Question

The value of solar constant in SI system is
(A). \[1340\dfrac{\text{watt}}{{{m}^{2}}}\]
(B). \[1340\dfrac{\text{kilowatt}}{{{m}^{2}}}\]
(C). \[1340\dfrac{{{m}^{2}}}{\text{watt}}\]
(D). \[1340\dfrac{\text{watt}}{m}\]

Answer 1

Hint: Solar constant \[({{G}_{sc}})\] is the measure of the mean of the flux density of solar irradiance per unit area. This irradiance is the total electromagnetic radiation from the Sun, which excludes the radiation by neutrinos. The Solar constant is not a physical constant like the Planck's constant (h) or the Avogadro number (NA). Billions of years ago it was significantly lower than the present value. In the past 400 year scale, it has shown a difference of 0.2 percentage.

Complete step by step answer:
The heat flux is defined as the heat energy flowing in unit time. Therefore its unit is Joules/second which is equal to watt. The heat flux density is measured for a unit surface that is coming out perpendicular to the surface. Hence the value is for a unit area giving us the value of the unit as wattage per metre square. Now by calculating the value of the solar irradiance from a distance of 1AU (Astronomical Unit), we get it in the range of thousands, yielding the correct unit as \[\dfrac{\text{watt}}{{{m}^{2}}}\]. The correct option is A.

Additional information:
Power is defined as the energy produced per unit time and has the unit wattage(W). One AU (Astronomical Unit) is roughly the distance between the Earth and the Sun which is equal to \[1.49597\times {{10}^{11}}m\]. The mean flux density is considered in this case because the emissions from the Sun are not equal from any surfaces in a period.

Note: The square in the given options of units has to be viewed carefully. The unit of wattage can be written in terms of Joules and seconds. The representations of constants are unique and therefore questions may include the representative alphabets instead of the constant name.