Conduction | Temperature Gradient | Thermal Conductivity | Physics Grade XI

Transfer of Heat

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Conduction | Temperature Gradient | Thermal Conductivity | Physics Grade XI

Conduction | Temperature Gradient | Thermal Conductivity

Transfer of Heat
Heat transfers from one point to another due to temperature difference between the points. Heat transfers through conduction, convection, and radiation.
 
Conduction
The process through which heat transfers through a material without the movement of the material is called conduction. In other words, when the heat is transferred by the molecules by the collision between rapidly vibrating molecules from the hotter end to the cooler end, then the process is called Conduction.
 
Temperature Gradient
Suppose a metal rod AB of length L whose ends are in the thermal contact with a hot reservoir at a temperature θ1 and cold reservoir at temperature θ2. The sides of the rod are covered with insulating materials, and so heat transfer is along the rod from A to B, not through the sides as shown in the figure.
Temperature gradient illustration
Temperature gradient graphThe rate of fall in temperature with distance in the direction of heat flow is called the temperature gradient. The temperature gradient of uniform rod is
     Temperature gradient = (θ1 – θ2)/ L
Between any two points, C and D, at distance x
     Temperature gradient = (θC – θD)/ x
 
Thermal conductivity
Thermal conductivity of the material of a body is defined as the amount of heat that flows in one second across the opposite faces of a unit cube, whose opposite faces are kept at a difference of 1K. The coefficient of thermal conductivity is represented by k. its SI unit is Wm-1k-1
 
     Mathematically, Q/t = k A(θ1 – θ2)/ x
When A = 1m2, θ1 – θ2 = 1K and x = 1, then k = Q/t
 
Application of Conduction
  1. In winter, iron chairs appear to be colder than wooden chairs: Iron is good conductor of heat. When we touch them, they absorb the heat from our body. Since our body loses heat, we feel cold. On the other hand, wood is a bad conductor of heat and so there is no transfer of heat from our body to the wooden chair. Hence iron chairs appear cooler than wooden chairs.
  2. Ice is packed in sawdust: Sawdust from wood is a bad conductor of heat also air trapped inside doesn’t allow transfer of heat from the surroundings to the ice.  Hence ice doesn’t melt for which it is kept in sawdust.
  3. Eskimos make double-walled houses of the blocks of ice: The air enclosed between the double walls is a bad conductor of heat. It reduces the transmission of heat from the house to old surroundings. Therefore, the people living in the houses feel warmer.

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