# ***************************************************************************
# *   Copyright (c) 2020 Bernd Hahnebach <bernd@bimstatik.org>              *
# *                                                                         *
# *   This file is part of the FreeCAD CAx development system.              *
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# *   This program is free software; you can redistribute it and/or modify  *
# *   it under the terms of the GNU Lesser General Public License (LGPL)    *
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"""Collection of natural constants for the Fem module.

This module contains natural constants for the Fem module.
All constants are in SI units.
"""


__title__ = "FEM collection of natural constants"
__author__ = "Bernd Hahnebach"
__url__ = "https://www.freecad.org"


def gravity():
    # https://en.wikipedia.org/wiki/Gravitational_acceleration
    return "9.80665 m/s^2"


def stefan_boltzmann():
    # https://en.wikipedia.org/wiki/Stefan-Boltzmann_constant
    return "5.67037e-8 W/(m^2*K^4)"


def vacuum_permeability():
    # https://en.wikipedia.org/wiki/Vacuum_permeability
    return "1.256637e-6 N/A^2"


def vacuum_permittivity():
    # https://forum.freecad.org/viewtopic.php?f=18&p=400959#p400959
    # https://en.wikipedia.org/wiki/Permittivity#Vacuum_permittivity
    return "8.85419e-12 F/m"


def boltzmann_constant():
    # https://en.wikipedia.org/wiki/Boltzmann_constant
    return "1.38065e-23 J/K"


"""
from FreeCAD import Units
from femtools import constants
Units.Quantity(constants.gravity()).getValueAs("mm/s^2")

"""

# TODO: a unit test to be sure these values are returned!