def constant_charge_single_potential(sigma0, radius, kappa, epsilon):
    """
    It computes the surface potential on a sphere at constant charged, immersed
    in water.

    Arguments
    ----------
    sigma0 : float, constant charge on the surface of the sphere.
    radius : float, radius of the sphere.
    kappa  : float, reciprocal of Debye length.
    epsilon: float, water dielectric constant.

    Returns
    --------
    phi  : float, potential.
    """

    dphi = -sigma0 / epsilon
    phi = -dphi * radius / (1. + kappa * radius)  # Surface potential

    return phi