# Photoelectric effect problem -- Determine the heat delivered to the interaction space

Homework Statement:
Determine the heat delivered to the interaction space in three minutes, if the amount of 5.6.10^18 photons per 1 s hits the area of 1 cm2 and the absorption coefficient for a given wavelength is 0.32.
Relevant Equations:
E=hc/wavelength
N(interaction)=N0(1-e^-µx) and E(ab)= N(interaction) x µ(ab) x E
Hi Guys !
Previous question was to determine the energy of photon with wavelength = 820nm
I found that E = 2.42x10^-19 J = 1.51 ev using the equation E=hc/wavelength
- I tried to solve the above problem with various equations but all failed.
The closest equation i tried is N(interaction)=N0(1-e^-µx) and E(ab)= N(interaction) x µ(ab) x E
Please i want to know if i'm on the right equation to solve the problem ?!!

kuruman
Homework Helper
Gold Member
Although μ in the context you present is called the absorption coefficient, it doesn't look like you want to go there because no distance x is given through which photons move. Also, the number 0.32 has no units. I suspect it is an absorption efficiency, i.e. 32 % of the incident photons are absorbed and converted into heat.

Thank you for your response Kuru !
Exactly, x unknown here and i was thinking that 0.32 was in meter.
Is there any other equation to solve it ? Because the next question asking if the heat is enough to ignite a paper of 0.1mm thick. i don't see the interconnection between the questions in the problem !

kuruman
Homework Helper
Gold Member
I gave you my interpretation on the basis of the statement of the problem that you posted. Is this statement exactly as was given to you or is the original statement different, e.g. from another language that you translated?

- The problem is assembly of three questions (it's exactly as was given in english)
the first was to calculate the energy of the photon the second is the above and the third is (Is the supplied heat enough to ignite a sheet of classic office paper 0.1 mm thick and with a (specific surface area) weight of 80 g.m-2? (cp = 1.34 kJ.kg-1.K-1))

kuruman
Homework Helper
Gold Member
Perhaps the answer to the second part is intended to be a mathematical expression in the form $$N= N_0\left (1-e^{-\mu~x}\right)$$where ##N## is the number of photons absorbed to a depth ##x## from the surface. You have a length of 1 mm in part 3 so you can calculate the number of photons absorbed. The question remains whether 0.32 is in m-1, cm-1 or mm-1. Perhaps you can do a web search and find out what's reasonable and what isn't. While you're at it, look up the combustion temperature for paper. You will need it for part 3.

meher4real
Perhaps the answer to the second part is intended to be a mathematical expression in the form $$N= N_0\left (1-e^{-\mu~x}\right)$$where ##N## is the number of photons absorbed to a depth ##x## from the surface. You have a length of 1 mm in part 3 so you can calculate the number of photons absorbed. The question remains whether 0.32 is in m-1, cm-1 or mm-1. Perhaps you can do a web search and find out what's reasonable and what isn't. While you're at it, look up the combustion temperature for paper. You will need it for part 3.
That's exactly why i asked for help here, bacause i've tried using that equation but the result is to big
I took 0.32 m^-1 as absorption coeff, 0.1x10^3 m as length, 5.6x10^18 x 180s as N0
i get from that 1.01x10^21 ?
Then i'm thinking on this equation to determine the E(absorption) = N(interactions) x µ x E(photon) ?
Any thoughts ?

kuruman
Homework Helper
Gold Member
1 mm =1×10-3m not 103m (which is 1 kilometer). Would that make a difference?

meher4real
I think it's still big value in my opinion (i calculate it for 3mins not for 1s)
3.23x10^17

kuruman
Homework Helper
Gold Member
I think it's still big value in my opinion (i calculate it for 3mins not for 1s)
3.23x10^17
What does the number 3.23x10^17 represent? There are no units.

What does the number 3.23x10^17 represent? There are no units.

Sorry for the delay, i was working.
I think it's "ev"
Did my result looks legit ? and what equation do i need to use to dermine the enough heat using weight ? i'm lost here !

kuruman
Homework Helper
Gold Member
OK, it's eV. What kind of energy is it? What equation did you use to get it?

To find if there is enough heat, you need to find the temperature change of the paper if all the energy of the photons that are absorbed goes into raising the temperature of the paper. What equation do you know that relates heat added to temperature change?

meher4real
OK, it's eV. What kind of energy is it? What equation did you use to get it?

