Work and heat express physical processes of supply or removal of energy, while the internal energy U is a mathematical abstraction that keeps account of the changes of energy that befall the system. The term Q is the quantity of energy added or removed as heat in the thermodynamic sense, not referring to a form of energy within the system. Likewise, W denotes the quantity of energy gained or lost through thermodynamic work. Internal energy is a property of the system, while work and heat describe the process, not the system. Thus, a given internal energy change, Δ Δ U , can be achieved by different combinations of heat and work. Heat and work are said to be path dependent, while change in internal energy depends only on the initial and final states of the system, not on the path between. Thermodynamic work is measured by change in the system, and is not necessarily the same as work measured by forces and distances in the surroundings, though, ideally, such can sometimes be arranged; this distinction is noted in the term ' isochoric work ', at constant system volume, with Δ Δ V = 0 , which is not a form of thermodynamic work.