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Offline jbvaliquette

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Need help
« on: September 24, 2007, 07:36:56 PM »
if the specific heat of water is 4.184 J/g degrees celcius how many photons of blue light (410nm) are required to raise the temperature of a 5 gallon bird bath 3 degess celcius assuming complete transfer of energy to the water and no influence from the bird bath container.

i just dont know the steps and how the gallons and degress effect it. 

Offline Padfoot

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Re: Need help
« Reply #1 on: September 24, 2007, 07:50:12 PM »
Changing the font colour does not make it a different post...
But I suppose I'll still help:
Use specific heat, change in temp and mass to calculate energy needed (if this doesn't help review what specific heat is).
Energy of photon=hf=h*c/(410*10-9) for photons of the blue light.  How many photons are therefore required?

Offline jbvaliquette

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Re: Need help
« Reply #2 on: September 24, 2007, 10:02:58 PM »
i still do not understand how the gallons or degrees will effect the answer need step by step info in a plug and chug format if not willing to give answer.

Offline jbvaliquette

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Re: Need help
« Reply #3 on: September 24, 2007, 10:17:45 PM »
got answers of .000723 and 4.84829*10^-37 not sure on my answers any help

Offline Padfoot

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Re: Need help
« Reply #4 on: September 24, 2007, 10:54:55 PM »
No, neither are right, you should only have 1 answer as well. The number of photons should definitely be greater than 1.

What did you do?

Use E=m*C*delta T to find the Energy that is needed to raise the 5 gallons of water by 3 degrees.
(For m, convert 5 gallons to grams)
Number of photons needed= this value/energy of the photons (see my last post)

I think confusion lies with Specific Heat...
C=specific heat=4.184.  This means that it takes 4.184J of energy to raise the temp of 1g of water 1 degree.  For every gram of water you need 4.184J to raise it's temp by 1 degree, (4.184*2)J to raise its temp by 2 degrees and so on. 

Offline jbvaliquette

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Re: Need help
« Reply #5 on: September 24, 2007, 11:10:01 PM »
how do you convert gallons to grams when one is a volume and the other is a mass? and what is delta T?

Offline jbvaliquette

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Re: Need help
« Reply #6 on: September 24, 2007, 11:33:16 PM »
so 5 gallons equal 18143.6948 grams but what is delta t

Offline Padfoot

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Re: Need help
« Reply #7 on: September 24, 2007, 11:40:06 PM »
delta t is change in temperature (delta means change)
So in your problem, this is 3.

Offline jbvaliquette

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Re: Need help
« Reply #8 on: September 24, 2007, 11:43:01 PM »
so 18143.6948*4.184*3=227740 is that the answer doesnt seem right

Offline jbvaliquette

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Re: Need help
« Reply #9 on: September 24, 2007, 11:59:54 PM »
or is the answer 3.8283*10^-10 if so what in what unit

Offline Padfoot

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Re: Need help
« Reply #10 on: September 25, 2007, 12:33:18 AM »
Your second answer is wrong - it is much to low.
Assuming your conversion from gallons is right (I don't know it off the top of my head - you could probably google it), what you have posted directly after my last reply is the Energy that is required to raise the temp of the water by 3 degrees.  You still have to find out how many photons are needed to do this.  The number of photons needed will be this value divided by the energy of the photons.  Do you know what the energy of the photons are?  I put it in one of my earlier posts...

Energy of photon = h*c/wavelength
     = 6.626*10-34*3*108/(410*10-9)

(h= Planck's constant, c= speed of light in vacuum)

You are finding out how many photons are needed - the unit for your answer is therefore photons.

*Also, after doing this calculation, convert your answer to the next integer (whole number).  Can't have a fraction of a photon.

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Re: Need help
« Reply #11 on: September 25, 2007, 03:04:58 AM »
if the specific heat of water is 4.184 J/g degrees celcius how many photons of blue light (410nm) are required to raise the temperature of a 5 gallon bird bath 3 degess celcius assuming complete transfer of energy to the water and no influence from the bird bath container.

i just dont know the steps and how the gallons and degress effect it. 

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Offline jbvaliquette

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Re: Need help
« Reply #12 on: September 25, 2007, 07:11:39 AM »
i got 4.84829^-37 or 4.69732^41

Offline jbvaliquette

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Re: Need help
« Reply #13 on: September 25, 2007, 07:22:40 AM »
dont i have to make the 410nm into meters?

Offline Padfoot

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Re: Need help
« Reply #14 on: September 25, 2007, 08:03:57 AM »
After doing the calculations I described in my last post your final answer is 4.6976 x 1023 photons which resembles your second answer.
   
Yes, for the Energy of photon equation, wavelength is in meters.  If you see my previous post, I have 410*10-9 for wavelength.
(nano=10-9)

Hope that helps!

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