On 2007-04-25 09:33:53 +0100, Grunff said:

Andy Hall wrote:

But was that the material or the power rating? It seems a bit
surprising that stainless steel can radiate/convect *that* much heat to
make much of a difference with a 2-3kW heat source.

This puzzled me at the time. The stainless kettle was a Morphy Richards
3.1kW, which replaced a (much faster) 3kW Kenwood.

To start with I thought it might be thermal mass, but raising 300 grams
of stainless steel from 20C to 80C takes about 9kJ, which the element
can do in 3 seconds, so that's not the issue. I therefore assumed that
it must be convective losses from the surface.

I wonder if one could work out the convection and radiation losses for both.

The energy equation is pretty simple. We know that there is not an
issue of the heat being transferred into the water since it has nowhere
else to go. Therefore if one takes a given amount of water in both
cases and heats it from temperature T to 100 then the energy used in
the time difference would account for the difference in loss.

It's not possible that there was less water in the plastic kettle?
These tend to be jugs whereas the metal ones are usually more
traditional kettle shaped. Hmm.... thinks difference in surface
area of short squat cylinder vs. tall one?

As for the toaster, I think it's harder to work out. Here the heat
transferred to the bread would depend on what's behind the element
(insulation and reflection of some kind?) and proximity of element to
bread.