Physics · Thermal Properties Of Matter · NEET
Both describe the same thing, but W m^-1 K^-1 is the standard SI form. Thermal conductivity relates to the RATE of heat flow (heat per second = power), so the unit uses watt (W), not joule (J). If you use joule, you must include time: J s^-1 m^-1 K^-1. Since 1 W = 1 J s^-1, the two are identical. Plain J m^-1 K^-1 (with no seconds) is WRONG because it forgets time.
Start from H = K A (delta T)/L, where H is the rate of heat flow. H is heat energy per second, so its unit is joule per second = watt. When you rearrange for K, watt stays in the numerator. So the unit naturally becomes watt divided by (metre and kelvin): W m^-1 K^-1.
K = H L / (A delta T). Dimensions: H (power) = [M L^2 T^-3], L = [L], A = [L^2], delta T = [K]. So K = [M L^2 T^-3][L] / ([L^2][K]) = [M L T^-3 K^-1]. This matches the NCERT summary value [M L T^-3 K^-1].
They belong to different quantities. W m^-1 K^-1 is thermal conductivity K (used in conduction through a rod, formula H = KA delta T / L). W m^-2 K^-1 is a heat transfer coefficient (used in convection/cooling, where per unit AREA matters). Do not mix them up: conductivity has metre to the power minus one, not minus two.
No. K is a material property, not a shape property. In H = KA delta T / L the length and area cancel out in a way that leaves K with the fixed unit W m^-1 K^-1 for any rod. Two rods of the same material have the same K even if their lengths differ.
The SI unit of thermal conductivity is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Watt per metre per kelvin, written W m^-1 K^-1. An equivalent form is J s^-1 m^-1 K^-1 because 1 watt equals 1 joule per second.
In the CGS system it is cal s^-1 cm^-1 (degree C)^-1, that is calorie per second per centimetre per degree Celsius. For NEET, the SI unit W m^-1 K^-1 is what you must remember.
A high K means the material conducts heat quickly, so metals like copper and silver have large K. A low K means the material is a good insulator, like wood or air.
Yes. K = H L / (A delta T) gives [M L^2 T^-3][L] / ([L^2][K]) = [M L T^-3 K^-1], as listed in the NCERT chapter summary.
Because 1 watt = 1 joule per second. Replacing W with J s^-1 in W m^-1 K^-1 gives J s^-1 m^-1 K^-1. Both forms are correct and identical.
