Thermal physics & fluids
Start with the model and its assumptions. These relationships use SI units unless you convert every quantity consistently.
Thermal physics & fluids
| Relationship | Formula | When it applies |
|---|---|---|
| Temperature & heat | T/K = T/°C + 273.15; Q = mcΔT; Qphase = mL | A temperature difference has the same numerical value in K and °C. |
| Heat transfer | Pcond = kAΔT/ℓ; Prad,net = εσA(T⁴ − Tenv⁴) | Radiation temperatures must be in kelvin. |
| Expansion | Δℓ = αℓ₀ΔT; ΔV ≈ 3αV₀ΔT | Small changes in isotropic solids; other substances use their volume expansion coefficient β. |
| Ideal gas | PV = nRT | Absolute pressure, temperature in kelvin; ideal dilute-gas approximation. |
| Hydrostatics & buoyancy | P = P₀ + ρgh; FB = ρfluid gVdisplaced | Use the fluid density, not the object density, for buoyancy. |
| Continuity & Bernoulli | A₁v₁ = A₂v₂; P + ½ρv² + ρgy = constant | Steady incompressible flow; Bernoulli along a streamline with negligible losses. |
Find definitions and the original reference tables in the glossary →