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Heat Transfer formula sheet

Every formula in the Heat Transfer chapter for JEE Main, with one line on when it applies. Topic 1 is open to everyone; sign in free to read all 19 and download the PDF.

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How we count: 7× asked means 7 past JEE Main questions in the PrepWiser bank needed that formula, and "last" is the latest year one appeared. ★ marks formulas asked 8 or more times.

Conduction

Heat current (Fourier's law)
5× asked, last 2025
H=dQdt=KA ΔTL,H=−KAdTdxH=\frac{dQ}{dt}=\frac{KA\,\Delta T}L,\qquad H=-KA\frac{dT}{dx}
Steady state, no side loss. Box: total area of all walls. Melting rate =H/Lf=H/L_f.
Thermal resistance
5× asked, last 2025
R=LKA,H=ΔTRR=\frac L{KA},\qquad H=\frac{\Delta T}R
Like Ohm's law; same series and parallel rules. Varying area: R=∫dx/KAR=\int dx/KA.
Slabs in series
7× asked, last 2025
R=R1+R2,Keq=L1+L2L1/K1+L2/K2R=R_1+R_2,\qquad K_{\text{eq}}=\frac{L_1+L_2}{L_1/K_1+L_2/K_2}
Same HH in each; drops ∝ RR. Equal lengths: Keq=2K1K2/(K1+K2)K_{\text{eq}}=2K_1K_2/(K_1+K_2).
Slabs in parallel
1× asked, last 2025
1R=1R1+1R2,Keq=K1A1+K2A2A1+A2\frac1R=\frac1{R_1}+\frac1{R_2},\qquad K_{\text{eq}}=\frac{K_1A_1+K_2A_2}{A_1+A_2}
Same end temperatures; currents add. Equal areas: (K1+K2)/2(K_1+K_2)/2.
Junction temperature
7× asked, last 2025
θ=θ1R2+θ2R1R1+R2,∑iθi−θRi=0\theta=\frac{\theta_1R_2+\theta_2R_1}{R_1+R_2},\qquad \sum_i\frac{\theta_i-\theta}{R_i}=0
Series slabs; or rods meeting at a joint. Nearer the outer temperature of the smaller-RR slab.
Radial flow through a shell
1× asked, last 2021
Rsph=r2−r14πKr1r2,Rcyl=ln⁡(r2/r1)2πKLR_{\text{sph}}=\frac{r_2-r_1}{4\pi Kr_1r_2},\qquad R_{\text{cyl}}=\frac{\ln(r_2/r_1)}{2\pi KL}
Inner r1r_1, outer r2r_2; H=Δθ/RH=\Delta\theta/R.
Growth of ice on a pond
dxdt=KθρLfx,t=ρLf2Kθ(x22−x12)\frac{dx}{dt}=\frac{K\theta}{\rho L_fx},\qquad t=\frac{\rho L_f}{2K\theta}(x_2^2-x_1^2)
Air at −θ-\theta °C. 0→x0\to x, x→2xx\to2x, 2x→3x2x\to3x: times 1:3:51:3:5.

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12 more formulas in 4 topics

Convection, Radiation and Stefan–Boltzmann law, Wien's law and black-body spectrum and Newton's law of cooling. Read the first part free. Sign in free with Google to read the rest and download the PDF.

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Convection

  • Convection

Radiation and Stefan–Boltzmann law

  • Absorptivity, reflectivity, emissivity
  • Kirchhoff's law
  • Stefan–Boltzmann law3×
  • Net exchange with surroundings1×
  • Rate of cooling
  • Radiation at a distance1×

Wien's law and black-body spectrum

  • Wien's displacement law
  • Energy distribution curve

Newton's law of cooling

  • Newton's law of cooling11×
  • Exponential cooling6×
  • Average-temperature form4×

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About 30 past JEE Main questions from this chapter, with instant answer checking and step-by-step solutions where available. Free, and you can start without signing up.

Practise Heat Transfer

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Version 1, October 2026. Free to share for study, as it is. Please don't sell it or remove the PrepWiser name.

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