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Planck's constant, denoted h, is a physical
constant that is used to describe the sizes of quanta. It plays a central role in
the theory of quantum mechanics, and is named after Max Planck, one of the founders of quantum theory. It has a value of approximately
- .
Planck's constant has units of energy multiplied by time, which are the units of action. These units may also be written as momentum times distance (N·m·s), which are the units of
angular momentum.
A closely-related quantity is the reduced Planck constant (sometimes called Dirac's constant):
-
where π is the constant pi. This constant is pronounced as "h-bar".
Planck's constant is used to describe quantization, a phenomenon occurring in microscopic particles such as electrons and photons in which certain physical properties occur in fixed amounts rather than assuming a continuous range of
possible values. For instance, the energy E carried by a beam of light with constant frequency ν can
only take on the values
-
It is sometimes more convenient to use the angular frequency
ω=2πν, which gives
-
Many such "quantization conditions" exist. A particularly interesting condition governs the quantization of angular momentum. Let J be the total angular momentum of a system
with rotational invariance, and Jz the angular momentum measured along any given direction. These quantities
can only take on the values
-
Thus, may be said to be the "quantum of
angular momentum".
Planck's constant also occurs in statements of Heisenberg's
uncertainty principle. The uncertainty in any position measurement, Δx, and the uncertainty in a momentum measurement
along the same direction, Δp, obeys
-
There are a number of other such pairs of physically measurable values which obey a similar rule.
On some browsers, the Unicode symbol ℎ (ℎ) is rendered as Planck's
constant, and the symbol ℏ (ℏ) is rendered as Dirac's constant.
See also
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