The ‘specific heat’ (or ‘specific heat capacity’) of a material is the quantity of (heat) energy needed to raise the temperature of a unit quantity of the material by 1 degree of temperature1. This ‘unit quantity’ may be expressed either as mass or volume. In the SI system, the preferred units are:
kilojoules [kJ] for the energy
kilograms [kg] or cubic metres [m³] for the quantity
kelvin [K] for the temperature degree2
So the preferred unit for specific heat is kJ·kg−1·K−1, or kJ/(kW·K). The specific heat of solids and liquids varies slightly, depending on parameters such as pressure and temperature, but only for the most accurate work do the values need to be adjusted for the conditions.
1 The value of specific heat defined in this way, and measured by calorimetry, is actually an approximation to the true value. This is the derivative of the curve of the internal energy in a sample (due to random motion of atoms) as a function of temperature, normalised by dividing by the mass of the sample. However, as the energy curve is fairly linear, the approximation is sufficiently accurate.
2 The Kelvin is numerically the same size as the degree Centigrade (°C), but has a different starting point (absolute zero, at approximately −273.15°C)
Approximate values of the specific heat of typical materials are given in Table 1. Low values show that only a small amount of energy is needed to produce a temperature change, whereas large values indicate that much more energy is needed. If a material has strong intermolecular forces (such as hydrogen bonding in water) then the specific heat is likely to be higher.
| Material | Specific heat (kJ·kg−1·K−1) |
|---|---|
| air3 | 1.005 |
| aluminium (pure) | 0.896 |
| copper (pure) | 0.383 |
| FR-4 | 0.60 |
| glass | 0.60 |
| lead | 0.13 |
| nylon | 1.70 |
| polythene | 2.20 |
| silicon | 0.81 |
| solder (Sn63) | 0.18 |
| steel (typical) | 0.486 |
| water4 | 4.20 |
3 Note that gases always have two values of specific heat, one (as in this case) when the pressure of the gas is kept constant, and another (related) value for when the volume is kept constant.
4 The value will depend on the temperature and on the presence of contamination or dissolved gases.
Note that, whilst the values are usually given in terms of unit weight, in many cases the value per unit volume may be more appropriate, for example, when comparing the heat capacity of a product made in different materials.
Other units often still seen, especially when the source is American, are °F for the degree and the British Thermal Unit5 or calories6 for the energy. There are useful conversion calculators at http://www.gordonengland.co.uk/conversion/specific_heat.htm.
5 Before 1929 the British Thermal Unit ( BTU) was defined as the amount of heat required to raise the temperature of 1 pound of water 1 degree Fahrenheit (from 59.5 deg F to 60.5 deg F). In 1929 it was redefined in terms of electrical units and is equivalent to 251.996 calories, 778.26 ft-lb, or approximately one-third watt-hours.
6 A calorie is a unit of heat energy, originally defined as the amount of energy, as heat (calor in Latin means heat), required to raise the temperature of 1g of liquid water from 14.5°C to 15.5°C. Today a calorie is defined in mechanical rather than thermal terms. In this system, 1 calorie (cal) equals 4.184 watt-seconds (W∙s), or joules (J). The term ‘calorie’ used by nutritionists and weight-watchers is the kilo-calorie (kcal, or Cal), which is 1,000 times bigger.
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