Suppose some fixed, external potential energy U generates a conservative force F ( x ) = − − ∇ ∇ U ( x ) (for example, an electric force) on a particle located at a given position x . We assume that the particle would respond by moving with velocity v ( x ) = μ μ ( x ) F ( x ) (see Drag (physics)). Now assume that there are a large number of such particles, with local concentration ρ ρ ( x ) as a function of the position. After some time, equilibrium will be established: particles will pile up around the areas with lowest potential energy U , but still will be spread out to some extent because of diffusion. At equilibrium, there is no net flow of particles: the tendency of particles to get pulled towards lower U , called the drift current, perfectly balances the tendency of particles to spread out due to diffusion, called the diffusion current (see drift-diffusion equation).