a Joule (J) is an SI Unit for energy: equal 1 kg x m^2/s^2. a Photon (quantum) is the smallest possible packet of electromagnetic radiation with an energy equal to hv. Solution a Joule (J) is an SI Unit for energy: equal 1 kg x m^2/s^2. a Photon (quantum) is the smallest possible packet of electromagnetic radiation with an energy equal to hv..