Question

If $l$ is the length, $A$ the area of cross-section and $K$ the thermal conductivity, then the thermal resistance of the block is given by

• A. $Kl/A$
• B. $l/KA$
• C. $AK/l$
• D. $A/Kl$

Answer 1

Answer: (B)

$l/KA$

The rate of heat flow also known as heat current of a block is
$\frac{dQ}{dt}=\frac{KA(T_2-T_1)}{l}$
where $K$ is the effective thermal conductivity, $T_2-T_1$ is the temperature difference between the ends of the block, $A$ is area and $l$ is the length of the block
In other words
$\frac{dQ}{dt}=\frac{(T_2-T_1)}{R}$
where $R$ is thermal resistance
By comparing above two equations
$R=\frac{l}{KA}$
option (B) is the correct answer.