Differential equations arise, as we have seen, whenever a state variable such as temperature, concentration, velocity, or electrical current, varies with time or distance, or both. The state variables usually assume the role of the dependent variable, whereas time and distance become the independent variables. As we have seen in Chapter 1, some exceptions to this rule may occur. For example, distance may be used as a dimension to describe a time-varying mass or volume, or the deflection of a physical structure. These exceptions, however, are relatively few in number, and the original definitions given earlier apply in general.