Question

In steady-state heat conduction through a plane wall, which of the following remains constant?

• A. Temperature
• B. Heat flux
• C. Thermal conductivity
• D. Wall thickness

Hint:

Logic Tip: "Steady-state" is like a perfectly flowing river. The water (heat energy) doesn't pile up anywhere, so the amount of water flowing past any point per second (Heat Flux) is exactly the same.

Answer 1

The Correct Option is B

Concept:
"Steady-state" is a fundamental assumption in heat transfer problems. It means that the conditions (temperatures, heat flow rates) at any specific point within the system do not change over time.

Step 1: We are analyzing a solid plane wall with a hot side ($T_1$) and a cold side ($T_2$). Heat is conducting through the wall from hot to cold.

Step 2: Because it is steady-state, no thermal energy is being stored or depleted within the wall material itself. Therefore, the total amount of heat energy entering the hot side per second ($Q_{in}$) must exactly equal the amount of heat energy leaving the cold side per second ($Q_{out}$).

Step 3: Heat flux ($q''$) is the rate of heat transfer per unit area ($Q/A$). Since the heat transfer rate ($Q$) is constant, and the cross-sectional area ($A$) of a plane wall is constant, the heat flux must be constant everywhere throughout the wall.

Step 4: * Temperature is not constant; it drops linearly from the hot side to the cold side. * Thermal conductivity can vary with temperature (though often assumed constant for simple problems, it is not guaranteed to be constant by the definition of steady-state). * Wall thickness is a physical dimension, not a flow variable.