What is a mathematical description of a continuous physical phenomenon in which a dependent variable is a function of more than one independent variable?

• A. Ordinary Differential Equation
• B. Integral Equation
• C. Algebraic Equation
• D. Partial Differential Equation

Answer: (D)

When a quantity depends on more than one independent variable, you need derivatives with respect to each of those variables to describe how it changes in each direction. That leads to a partial differential equation, which governs how the dependent field evolves across space (and possibly time) rather than along a single variable alone.

For example, the temperature in a rod is a function u(x,t) of position x and time t. Its evolution is described by the heat equation, a partial differential equation: ∂u/∂t = α ∂²u/∂x². This shows how the temperature changes in time and across position, requiring partial derivatives with respect to both variables.

An ordinary differential equation, in contrast, involves a function of a single independent variable (like time). An integral equation relates the unknown function to an integral of itself, and an algebraic equation is a static relation without derivatives. Thus, the description of a continuous phenomenon with multiple independent variables is best captured by a partial differential equation.