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Compared
to other substances, water has a huge specific heat capacity. This
means it takes a relatively large amount of heat transferred to it to
raise its temperature. Specific heat capacity is defined as the amount
of heat energy required to raise the temperature of one kilogram of a
material by one degree kelvin. I.e. 1 °K (which is the same as 1 °C).
Whereas the zero point on the Celsius temperature scale starts at the
freezing point of water which is 0 °C, the Kelvin temperature scale
starts at absolute zero when the motion of all atoms ceases. 0 °K is
equivalent to −273.15 °C. However a difference in temperature of 1 °C = 1
°K, so the divisions on both scales are the same magnitude.
What are calories?
The
joule is the metric unit of energy, although the calorie is still used
in some countries and in the context of food or heating systems (The
British Thermal Unit or BTU is also used). The kilocalorie, also known
as the kilogram calorie, great calorie or large calorie is what's marked
on food products however and is one thousand small calories. One large
calorie is the energy required to heat 1 l of water by 1 °C. One full
sized Mars bar has 260 calories so it has enough energy to boil two full
kettles of water if burned and all the energy transferred as heat to
the water
This
is how the calorific heat content of materials was traditionally
measured in a lab: Burn a known weight of material in an insulated
chamber called a bomb calorimeter and measure by how much it raises the
temperature of a known quantity of water.
The Joule, the metric unit of energy
Back
to metric. Energy in the SI system is measured in joules. The symbol
for joules is J, watts is W and seconds is S, so 1 J equals 1 W for 1 S.
A one bar electric fire with a 1000 W bar uses 1000 J in 1 S. Similarly
a 20 W LED bulb uses 20 J in 1 S.
Water
has a specific heat capacity of 4200 J/kg °K. (read as "joules per
kilogram per degree kelvin). Compare this to iron for instance which
only has a SHC of 451 J/kg °K. Soil has a SHC of between 800 and 1480
J/kg °K, depending on whether it's dry or wet. The high SHC of water has
several consequences. It's responsible for our mild climate because the
Atlantic Ocean holds a lot of heat and keeps us relatively warm in
winter. In the summer, the ocean prevents the air temperature from
becoming too high. However regions that are a long distance inland and
far from the ocean have a continental climate because the land surface
has such a relatively low SHC and doesn't hold so much heat. The low SHC
causes the ground surface to heat up fast in summer so ambient air
temperature becomes high, but in winter, temperatures plummet.
Sea breezes.
The
difference in SHC of sea versus land is responsible for how sea breezes
change direction between day and night. In the daytime, land heats up
quicker than the sea and air moves towards land to replace air that
rises as it increases in temperature. This is because the air becomes
less dense as it expands and therefore more buoyant, just like a helium
or hot air balloon rises upwards (that's how your fireplace and chimney
work also). During the night the opposite happens: Land cools down
quicker than the sea, so rising air over the sea causes air movement
away from the land.
What material has the highest specific heat capacity?
Hydrogen has a SHC of 14,307 kJ/kg K, over three times that of water.
On the Engineering Toolbox site. Specific heat capacity of various materials.