When do I use the the following energy formulas?

Question

q = m * delta t * Cp

where q is energy, delta t is temp. change and Cp is specific heat.

q = m * delta H of phase change

where delta H is the change in heat.

Answer 1

The first equation is used when there is no change of phase, and energy causes a change in temperature.

The second equation calculates the energy when there is a phase change (and as a result there is no temperature delta).

Answer 2

Physics, or Chemistry. Or just about any time a specific heat is mentioned. Not much more I can tell you without you being more specific.