Temperature
The Earth - We are all affected by a property of the Universe called temperature
The next SI Unit is the second (S). Time is something we all feel passing but it can in fact be
bent and twisted and is actually part of the very fabric of the Universe. Other SI units are
available from the menus at the top of the page.
Unlike some of the other SI units temperature is one of those things that we instinctively have
an understanding for. Temperature-sensitive nerve endings in our skin send signals to the
brain to tell us when we are too hot or too cold. But how cold is "cold" and is it possible to lower
temperatures indefinitely?
The two most widely used measurements of temperature are Fahrenheit and Celsius. The first
was proposed in 1724 and is still the official scale in the United States. On this scale the
freezing point of water is 32 degrees Fahrenheit (°F) and the boiling point 212 °F. The other is
called the Celsius scale which is also defined by the freezing and boiling points of water. In this
scale water freezes at close to 0 degrees Celsius (°C) and boils at close to 100 °C. The reason
I say "close to" is because the modern definition of Celsius is quite complex, but we don't need
to worry about it here. For our purposes we can say that water freezes at °0 C and boils at 100
°C.
These two scales work very well on an everyday basis. We know that it's cold when the
temperature is 5 °C (41 °F) and that it's hot when the temperature is 30 °C (86 °F). However,
temperature is unusual in that it has an absolute starting point.
Ice forms at 0 degrees Celsius (°C)
If we consider the SI unit for length the starting and end points are not fixed. A two metre
ruler can be placed anywhere and it will still measure two metres. Temperature is different
and has a fixed starting point called absolute zero, which is defined as zero kelvin (0 K) and
is the equivalent of –273.15 °C (–459.76 °F).
Nothing, no matter how much it's cooled, can be as cold or colder than this temperature. This
is because temperature is a consequence of the movement of particles – the faster a particle
moves the hotter it is, and the slower a particle moves the colder it is, and it's not possible for
a particle to be absolutely still. Particles, all particles, such as molecules, atoms, electrons
and so on, would cease to move at all at absolute zero and so it's not possible to lower their
temperature to this point or any further.
An important point is that the kelvin scale has the same graduation scale as Celsius. That is,
a temperature difference of 10 degrees on the kelvin scale is the same as a temperature
difference of 10 degrees on the Celsius scale. To convert from one to the other we simply
add or subtract plus or minus 273.15. For example, 30 degrees C is:
30 + 273.15 = 303.15 kelvin
Note that it's incorrect to say "degrees kelvin" and the unit is just K, not °K.
Below are are some examples of temperatures and their equivalents in other scales (note that
beyond a certain point the percentage difference is so small that it makes sense to simply
consider kelvin and °C as being equivalent):
Absolute zero
0 K
–273.15 °C / –459.76 °F
Boiling point of water
373.134 K
99.984 °C / 212 °F
Incandescent lamp
2500 K
2,000 °C / 3632 °F
The Sun’s surface
5,778 K
5,505 °C / 9941 °F
A lightning bolt
28,000 K
28,000 °C / 50,000 °F
The Sun’s core
16 million K
16 million K
An H-bomb’s core
350 million K
350 million K
Particle collisions in
1 – 10 trillion K
1 – 10 trillion K
nuclear colliders
The Universe 5.4 x 10
–44
1.417 x 10
32
K
seconds after the Big Bang
Finally, temperature sensitive nerve endings were mentioned at the start of the page. Here's a
fun little experiment to try. Gently touch different spots on the back of your hand using the tip of
a pencil or a fork's prong. Most of the time you will only feel a little pressure but every so often
you will touch a temperature sensing nerve-end which will feel noticeably cold.
Click for kelvin to Celsius and Fahrenheit to Celsius converters.
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Temperature
The Earth - We are all affected by a property of the
Universe called temperature
The next SI Unit is the second (S). Time is something we all feel
passing but it can in fact be bent and twisted and is actually part
of the very fabric of the Universe. Other SI units are available
from the menus at the top of the page.
Unlike some of the other SI units temperature is one of those
things that we instinctively have an understanding for.
Temperature-sensitive nerve endings in our skin send signals to
the brain to tell us when we are too hot or too cold. But how cold
is "cold" and is it possible to lower temperatures indefinitely?
The two most widely used measurements of temperature are
Fahrenheit and Celsius. The first was proposed in 1724 and is
still the official scale in the United States. On this scale the
freezing point of water is 32 degrees Fahrenheit (°F) and the
boiling point 212 °F. The other is called the Celsius scale which is
also defined by the freezing and boiling points of water. In this
scale water freezes at close to 0 degrees Celsius (°C) and boils
at close to 100 °C. The reason I say "close to" is because the
modern definition of Celsius is quite complex, but we don't need
to worry about it here. For our purposes we can say that water
freezes at °0 C and boils at 100 °C.
These two scales work very well on an everyday basis. We know
that it's cold when the temperature is 5 °C (41 °F) and that it's hot
when the temperature is 30 °C (86 °F). However, temperature is
unusual in that it has an absolute starting point.
Ice forms at 0 degrees Celsius (°C)
If we consider the SI unit for length the starting and end points
are not fixed. A two metre ruler can be placed anywhere and it
will still measure two metres. Temperature is different and has a
fixed starting point called absolute zero, which is defined as zero
kelvin (0 K) and is the equivalent of –273.15 °C (–459.76 °F).
Nothing, no matter how much it's cooled, can be as cold or
colder than this temperature. This is because temperature is a
consequence of the movement of particles – the faster a particle
moves the hotter it is, and the slower a particle moves the colder
it is, and it's not possible for a particle to be absolutely still.
Particles, all particles, such as molecules, atoms, electrons and
so on, would cease to move at all at absolute zero and so it's not
possible to lower their temperature to this point or any further.
An important point is that the kelvin scale has the same
graduation scale as Celsius. That is, a temperature difference of
10 degrees on the kelvin scale is the same as a temperature
difference of 10 degrees on the Celsius scale. To convert from
one to the other we simply add or subtract plus or minus 273.15.
For example, 30 degrees C is:
30 + 273.15 = 303.15 kelvin
Note that it's incorrect to say "degrees kelvin" and the unit is just
K, not °K.
Below are are some examples of temperatures and their
equivalents in other scales (note that beyond a certain point the
percentage difference is so small that it makes sense to simply
consider kelvin and °C as being equivalent):
Finally, temperature sensitive nerve endings were mentioned at
the start of the page. Here's a fun little experiment to try. Gently
touch different spots on the back of your hand using the tip of a
pencil or a fork's prong. Most of the time you will only feel a little
pressure but every so often you will touch a temperature sensing
nerve-end which will feel noticeably cold.
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