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.. _physical_types:
Physical Types
**************
A physical type corresponds to physical quantities with dimensionally
compatible units. For example, the physical type *mass* corresponds to
physical quantities with units that can be converted to kilograms.
Physical types are represented as instances of the |PhysicalType| class.
Accessing Physical Types
========================
.. EXAMPLE START: Accessing Physical Types
Using :func:`~astropy.units.get_physical_type` lets us acquire |PhysicalType|
instances from strings with a name of a physical type, units, |Quantity|
instances, objects that can become quantities (e.g., numbers), and
|PhysicalType| instances.
>>> import astropy.units as u
>>> u.get_physical_type('speed') # from the name of a physical type
PhysicalType({'speed', 'velocity'})
>>> u.get_physical_type(u.meter) # from a unit
PhysicalType('length')
>>> u.get_physical_type(1 * u.barn * u.Mpc) # from a Quantity
PhysicalType('volume')
>>> u.get_physical_type(42) # from a number
PhysicalType('dimensionless')
The physical type of a unit can be accessed via its
:attr:`~astropy.units.UnitBase.physical_type` attribute::
>>> u.coulomb.physical_type
PhysicalType('electrical charge')
>>> (u.meter ** 2).physical_type
PhysicalType('area')
.. EXAMPLE END
Using Physical Types
====================
.. EXAMPLE START: Using Physical Types
An equality comparison between a |PhysicalType| and a string will return
`True` if the string is a name of the |PhysicalType|::
>>> acceleration = u.get_physical_type(u.m / u.s ** 2)
>>> acceleration == 'acceleration'
True
Some units may correspond to multiple physical types because compatible
units can be used to quantify different phenomena::
>>> u.get_physical_type('pressure')
PhysicalType({'energy density', 'pressure', 'stress'})
We can iterate through the names of a |PhysicalType|::
>>> for name in u.J.physical_type: print(name)
energy
torque
work
We can test for membership or equality with a string that has the name
of a |PhysicalType|::
>>> 'energy' == u.J.physical_type
True
>>> 'work' in u.J.physical_type
True
.. EXAMPLE END
Dimensional Analysis
====================
.. EXAMPLE START: Dimensional Analysis With Physical Types
|PhysicalType| instances support multiplication, division,
and exponentiation. Because of this, they can be used for
dimensional analysis::
>>> length = u.get_physical_type('length')
>>> time = u.get_physical_type('time')
>>> length ** 2
PhysicalType('area')
>>> 1 / time
PhysicalType('frequency')
Dimensional analysis can be performed between a |PhysicalType| and a
unit or between a |PhysicalType| and a string with a name of a
|PhysicalType|::
>>> length ** 2 / u.s
PhysicalType({'diffusivity', 'kinematic viscosity'})
>>> length / 'time'
PhysicalType({'speed', 'velocity'})
.. EXAMPLE END
|
