ANS:C - both 'a' & 'b'

Stefan's Law describes the mass transfer by diffusion. Explanation:

1. Stefan's Law:
   • Stefan's Law, formulated by the Austrian physicist Josef Stefan, describes the rate of mass transfer of a component through a stationary phase (such as a gas or liquid) by molecular diffusion.
   • It states that the rate of mass transfer (molecular flux) of a component is directly proportional to the concentration gradient of that component in the direction of diffusion.
   • Mathematically, Stefan's Law is expressed as: 𝐽=−𝐷𝑑𝐶𝑑𝑥J=−DdxdC​ where:
     • 𝐽J is the mass flux (amount of mass transferred per unit area per unit time),
     • 𝐷D is the diffusion coefficient (a measure of the ability of the component to diffuse through the medium), and
     • 𝑑𝐶𝑑𝑥dxdC​ is the concentration gradient of the component in the direction of diffusion.
2. Diffusion:
   • Diffusion is the process by which molecules move from regions of higher concentration to regions of lower concentration, resulting in the equalization of concentration throughout a medium.
   • It occurs due to random molecular motion and does not require bulk flow or the movement of the medium itself.
   • Diffusion is responsible for the transport of molecules within gases, liquids, and solids, and it plays a crucial role in various natural and industrial processes, including gas exchange in biological systems, chemical reactions, and mass transfer in separation processes.
3. Bulk Flow:
   • Bulk flow, also known as convective transport, involves the movement of a fluid as a whole, typically driven by pressure differences or mechanical forces.
   • Unlike diffusion, which relies on random molecular motion, bulk flow requires the movement of the entire medium and can transport mass or heat more rapidly over long distances.
   • Bulk flow is characterized by organized, directed movement and is often associated with fluid flow in pipes, channels, or vessels.

In summary, Stefan's Law specifically describes the mass transfer by diffusion, which involves the movement of molecules through a medium due to concentration gradients. It does not apply to mass transfer by bulk flow, which involves the movement of the entire fluid medium as a result of external forces. Therefore, the correct answer is neither 'a' nor 'b'.