## Sapling #5

$E=hv$

IreneSeo3F
Posts: 82
Joined: Wed Sep 30, 2020 9:31 pm

### Sapling #5

I am confused with number 5 on this week's sampling. The question asks: A liquid is exposed to infrared radiation with a wavelength of 5.68x10^-4cm. Assume that all the radiation is absorbed and converted to heat. How many photons are required for the liquid to absorb 17.97J of heat?
How can I start this question?
Last edited by IreneSeo3F on Sun Oct 25, 2020 11:44 pm, edited 1 time in total.

Samudrala_Vaishnavi 3A
Posts: 93
Joined: Wed Sep 30, 2020 9:34 pm

### Re: Sampling #5

At first glance, this is what I would do.
I would use the wavelength in m (convert cm to using 10^-2) and then plug in just Planck's constant, the constant speed of light, and the wavelength given into the E=hC/wavelength equation. Once I get the number of Joules (E) of energy the photon can potentially produce, I would divide it by the number of joules we need (given) to get the number of photons.

Posts: 142
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### Re: Sampling #5

In addition to the steps given about calculating for the Joules of energy, an important concept to remember that you can apply when calculating for other problems in the future is to always look at the units. When you calculate for energy after finding the wavelength in meters and plugging in your values into E=hc/wavelength, this energy is in Joules per photon. Now, the question is asking for the energy in photons only, so to get this, you would have to cancel out "Joules" in the units somehow. To do this, we need to take the energy we were given, 17.97 J of heat, and divide that by the value you got in Joules per photon, and you'll see that the units for Joules cancels out and you get the value of the energy in photons.

This method of looking at the units can be very helpful, especially when the answer needs to be in certain units. I hope this helps!

Crystal Yu 1D
Posts: 163
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### Re: Sampling #5

You would first solve for the Energy of photon. Then, you can use conversion factors to find the number of photons required for the energy given. I have attached a photo of my work below. The numbers are different, but I think it will help you visualize the steps! Hope this helps :)

George_Yin_3I
Posts: 50
Joined: Wed Sep 30, 2020 9:42 pm

### Re: Sampling #5

By using c=lambda*v, and E=hv, the answer can be obtained.
It is a two-step calculation.

rhettfarmer-3H
Posts: 96
Joined: Wed Sep 30, 2020 9:59 pm

### Re: Sampling #5

At first it not a usually problem we see but just think what we are given. We are given the energy in the water and wavelength in nm. SO first convert nm to m by mutiplying by 10^9. Then find the amount of energy per photon. E=hc/lamanda. Here we are given the joules/photon. Now we can divide the energy they gave in water by joules/photon and we will get just photons. that it folks.

Jacquelyn Challis 2B
Posts: 73
Joined: Wed Sep 30, 2020 9:42 pm

### Re: Sapling #5

I'm still stuck on this question, I understand the steps but I can't figure out how to find c for the equation E=hc/wavelength?

Julianna Shimabukuro 1D
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### Re: Sapling #5

Hello! C is the speed of light, a constant that we are given in the equations sheet. C = (3.00 x 10^8) m/s