start studying soon!

Also, what is density measured in? Kg/M^2?

Thanks in advance!

[TALewis]

Density is mass per unit volume. In the SI system, it's kg/m^3.

[thingsfallapart]

bump bump.. could someone answer the convection current/conduction/radiation question?

[HallsofIvy]

"convection" is heat transferred by an air (or water) current: the air heats up, moves to another place, heats up that place.

"conduction" is heat moving from one solid object to another: a hot piece of metal is place against a cold piece of metal and heat moves from one to the other.

"radiation" is heat moving from one place to another as electro-magnetic (mainly infra-red) radiation.

[Gokul43201]

Radiation is basically like light. Light carries energy, and it does not require a medium for transportation (light can travel through vacuum). When the light falls on some object, it transfers its energy (heat) to the object.

Here's an example to explain convection. Imagine a pan of water on a hotplate. The water is getting heated from below. The lower parts of the water get hot because they are touching the hot bottom of the pan (conduction coming next). The water near the top is relatively cooler. Since hot things are lighter and colder things are denser, the hot water rises to the top and the cold water sinks to the bottom. So, this way, the heat has been carried from the bottom to the top. Now again the water at the bottom gets hotter, while the water near the surface cools off, and the cycle repeats itself continuously.

Conduction, is transfer of heat by the atoms of a solid or liquid. When something is hot, its atoms are vibrating furiously. In a solid, all the atoms are "connected" to each other by spring-like forces. So, when one atom starts jiggling, so will its neighbours - just like a wave travelling down a slinky toy. How fast the jiggling (heat) get transfered depends on how strongly connected the atoms are. We know that atoms in a metal (like copper or silver) are more tightly bonded than the atoms in plastic or rubber or sand or glass. That's why metals are better conductors of heat. Also the bonding in liquids is extremely weak (there's almost no bonding at all) that's why liquids depend on covection, rather than conduction, to transfer heat.