## Energy of photon

Rose_Malki_3G
Posts: 88
Joined: Wed Sep 30, 2020 10:02 pm

### Energy of photon

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?

darchen3G
Posts: 83
Joined: Wed Sep 30, 2020 9:39 pm

### Re: Energy of photon

The units for energy is always Joules (J).

Sabina House 2A
Posts: 69
Joined: Wed Sep 30, 2020 9:35 pm
Been upvoted: 3 times

### Re: Energy of photon

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.

Hannah Rim 2D
Posts: 58
Joined: Wed Sep 30, 2020 9:48 pm
Been upvoted: 1 time

### Re: Energy of photon

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

Jenny Lee 2L
Posts: 69
Joined: Thu Oct 08, 2020 12:15 am

### Re: Energy of photon

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.

Natalie Phan 3G
Posts: 50
Joined: Wed Sep 30, 2020 9:48 pm

### Re: Energy of photon

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.