Convection Currents
Natural convection — heated fluid rises, cools, sinks.
Key Notes
Convection: heat transfer by bulk motion of a FLUID (liquid or gas).
Natural convection: driven by density differences (hot fluid rises, cold fluid sinks).
Forced convection: pump or fan moves the fluid.
Doesn't occur in solids (no bulk fluid motion) or vacuum.
Examples: boiling water, atmosphere circulation (winds), ocean currents, room heating with radiator.
Newton's law of cooling: rate of heat loss ∝ (T_body − T_surroundings) — assumes convective cooling.
Convection coefficient h depends on fluid speed, geometry, fluid properties. Typical: natural air h ~ 5 W/m²·K; forced air h ~ 50; water h ~ 500.
Rayleigh number determines whether convection occurs vs pure conduction in a fluid layer.
Formulas
Convective heat transfer rate
h = convection coefficient (W/m²·K), A = area, T_s = surface T, T_∞ = bulk fluid T.
Newton's law of cooling
Body's T approaches T_∞ exponentially: T(t) = T_∞ + (T₀ − T_∞)·e^(−kt).
Rayleigh number
Dimensionless; Ra > ~1700 ⇒ convection starts.
Important Points
Convection needs a fluid (liquid or gas) — not in solids or vacuum.
Natural convection: hot fluid rises (less dense), cold sinks. Drives Hadley cells in atmosphere.
Forced convection (fan, pump): faster but needs energy input.
Free convection coefficients are LOW (~few W/m²·K). Forced is much higher.
Hot air rises ⇒ heat radiators are placed near the floor; AC vents near the ceiling.
Convection currents drive global weather patterns and ocean circulation.
Convection Currents notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 11 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.