 For some simple materials, like an ideal gas, one can derive from basic theory the equation of state formula_55 and even the specific internal energy formula_56 In general, these functions must be determined experimentally for each substance.The absolute value of this quantity is undefined, and (for the purposes of thermodynamics) the state of "zero internal energy" can be chosen arbitrarily.However, by the law of conservation of energy, any infinitesimal increase formula_57 in the total internal energy formula_58 must be matched by the net flow of heat energy formula_59 into the sample, plus any net mechanical energy provided to it by enclosure or surrounding medium on it.