09-25-2011, 10:58 PM

Ok, I had a long drive yesterday, during which I tried to re-explain a couple of issues that I think are still "outstanding"...

W/m2 instantly sprang to my mind, the below is hopefully a clear explanation of my "issue" with the use of W/m2.

What is missing from a W/m2. - The amount of energy transferred.

Yes, a Watt = Joule per second, but no a W/m2 does not NECESSARILY, or even usually, equal a Joule per second, why?

Amount.

W/m2 is power, like voltage is, but volts are no use without the amount of current, or rather amps.

I can say I have a wire carrying 6 volts, but what is the current???? How much work will the 6 volt wire be able to do?

It is the same with W/m2, it is a power figure.

240W/m2 means as much as 6 volts does.

That is THE POINT, that is why you can not do grey body calculations using W/m2.

The amount being transferred at that power is never given.

It is also the reason why W/m2 is so often compared and used incorrectly, you are assuming the same "size" of emitter and receiver.

When, for example, conduction of a receiver is included to it's volume, W/m2 fail for this reason.

For example,

How hot should the earth's surface be for a measured solar input?

S&B law is too simply applied, it is different for grey bodies, ie, the real world..

Amount as well as power needs to be taken into account,

as well as relative receiver "size" and most notably conduction.

W/m2 instantly sprang to my mind, the below is hopefully a clear explanation of my "issue" with the use of W/m2.

What is missing from a W/m2. - The amount of energy transferred.

Yes, a Watt = Joule per second, but no a W/m2 does not NECESSARILY, or even usually, equal a Joule per second, why?

Amount.

W/m2 is power, like voltage is, but volts are no use without the amount of current, or rather amps.

I can say I have a wire carrying 6 volts, but what is the current???? How much work will the 6 volt wire be able to do?

It is the same with W/m2, it is a power figure.

240W/m2 means as much as 6 volts does.

That is THE POINT, that is why you can not do grey body calculations using W/m2.

The amount being transferred at that power is never given.

It is also the reason why W/m2 is so often compared and used incorrectly, you are assuming the same "size" of emitter and receiver.

When, for example, conduction of a receiver is included to it's volume, W/m2 fail for this reason.

For example,

How hot should the earth's surface be for a measured solar input?

S&B law is too simply applied, it is different for grey bodies, ie, the real world..

Amount as well as power needs to be taken into account,

as well as relative receiver "size" and most notably conduction.

The whole aim of practical politics is to keep the populace alarmed

(and hence clamorous to be led to safety)

by menacing it with an endless series of hobgoblins, all of them imaginary.

H. L. Mencken.

The hobgoblins have to be imaginary so that

"they" can offer their solutions, not THE solutions.

(and hence clamorous to be led to safety)

by menacing it with an endless series of hobgoblins, all of them imaginary.

H. L. Mencken.

The hobgoblins have to be imaginary so that

"they" can offer their solutions, not THE solutions.