In geometry, an intrinsic equation of a curve is an equation that defines the curve using a relation between the curve's intrinsic properties, that is, properties that do not depend on the location and possibly the orientation of the curve. Therefore an intrinsic equation defines the shape of the curve without specifying its position relative to an arbitrarily defined coordinate system. The intrinsic quantities used most often are arc length s, tangential angle \theta, curvature \kappa or radius of curvature, and, for 3-dimensional curves, torsion \tau. Specifically: The equation of a circle (including a line) for example is given by the equation where s is the arc length, \kappa the curvature and r the radius of the circle. These coordinates greatly simplify some physical problem. For elastic rods for example, the potential energy is given by where B is the bending modulus EI. Moreover, as, elasticity of rods can be given a simple variational form.