physunits 0.0.8

Physical units as global variables for simple numerical propagation

physunits

Simple package with common physical unit definitions.

Installation

Add this package to your existing python environment with pip pip install physunits, or alternatively follow your distribution specific instructions.

Usage

An example usage of this package is:

>> from physunits import *
>> print(f"{1000 * cm / um:.2f} is how many microns there are in a thousand centimeters")
10000000.00 is how many microns there are in a thousand centimeters

The package tries to include only the most used unit prefixes, but you can generate other units, for example:

>> from physunits import *
>> minute = 60 * s
>> hour = 60 * minute
>> day = 24 * hour
>> print(f'There are approximately {365*day / hour} hours in a year.')
There are approximately 8760.0 hours in a year.

The unit prefixes act as simple globals. If only a subset of the supported units is desired, manually importing them as

from physunits import mm, nm, um

is good enough.

Another example where physunits may be useful is illustrated by the following script dealing with a voltage trace:

import numpy as np
import matplotlib.pyplot as plt
from physunits import *

f = 2.12*kHz
omega = 2*np.pi*f
t = np.linspace(-670*us, 410*us, 2**10)
my_voltage = (1.5*mV) * np.sin(omega*t)

# Now plot voltage in mV vs time in ms
plt.plot(t/ms, my_voltage/mV)
plt.xlabel(r't (ms)')
plt.ylabel(r'V (mV)')
plt.show()

which outputs the following plot:

Description

All units are referenced to the SI. This means that base, derived, freedom (imperial), and other units including prefixes are always relative to the SI base units. An example where this is manifest is that when imported, the unit kg takes on the numerical value 1.0, regardless of the prefix indicating the numerical factor of 1000.

Unit	Supported
length	nm, um, mm, cm, m, km, inch, ft, yd
time	ps, ns, us, ms, s
mass	ng, ug, mg, g, kg, lb, oz
temperature	nK, uK, mK, K
angle*	deg, rad, mrad
frequency	mHz, Hz, kHz, MHz, GHz, THz
voltage	pV, nV, uV, mV, V, kV
charge	C, mC, uC, nC
current	nA, uA, mA, A
resistance	ohm, mohm, kohm, Mohm
capacitance	F, mF, uF, nF, pF
inductance	H, mH, uH, nH
energy**	mJ, J, kJ, meV, eV, keV, MeV, GeV, TeV
power	nW, uW, mW, W, kW, MW
pressure	mPa, Pa, kPa, MPa, atm, psi, Torr, mBar, Bar
rel concentration	ppm, ppb

If you want a unit to be supported, feel free to open an issue.

• We all know that if there was an SI unit for angles, it would be the radian, so in physunits, it takes the value rad = 1.0 , and degrees are defined relative to it. This works nicely in all trig functions. Special thanks to chrisjbillington for pointing this out.

** While the electronvolt (eV) is not a part of the SI, the SI recognizes its use. Furthermore, the latest SI redefinition fixed the value of the electron charge constant, effectively fixing the conversion between eV and J.

Contact and support

Please report, fork, test, contribute, or create an issue directly on the project's repository.