Question

When thermal resistances are arranged in series, the overall thermal resistance is equal to

• A. Product of individual resistances
• B. Average of individual resistances
• C. Sum of individual resistances
• D. Difference of individual resistances

Hint:

Logic Tip: If you have to run through three muddy fields (resistances) one after the other, your total struggle is the sum of the struggle from the first, second, and third fields combined.

Answer 1

The Correct Option is C

Concept:
Heat transfer through multiple layers of different materials (like a composite wall) is mathematically modeled using the concept of a "thermal circuit," which behaves exactly like an electrical circuit.

Step 1: In electrical circuits, when resistors are placed end-to-end (in series), the total resistance to the current flow is simply the sum of all individual resistors ($R_{total} = R_1 + R_2 + R_3...$).

Step 2: In heat conduction, heat flow ($Q$) acts like electrical current ($I$), temperature difference ($\Delta T$) acts like voltage ($V$), and thermal resistance ($R_{th}$) acts like electrical resistance ($R$). This is expressed as $Q = \frac{\Delta T}{R_{th}}$.

Step 3: When heat passes through a composite wall (e.g., brick, then insulation, then drywall), the heat must sequentially travel through every single layer.

Step 4: Because the heat is forced to push through each layer one after the other, the total resistance opposing the heat flow is the simple algebraic sum of the individual thermal resistances of each layer.