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### Energy of photon

Posted: **Tue Oct 27, 2020 1:25 pm**

by **Rose_Malki_3G**

In the equation Ep - (threshold energy) = (kinetic energy)

what are the units of each energy? Is Ep in units of J/photon or J/mol of photon? And if it is in units of J/photon, would you just multiply by Avogadro's number to get it in J/mol of photon?

### Re: Energy of photon

Posted: **Tue Oct 27, 2020 1:43 pm**

by **darchen3G**

The units for energy is always Joules (J).

### Re: Energy of photon

Posted: **Tue Oct 27, 2020 1:50 pm**

by **Sabina House 2A**

I believe the energy of Ep is usually given in J/photon, but pay attention to the question to make sure your units match for each of the values in that equation. If you have the energy in J/photon, you can multiply by Avogadro's number to get J/mol of photon.

### Re: Energy of photon

Posted: **Wed Oct 28, 2020 3:40 pm**

by **Hannah Rim 2D**

Yes, you are correct; Ep is Joules/photon. If you want to convert this to the unit, joules/mol, you can multiply the quantity by Avogadros number (6.022*10^23). This conversion works because there are about 6.022*10^23 photons in a mol. With dimensional analysis, you can cancel out the photons to get your answer in terms of mols

### Re: Energy of photon

Posted: **Wed Oct 28, 2020 3:55 pm**

by **Jenny Lee 2L**

Hello! In this equation, Ep is the energy of the photon (Joules per photon) and you would use Avogadro's number to find it in Joules/mole or use SI conversion to get it to kJ. However, the threshold energy and kinetic energy is usually just in Joules.

### Re: Energy of photon

Posted: **Wed Oct 28, 2020 4:47 pm**

by **Natalie Phan 3G**

For the energy of incoming light it's in J/photon, but be careful with your units because sometimes problems will give you kJ or ask to convert into eV. And whenever a problem is asking for moles/atoms, use Avogadro's number to convert.