Physics · Thermal Properties Of Matter · NEET
Conduction is the transfer of heat between neighbouring parts of a body because of their temperature difference. When one end of a metal rod is put in a flame, the molecules there vibrate faster and pass their extra energy to the next molecules through collisions. This energy travels along the rod to the cold end. Note that the molecules themselves do not travel; only the heat (energy) moves. This is why conduction is the main way heat moves through solids.
K is a number that tells how easily a material lets heat pass through it. In the formula H = K A (T1 - T2) / L, K is the constant of proportionality. A large K (like copper or silver) means heat flows quickly; a small K (like wood, glass or air) means the material is a good insulator. K is a property of the material itself, so it does not change if you cut the rod shorter or make it thicker.
Heat current H is the rate of heat flow, measured in watt (joule per second). It depends on the rod you choose - its length L, its area A, and the temperature difference across it. Thermal conductivity K is a fixed property of only the material (copper, iron, glass). So H can be different for two copper rods of different sizes, but K is the same for both because they are the same material.
Metals have free electrons that move easily and carry energy quickly from the hot end to the cold end, so their K is large. In wood, plastic and other non-metals heat can only pass slowly through molecular vibrations, so their K is small. That is why a metal spoon in hot tea gets hot fast, but a wooden or plastic handle stays cool - the wood conducts heat poorly.
No. K depends only on the material, not on the shape or size of the rod. The rate of heat flow H does depend on length and area (longer rod means less flow, wider rod means more flow), but K itself stays the same. Students often confuse H and K here - remember K is intrinsic to the material, like density.
The full form of the conduction law is H = -K A (dT/dx). The minus sign shows that heat flows from high temperature to low temperature, that is, opposite to the direction in which temperature increases. For NEET numericals you usually use the simple form H = K A (T1 - T2) / L with (T1 - T2) as the positive temperature difference, so the sign is already taken care of.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
In steady state, H = K A (T1 - T2) / L. Here H is heat flow rate in watt, K is thermal conductivity, A is area of cross-section, (T1 - T2) is the temperature difference between the ends, and L is the length of the rod.
The SI unit of K is watt per metre per kelvin, written as W m^-1 K^-1 (same as J s^-1 m^-1 K^-1). This comes directly from rearranging H = K A (T1 - T2) / L to K = H L / (A (T1 - T2)).
Among common materials, silver and copper have the highest thermal conductivity, so they are the best conductors. Air, wood and thermocol have very low K, so they are good insulators.
Steady state is the condition where the temperature at every point of the rod stops changing with time. Then the same heat current H flows through every cross-section, and we can use H = K A (T1 - T2) / L. Before steady state, some heat is still being absorbed to warm the rod itself.
For NEET purposes thermal conductivity K is treated as a scalar property of the material. The direction of heat flow is handled separately by the temperature gradient in the law H = -K A (dT/dx).
