Measuring K & Convection
A lab measurement of K, and heat that rides along with moving fluid.
Lesson at a glanceShow the key points
- Searle: lagged rod heated by steam, cooled by water in a coil; all conducted heat warms the water: K = x ms(θ₃ − θ₄)/[A(θ₁ − θ₂)t] (28.4).
- Convection: heat carried by moving material; forced (pump, fan) or natural (density differences).
- Ponds cool to 4°C throughout by convection, then freeze from the top; the bottom stays at 4°C.
- The body moves heat by forced convection (heart and blood).
How to measure a metal’s conductivity in the lab, and the second route for heat: carried bodily by moving fluid.
Searle’s method for a good conductor
A metal rod has one end in a steam chamber. Near the other end a copper tube is coiled round it, carrying a steady stream of water that enters at the far end and leaves nearer the steam. Two holes drilled in the rod, a distance x apart and filled with mercury, hold thermometers (θ₁, θ₂); two more measure the water out and in (θ₃, θ₄). Everything is lagged with felt or wool so no heat escapes from the sides.
Once all four readings are steady, collect the water flowing out for a time t (mass m) and measure the rod’s cross-section A. All the heat conducted along the rod ends up in the water:
A rod of radius 2.0 cm has thermometer holes 8.0 cm apart reading 18°C apart. In 5.0 min, 0.30 kg of water flows through the coil and warms by 4.0°C. Find K.
Convection
In convection the heated material itself moves and carries the heat with it. A hot-air blower heats air and a fan pushes it out. Water on a stove is heated at the bottom by conduction through the pan; it expands, becomes less dense, and rises, while cooler water sinks.
- Forced convection: the fluid is driven by a fan or pump.
- Natural (free) convection: the fluid moves because of density differences.
Ponds in winter
As the surface of a pond cools, the denser cold water sinks and warmer water rises to be cooled, until all the water reaches 4°C. Then surface water cooled further expands, stays on top and finally freezes. After that heat escapes only by conduction through the ice, a poor conductor, so freezing is slow and the bottom stays at 4°C: fish survive.
Inside the body
The body moves heat mainly by forced convection: the heart pumps blood around. It loses heat to the surroundings by conduction, convection and radiation, at a rate depending on clothing, sweating, air temperature, wind and humidity, yet keeps a remarkably steady temperature.
- 1Searle's method: K = x ms(θ₃ − θ₄)/[A(θ₁ − θ₂)t] from a lagged rod heated by steam and cooled by flowing water.
- 2Convection: heat carried by moving fluid; natural (density-driven) or forced (fan, pump, heart).
- 3Ponds freeze from the top and stay at 4°C below.
Check yourself
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