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Name: Brent
Status: student
Age: 10
Location: N/A
Country: N/A
Date: 2000


Question:
Which will heat faster, sand soil or water? And why?


Replies:
Water [liquid] will heat faster than dry sand for two reasons:


1. Water has less capacity [heat capacity, Cp] to hold heat than dry sand/air.

2. Water transfers heat faster than sand/air. That is, the [thermal conductivity, Q'] of water is larger than dry sand/air.

Here are the numbers:
SUBSTANCE               Cp(cal/gm*K)               Q'(cal/(cm^2*sec*(K/cm))
    
    SiO2                 45*10^-3                   32.00*10^-4
    air                 389*10^-3                    0.57*10^-4
    H2O(liq)             56*10^-3                   15.00*10^-4

1. While the heat capacity, Cp, of water and sand are about the same, the heat capacity of air, a gas, is about 10 times larger. The Cp of gases is generally larger than the Cp of condensed phases because the heat energy is used to expand the gas, as well as increasing the temperature of the gas. In contrast, in condensed phases (solids and liquids), almost all of the heat is used to increase the temperature of the substance, since solids and liquids expand very little when heated. That means Cp is a smaller number.

2. More importantly, the sand particles are "point contacts", so the effective cross sectional area available to transfer heat from particle-to-particle is very small and circuitous. This slows the transfer of heat. In addition, the interstitial air phase is no help because it is a poor conductor of heat.

Neither of these limitations are present in the case of a continuous liquid water phase.

Vince Calder



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