def get_eccF(r1_norm, r2_norm, c_norm):
    """
    Computes the eccentricity component along the chord. This value is kept
    constant for all the problem as long as the boundary conditons are not
    changed.

    Parameters
    ----------
    r1_norm: float
        Norm of the initial vector position.
    r2_norm: float
        Norm of the final vector position.
    c_norm: float
        Norm of the chord vector.

    Returns
    -------
    ecc_F: float
        Eccentricity component along the chord direction.

    Notes
    -----
    Equation (3) from Avanzini's report [1].

    """
    ecc_F = (r1_norm - r2_norm) / c_norm
    return ecc_F