To find if there is enough heat, you need to find the temperature change of the paper if all the energy of the photons that are absorbed goes into raising the temperature of the paper. What equation do you know that relates heat added to temperature change?
As i mentioned before i used E=hc/wavelength to determine the Energy.
Can you explain more ? What do you mean by "To find if there is enough heat, you need to find the temperature change of the paper if all the energy of the photons that are absorbed goes into raising the temperature of the paper."

kuruman
Homework Helper
Gold Member
As i mentioned before i used E=hc/wavelength to determine the Energy.
When you use E = hc/λ, that is the energy of a single photon. Are you saying to me that 3.23x10^17 eV is the energy of a single photon? I think not. You already found that the energy of a single photon is 1.51 eV and I agree with that number. So what energy does 3.23x10^17 eV represent?
Can you explain more ? What do you mean by "To find if there is enough heat, you need to find the temperature change of the paper if all the energy of the photons that are absorbed goes into raising the temperature of the paper."
This suggestion is for part 3. What strategy will you follow to find if "the supplied heat (is) enough to ignite a sheet of classic office paper 0.1 mm thick and with a (specific surface area) weight of 80 g.m-2? (cp = 1.34 kJ.kg-1.K-1))"

When you use E = hc/λ, that is the energy of a single photon. Are you saying to me that 3.23x10^17 eV is the energy of a single photon? I think not. You already found that the energy of a single photon is 1.51 eV and I agree with that number. So what energy does 3.23x10^17 eV represent?

This suggestion is for part 3. What strategy will you follow to find if "the supplied heat (is) enough to ignite a sheet of classic office paper 0.1 mm thick and with a (specific surface area) weight of 80 g.m-2? (cp = 1.34 kJ.kg-1.K-1))"
I Think that 3.23x10^17 eV is the transmitted energy.

kuruman
Homework Helper
Gold Member
How did you calculate that number? Show me the equation and the numbers you put in it.

N(interaction)=N0(1-e^-µx)
where 0.32 m^-1 as absorption coeff, 0.1x10^-3 m as length (paper 0.1 mm thick), 5.6x10^18 x 180s as N0
Correction : I found 1.03x10^13 ev

kuruman
Homework Helper
Gold Member
I Think that 3.23x10^17 eV is the transmitted energy.
Is that what you want? How are you going to use the transmitted energy to find if there is enough energy to set the piece of paper on fire?

meher4real
Is that what you want? How are you going to use the transmitted energy to find if there is enough energy to set the piece of paper on fire ?
The only think i'm sure about at this point that i have to use (cp = 1.34 kJ.kg-1.K-1)) and weight on my next equation.
But don't know how because i'm thinking about : E(absorption) = N(interactions) x µ x E(photon)

kuruman
Homework Helper
Gold Member
The question in part 3 is:
Is the supplied heat enough to ignite a sheet of classic office paper 0.1 mm thick and with a (specific surface area) weight of 80 g.m-2? (cp = 1.34 kJ.kg-1.K-1))?

What strategy is needed to answer this question? By this I mean what do you need to know? Is calculating the absorbed energy sufficient? If not, what else do you need to know or look up or calculate? Hint: Not all things that absorb energy ignite. Why not?

meher4real
The question in part 3 is:
Is the supplied heat enough to ignite a sheet of classic office paper 0.1 mm thick and with a (specific surface area) weight of 80 g.m-2? (cp = 1.34 kJ.kg-1.K-1))?

What strategy is needed to answer this question? By this I mean what do you need to know? Is calculating the absorbed energy sufficient? If not, what else do you need to know or look up or calculate? Hint: Not all things that absorb energy ignite. Why not?
Sorry for the delay i was at work !
Thank you for your time Kuru, but as i said before i'm a little confused because i don't know an equation that needs CP? plus i find some problems understanding some scientific sentences, i'm trying my best by using different equations to solve it but still no results.
Only one problem left for my exam and this took me too much time.
I need your help with more details, i'm not a physician by the way and i used to study physics in french.

kuruman
Homework Helper
Gold Member
I am not asking to find an equation. I am asking for a plan. Once you have a plan, you will know where to look for equations. If this is an exam problem, the solution must come from you. Here is a simple question, no equation is needed. It's the first step towards understanding what comes next.
Question: As you know, a piece of paper does not ignite if it is just sitting there minding its own business. What must happen to it in order to ignite?

meher4real
I am not asking to find an equation. I am asking for a plan. Once you have a plan, you will know where to look for equations. If this is an exam problem, the solution must come from you. Here is a simple question, no equation is needed. It's the first step towards understanding what comes next.
Question: As you know, a piece of paper does not ignite if it is just sitting there minding its own business. What must happen to it in order to ignite?
Understood !
Must expose it to heat, means must absorb energy ?

kuruman