Now, the difference in temperature is simply this temperature minus this temperature, when you plot the things in, you get the rate of heat flow, okay? And this will become important only when you're dealing with conduction and nothing else. This formula only applies for conduction. The second way by which energy is transferred due to heat is called convection. Convection is a transfer of energy due to the movement of fluids. And fluids can both include gas and liquid, okay?