From the definition of heat capacity C comes the relation C = d Q / d T . Differentiating this equation with regard to time gives the identity (valid so long as temperatures in the object are uniform at any given time): d Q / d t = C ( d T / d t ) . This expression may be used to replace d Q / d t in the first equation which begins this section, above. Then, if T ( t ) is the temperature of such a body at time t , and T env is the temperature of the environment around the body: d T ( t ) d t = − − r ( T ( t ) − − T env ) = − − r Δ Δ T ( t ) where r = h A / C is a positive constant characteristic of the system, which must be in units of s − − 1 , and is therefore sometimes expressed in terms of a characteristic time constant t 0 given by: t 0 = 1 / r = − − Δ Δ T ( t ) / ( d T ( t ) / d t ) . Thus, in thermal systems, t 0 = C / h A . (The total heat capacity C of a system may be further represented by its mass- specific heat capacity c p multiplied by its mass m , so that the time constant t 0 is also given by m c p / h A ).