Heat
Energy transferred between a system and its surroundings solely because of a temperature difference.
Formula / equation
q = mcΔT
Unit
J
In depth
Heat is energy in transit, never a property that a body contains; once absorbed, it simply becomes internal energy. Heat flow is positive when energy enters the system. It always moves spontaneously from hotter to colder regions, which is one statement of the second law.
Examples in the real world
Warming 100 g of water by 10 °C requires 4180 J of heat.