Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Phil Kenyon on 12/01/2009 08:54:16

Title: How long does it take for heat to reach us from the Sun?
Post by: Phil Kenyon on 12/01/2009 08:54:16: Phil Kenyon asked the Naked Scientists:

Hi Chris,
Â
IÂ enjoy your sectionÂ in the Redi Direko show on Cape Talk and wondered if you could discuss the following?

I was thinking about what would happen if the sun suddenly 'went-out'!
Â
Would we have 8.5 minutes of sunlight and 14 years of heat?

299792 km/s - Speed of light

152000000 km to sun

507.02 seconds for sunlight to arrive

8.45 minutes for sunlight to arrive

0.34 km/s speed of sound

447058824 seconds for sound to arrive

Therefore 14 years for sound to arrive

Speed of sound = speed of heat

Therefore it takes 14 years for heat to arrive?

Regards,

Phil Kenyon
(Somerset West)

What do you think?
Title: How long does it take for heat to reach us from the Sun?
Post by: Chemistry4me on 12/01/2009 09:55:42: Quote from: Phil Kenyon on 12/01/2009 08:54:16
Speed of sound = speed of heat
Why? [???] [???]
Title: How long does it take for heat to reach us from the Sun?
Post by: lyner on 12/01/2009 10:41:45: Sound doesn't travel through a vacuum so 'heat' wouldn't get to us that way.
The 'heat' transfer you refer to is what can happen in gases liquids and solids - molecules vibrating and bumping into each other. The speed of sound in the medium affects the speed of heat conduction but:
The heat which arrives from the Sun is in the form of Infra red radiation (another wavelentgth of electomagnetic wave). (Same speed as light).
Title: How long does it take for heat to reach us from the Sun?
Post by: yor_on on 12/01/2009 10:51:40: As the sun flickers out there will be no more photons coming our way.
When those photons stop our 'light' goes out.
That will create a rapid cooling.

The only heat will be from our Earth's core.
That will not be enough to warm the surface and the air.
All the water falls out from the atmosphere.
And I suppose the air will break down into a supercooled gas?

very few organisms will survive this.
Perhaps inside the earth?
How fast Earth will cool depends on how much warmth it has before.
And how fast that heat will dissipate.
I would guess the planet would be dead in a couple of days.

As for heat in air?
You lost me there.
Title: How long does it take for heat to reach us from the Sun?
Post by: lightarrow on 12/01/2009 12:21:08: Quote from: sophiecentaur on 12/01/2009 10:41:45
The heat which arrives from the Sun is in the form of Infra red radiation (another wavelentgth of electomagnetic wave). (Same speed as light).
Sorry for being nitpicking, heat is in the form of all the electromagnetic radiation.
Title: How long does it take for heat to reach us from the Sun?
Post by: lyner on 12/01/2009 21:24:18: Feel free to pick all you like, my friend. I was being deliberately sloppy in an attempt to seem more approachable. I shall avoid it in future. :-)

But I could nitpick back and say that em radiation is not heat (i.e. internal energy) at all but electric and magnetic energy (or even the dreaded photons). Heat is a term which is used very sloppily.
Title: How long does it take for heat to reach us from the Sun?
Post by: lightarrow on 12/01/2009 23:23:53: Quote from: sophiecentaur on 12/01/2009 21:24:18
Feel free to pick all you like, my friend. I was being deliberately sloppy in an attempt to seem more approachable. I shall avoid it in future. :-)

But I could nitpick back and say that em radiation is not heat (i.e. internal energy) at all but electric and magnetic energy (or even the dreaded photons). Heat is a term which is used very sloppily.
Hmmm...yes, but, wouldn't you define heat as those energy exchanged between two bodies by virtue of a temperature difference of them? Just thinking.
Title: How long does it take for heat to reach us from the Sun?
Post by: yor_on on 13/01/2009 10:22:59: How about this definition of what would happen to our Earth if it cooled down by an absence of Sun :)

"The atmosphere on earth is about 10 km thick. Ten kilometer is how far atoms at room temperature can move against the gravitational force. If the temperature of the air were ten times smaller (which is about 30 K or -240 degrees Celsius), the atmosphere would be only 1 km thick. At 30 microkelvin, the atmosphere would shrink to a mere millimeter, and at 30 nanokelvin, the height of the atmosphere would be 1 micrometer, hundred times less than the thickness of the human hair. (Of course, air is not an ideal gas and would have liquified by then). "

"Freshly liquefied air consists of 78.1% nitrogen, 21.0% oxygen, 0.9% argon, and very small amounts of rare gases and hydrogen in solution. Its boiling point is approximately −195Â°C. Because of fractional evaporation, its oxygen concentration and its boiling point increase with time. ..As the temperature of liquid air rises, the nitrogen evaporates first at −195.8Â°C, the argon next at −185.7Â°C, and the oxygen last at −183Â°C."

Seems chilly.
the solarsystem seems to have a temperature around 40-50 K (-233 - -223 C.)
But I still don't know how long time it would take?
Black body radiation?
Title: How long does it take for heat to reach us from the Sun?
Post by: thedoc on 24/02/2009 16:42:24: Listen to the answer to this question on our podcast. (http://www.thenakedscientists.com/HTML/podcasts/show/2009.02.22/)
Title: None
Post by: mudder fukker on 22/02/2010 16:45:31: you suck my coch or die