quantitykind:HeatFlowRate
The rate of heat flow between two systems is measured in watts (joules per second). The formula for rate of heat flow is $\bigtriangleup Q / \bigtriangleup t = -K \times A \times \bigtriangleup T/x$, where $\bigtriangleup Q / \bigtriangleup t$ is the rate of heat flow; $-K$ is the thermal conductivity factor; A is the surface area; $\bigtriangleup T$ is the change in temperature and $x$ is the thickness of the material. $\bigtriangleup T/ x$ is called the temperature gradient and is always negative because of the heat of flow always goes from more thermal energy to less).