| Contents are: | |||
| Specific Heat Capacity | Thermal Conductance | Thermal Conductivity | Heat Flux Density |
| It also referred to as Specific Entropy For any particular substance, its Specific Heat Capacity is the measure of the quantity of (heat) energy needed to raise the temperature of a unit quantity of the substance through 1 degree of temperature. Note the 'unit quantity' may be either that of mass or volume. In the SI system use joules [J] for the energy kilograms [kg] OR cubic metres [m³] for the quantity kelvin [K] for the temperature degree. (see Note above) Other units often still seen are the British Thermal Unit [Btu] or calories for the energy, and the °F for the degree. Solids and liquids usually have only one value for their Specific Heat Capacity which can be used for most practical work. Only in the most accurate work is there a need to seek a value adjusted to the prevailing conditions (pressure and temperature usually). Gases however always have two values. One for the case when the pressure (of the gas) is kept constant, and one for when the volume is kept constant. | Approximate values in J / kg °K of the Specific Heat Capacities of some substances are
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| Also known as Heat Transfer Coefficient It is a measure of the rate at which heat energy flows through a surface. A typical use for this is in a building, when assessing the heat which is lost through the external walls. It is measured by the amount of energy which flows through a unit area, in unit time, when there is a unit temperature difference between the two sides of the surface. So, a logical expectation of its units would be something like *joules per square metre per second per degree kelvin* or J/m² s °K And, looking the 'older' types of units listed in the calculator it will be seen that they are of that form, like Btu/ft² sec °F | However, when looking at the SI units it will be seen that there appears to be no unit of time. So what has happened? In the SI the units *joule/second* are equal to (are the definition of) a watt (which is a measure of power) and that fact has been used in reducing the units for Conductance to |
| Very similar to, and often confused with, Conductance as described above. The difference to be noted is that Conductance involves area while Conductivity involves length. For any particular substance, its Thermal Conductivity is the measure of the quantity of (heat) energy which flows through a unit length, in unit time, when there is a unit temperature difference between the two ends of the length. The SI units for this measure are Thermal Conductivity can be thought of indicating how quickly the heat will get to you. Imagine holding a bar and putting one end of it in a fire (like a poker). The end you were holding would warm up until (eventually) it became too hot to hold. How long this would take would depend upon the length of the bar and the material of which it was made. The table shows that silver (with a high value) would be quicker in getting the heat to your hand than one made of steel (with a lower value). A bar of concrete would be much slower still. There are other materials whose values are even lower, but it might be rather difficult (or impossible) to use those. | Approximate values in W / m °K of the Thermal Conductivities of some substances are
THERMAL RESISTIVITY is the inverse of Conductivity. |
| Also known as Heat Flow Rate Intensity This is a measure of the rate at which heat energy flows into or out of a surface. It is expressed simply as the amount of energy which flows through a unit area, in unit time. So, a logical expectation of its units would be something like *joules per square metre per second* or J/m² s And, looking the 'older' types of units listed in the calculator it will be seen that they are of that form, like Btu/ft² sec | However, (as explained above in Thermal Conductance) when looking at the SI units it will be seen that there appears to be no unit of time. Why? In the SI the units *joule/second* are equal to (are the definition of) a watt (which is a measure of power) and that fact has been used in reducing the units for Flux Density to |
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