Question

Two cylindrical rods A and B made of different materials, are joined in a straight line. The ratio of lengths, radii and thermal conductivities of these rods are : $ \frac{L_A}{L_B} = \frac{1}{2} $, $ \frac{r_A}{r_B} = 2 $, and $ \frac{K_A}{K_B} = \frac{1}{2} $. The free ends of rods A and B are maintained at $ 400 \, K $, $ 200 \, K $, respectively. The temperature of rods interface is ____ K, when equilibrium is established.

Hint:

Treat heat flow problems involving composite materials as analogous to electrical circuits. Thermal resistance plays the role of electrical resistance, temperature difference is analogous to voltage difference, and the rate of heat flow (thermal current) corresponds to electrical current. For rods in series, the thermal resistance is additive, and the thermal current is the same through each rod.

Answer 1

Correct Answer: 360

Step 1: Define the thermal resistance of each rod.
The thermal resistance \( R_{th} \) of a cylindrical rod is given by \( R_{th} = \frac{L}{KA} \), where \( L \) is the length, \( K \) is the thermal conductivity, and \( A \) is the cross-sectional area of the rod. The cross-sectional area of a cylindrical rod with radius \( r \) is \( A = \pi r^2 \). For rod A: Length \( L_A \) Radius \( r_A \) Thermal conductivity \( K_A \) Area \( A_A = \pi r_A^2 \) Thermal resistance \( R_{th,A} = \frac{L_A}{K_A \pi r_A^2} \) For rod B:
Length \( L_B \) Radius \( r_B \) Thermal conductivity \( K_B \) Area \( A_B = \pi r_B^2 \) Thermal resistance \( R_{th,B} = \frac{L_B}{K_B \pi r_B^2} \)
Step 2: Use the given ratios to relate the thermal resistances.
We are given \( \frac{L_A}{L_B} = \frac{1}{2} \), \( \frac{r_A}{r_B} = 2 \), and \( \frac{K_A}{K_B} = \frac{1}{2} \). Consider the ratio of the thermal resistances: \[ \frac{R_{th,A}}{R_{th,B}} = \frac{\frac{L_A}{K_A \pi r_A^2}}{\frac{L_B}{K_B \pi r_B^2}} = \frac{L_A}{L_B} \cdot \frac{K_B}{K_A} \cdot \frac{\pi r_B^2}{\pi r_A^2} = \frac{L_A}{L_B} \cdot \frac{K_B}{K_A} \cdot \left(\frac{r_B}{r_A}\right)^2 \] Substitute the given ratios: \[ \frac{R_{th,A}}{R_{th,B}} = \left(\frac{1}{2}\right) \cdot \left(\frac{1}{1/2}\right) \cdot \left(\frac{1}{2}\right)^2 = \frac{1}{2} \cdot 2 \cdot \frac{1}{4} = \frac{1}{4} \] So, \( R_{th,A} = \frac{1}{4} R_{th,B} \), or \( R_{th,B} = 4 R_{th,A} \).
Step 3: Apply the concept of thermal current in series.
When the rods are joined in series, the rate of heat flow (thermal current \( I_{th} \)) through both rods is the same at equilibrium. Let the temperature of the interface be \( T \). The thermal current through rod A is given by: \[ I_{th} = \frac{T_1 - T}{R_{th,A}} = \frac{400 - T}{R_{th,A}} \] The thermal current through rod B is given by: \[ I_{th} = \frac{T - T_2}{R_{th,B}} = \frac{T - 200}{R_{th,B}} \] Equating the thermal currents: \[ \frac{400 - T}{R_{th,A}} = \frac{T - 200}{R_{th,B}} \] Substitute \( R_{th,B} = 4 R_{th,A} \): \[ \frac{400 - T}{R_{th,A}} = \frac{T - 200}{4 R_{th,A}} \] Multiply both sides by \( 4 R_{th,A} \): \[ 4(400 - T) = T - 200 \] \[ 1600 - 4T = T - 200 \] \[ 1600 + 200 = T + 4T \] \[ 1800 = 5T \] \[ T = \frac{1800}{5} = 360 \, K \] The temperature of the rods interface is \( 360 \, K \).