Dewar's flash illustration

In a Dewar's flask, heat flow from the resistor produces a change in the temperature of the water within the calorimetry cup.

Energy may transform from one form to another within a system or it may be exchanged between the system and its surroundings. Always watch for the movement of energy between a system and its surroundings.

Photon energy is often the means by which an electro-dynamic system transfers energy to its surroundings. The quintessential example is the emission of light from an excited atom as it returns to the ground state.

Another common situation involves heat loss from a resistor, usually initiated by collisions between current electrons and conductor atoms at the molecular level which leads to a temperature difference between the resistor and the environment leading to heat flow.