Transfer of Thermal Energy?

Question

I am wondering...Why is it more preferable for a household electric tea kettle to have a polished body? (Answer is supposed to be in terms of thermal energy).

Thanks!

Also, I don't understand the concept in this question:

A helium balloon is to be used for setting an altitude record. Why is it only partially inflated as it leaves the ground?

Answer 1

The oxidation of the metal on the tea kettle has a different termal conductivity than the unoxidized metal. It's also possible that the shiny surface helps insulate the kettle by reflecting radiation due to heat back into the kettle, but that would only apply to the interior of the tea kettle.

As for the balloon. As a hot air balloon rises in the atmosphere the pressure drops. From the ideal gas law (PV=nRT) if the pressure drops the volume will expand (everything else being equal). In other words the pressure pushing around the balloon on the ground is more than the pressure in the air, so the balloon is less inflated on the ground.