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When
you touch a spoon or walk on ceramic tiles, they feel cold, yet
expanded polystyrene packing feels warm as though it’s generating heat.
How come?
It’s because of the differing thermal conductivities of the two materials. Heat capacity also has a part to play.
So what are these physical properties?
A
piece of metal, tile and polystyrene left in a room for a sufficiently
long period will all have stabilised at a temperature equal to the
ambient temperature in the room. However metal and to a lesser extent
ceramic have a much higher thermal conductivity than expanded
polystyrene. Thermal conductivity is a measure of the rate at which a
material can transfer heat energy.
Difference between heat and temperature
• Heat
is the amount of thermal energy an object has. It's measured in joules.
2 kilos of a substance has double the thermal energy of 1 kilo of that
substance at the same temperature.
• Temperature
is a measure of the amount of activity of the atoms in a substance.
It's the movement of atoms: vibration, spin and translation (movement
from one place to another) that gives rise to the temperature of an
object. The greater the movement, the higher the temperature. If you've
ever been splashed by grease when cooking or hit by a spark from a fire,
they were at a temperature of several hundred degrees. However because
the splash or spark were small, they didn't have a lot of heat energy
and cooled down quickly, compared to having a larger amount of the
material making contact with the skin.
Back to the spoons
The
temperature of the surface of your finger is likely to be over 30
degrees Celsius, however the temperature in a room would be typically 10
degrees or more less than this. Heat always flows from a region of high
temperature to one of lower temperature. When you touch a spoon, heat
flows from your warm finger to the spoon and the cooling effect is
detected by temperature sensors in your skin. The cooling effect is
quicker for materials such as metal with a high thermal conductivity
because they transfer heat energy faster. The scenario is a little
different when touching expanded polystyrene. Because a room will be
colder than your finger, your finger is constantly cooling down as air
moves around it. However if the room temperature is constant,
equilibrium is eventually reached and the surface of your finger will
eventually settle at a temperature cooler than that under the surface of
the skin, assuming your body continues to provide heat. When you touch
a piece of polystyrene, because it has a low thermal conductivity, heat
flows into it at a very low rate. However if it covers the pad of your
finger, the heat loss from your finger to the surrounding air will be
reduced. That reduction is likely to be greater than the heat transfer
to the polystyrene, so your finger will tend to warm up. So it’s not the
polystyrene that’s generating heat, making it appear warm, but your
finger heating up because it no longer cools as it loses heat to the
surrounding air.
Thermal conductivity is measured in watts per meter-kelvin ( W/(m K) )
Joules and Kelvin
Temperature is measured in degree Celsius. The zero point on the scale is the freezing point of water and 100 degrees is the temperature water boils at. For scientific purposes, the Kelvin scale is used, and the only difference between it and the Celsius scale is that the zero point is shifted to absolute zero. Absolute zero is when all movement of atoms ceases. It has never been reached, but we have cooled substances to within tiny fractions of a degree above absolute zero. The divisions on the Celsius and Kelvin scales are the same, so a temperature change of 1 °C = 1 K. (Note that the ° symbol is not used with the unit’s symbol “K”.)
Why does it feel colder on damp days?
Higher
thermal conductivity of water versus air explains why damp days feel
colder than when air humidity is lower, even though the air temperature
is the same. Winter temperature in continental climates is often much
less than in Ireland, but because of the dry air, it never feels as
cold.
Heat capacity
This
is another property of matter that can influence how we perceive
cooling. It’s defined as the amount of heat required to change the
temperature of a substance by 1 K. Water compared to other liquids has a
huge heat capacity and requires lots of heat energy to raise its
temperature. The specific heat capacity of a substance is the amount of
heat energy required to raise 1 kg of that substance by 1 K. The SHC of
water is approximately 4200 J/(kg K). (Expressed as 4200 joules per
kilogram per degree kelvin.)
If
you dip your finger into water, since water has a high SHC, it
continues to drain heat energy, lowering the finger's temperature. A
liquid with a lower SHC such as olive oil would rise in temperature
faster, and if there was no mixing of the liquid, this would reduce the
thermal gradient, slowing heat loss. So olive oil wouldn't feel as cold
if you put your finger in it.
Resources
Thermal conductivities of various materials:
https://www.engineeringtoolbox.com/thermal-conductivity-d...
https://www.engineeringtoolbox.com/thermal-conductivity-d...
Specific heat capacities if different materials: