The rate of heat flow through a slab is proportional to:

The SI unit of thermal conductivity is:

Thermal resistance of a slab of thickness L, area A, and conductivity k is:

Two slabs of equal thickness and area, with conductivities k and 2k, are placed in series. The effective conductivity is:

Two slabs of thicknesses 3 cm and 5 cm with conductivities 3k and k respectively are in series. The ratio of temperature drops across them is: <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 400 160" style="max-width:400px;width:100%;height:auto;background:#faf9f5;border-radius:12px;border:1px solid rgba(20,20,19,0.10)"> <title>Two slabs in series with different k and L — compare temperature drops</title> <!-- Slab 1 --> <rect x="60" y="30" width="110" height="80" rx="2" fill="#c6613f" opacity="0.1" stroke="#14140f" stroke-width="1.5"/> <text x="115" y="60" text-anchor="middle" font-size="12" font-weight="600" fill="#14140f">$L_{1}$ = 3 cm</text> <text x="115" y="77" text-anchor="middle" font-size="12" fill="#14140f">k_{1} = 3k</text> <text x="115" y="96" text-anchor="middle" font-size="11" fill="#c6613f">$R_{1}$ = 3/(3k) = 1/k</text> <!-- Slab 2 --> <rect x="180" y="30" width="160" height="80" rx="2" fill="#788c5d" opacity="0.1" stroke="#14140f" stroke-width="1.5" stroke-dasharray="6,3"/> <text x="260" y="60" text-anchor="middle" font-size="12" font-weight="600" fill="#14140f">$L_{2}$ = 5 cm</text> <text x="260" y="77" text-anchor="middle" font-size="12" fill="#14140f">k_{2} = k</text> <text x="260" y="96" text-anchor="middle" font-size="11" fill="#788c5d">$R_{2}$ = 5/k</text> <!-- Result --> <text x="200" y="135" text-anchor="middle" font-size="12" font-weight="600" fill="#14140f">$\Delta$$T_{1}$/$\Delta$$T_{2}$ = $R_{1}$/$R_{2}$ = (1/k)/(5/k) = 1:5</text> <text x="200" y="152" text-anchor="middle" font-size="11" fill="#c6613f">Higher k → smaller temperature drop</text> </svg>