The power P (with unit Watt) which is developed in a conductor is proportional to both
voltage U (with unit Volt) and current I (with unit Ampère), so P = U x I Watt. The
amount of energy E that is developed during a time period t (in seconds) is
proportional to power and time: E = P x t Watt-second or Joule. Through a 230 volts,
40 Watt incandescent lamp a current of 174 mA (40 W = 230 V x 0,174 A) will flow in
steady state. An amount of energy of 40 Joule per second is applied to the filament in
that case and, when properly dimensioned, this energy will be sufficient to bring a
large number of atoms into vibration. This energy is then released again in a
continuous process of relaxation of individual atoms, releasing their energy in the
form of photons each time a vibration returns to a lower energy level.