Law: linear density (#853)

* Create law and test

* Add definitions

* Fix documentation

symplyphysics/laws/quantities/__init__.py

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+"""
+Quantities
+==========
+
+Physical quantities can often be categorized as being either *extensive* or *intensive*, according to
+how the property changes when the size or extent of the system changes.
+
+Intensive quantity
+    A quantity whose magnitude is independent of the size of the system.
+
+Extensive quantity
+    A quantity whose magnitude scales linearly with the size of the system,
+    i.e. it is additive with respect to its subsystems.
+"""

symplyphysics/laws/quantities/quantity_is_linear_density_times_length.py

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+"""
+Extensive quantity is linear density times length
+=================================================
+
+An extensive quantity of interest can be obtained by multiplying the corresponding linear
+density by length.
+"""
+
+from sympy import Eq, Symbol as SymSymbol
+from symplyphysics import (
+    units,
+    Quantity,
+    Symbol,
+    validate_input,
+)
+from symplyphysics.core.dimensions import assert_equivalent_dimension
+
+extensive_quantity = SymSymbol("extensive_quantity")
+"""
+Extensive quantity
+
+Symbol:
+    X
+"""
+
+linear_density = SymSymbol("linear_density")
+r"""
+Intensive linear density
+
+Symbol:
+    lambda_X
+
+Latex:
+    :math:`\lambda_X`
+"""
+
+length = Symbol("length", units.length)
+"""
+Length
+
+Symbol:
+    L
+"""
+
+law = Eq(extensive_quantity, linear_density * length)
+r"""
+X = lambda_X * L
+
+Latex:
+    :math:`X = \lambda_X L`
+"""
+
+
+@validate_input(length_=length)
+def calculate_extensive_quantity(
+    linear_density_: Quantity,
+    length_: Quantity,
+) -> Quantity:
+    result_expr = law.rhs.subs({
+        linear_density: linear_density_,
+        length: length_,
+    })
+    result = Quantity(result_expr)
+
+    assert_equivalent_dimension(
+        result,
+        "return",
+        "calculate_extensive_quantity",
+        linear_density_.dimension * units.length,
+    )
+
+    return result

test/quantities/quantity_is_linear_density_times_length_test.py

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+from collections import namedtuple
+from pytest import fixture, raises
+from symplyphysics import (
+    assert_equal,
+    units,
+    Quantity,
+    errors,
+)
+from symplyphysics.laws.quantities import quantity_is_linear_density_times_length as law
+
+Args = namedtuple("Args", "m q l")
+
+
+@fixture(name="test_args")
+def test_args_fixture() -> Args:
+    m = Quantity(4.4 * units.kilogram / units.meter)
+    q = Quantity(3.5 * units.coulomb / units.meter)
+    l = Quantity(0.1 * units.meter)
+    return Args(m=m, q=q, l=l)
+
+
+def test_mass_law(test_args: Args) -> None:
+    result = law.calculate_extensive_quantity(test_args.m, test_args.l)
+    assert_equal(result, 0.44 * units.kilogram)
+
+
+def test_charge_law(test_args: Args) -> None:
+    result = law.calculate_extensive_quantity(test_args.q, test_args.l)
+    assert_equal(result, 0.35 * units.coulomb)
+
+
+def test_bad_length(test_args: Args) -> None:
+    lb = Quantity(units.second)
+    with raises(errors.UnitsError):
+        law.calculate_extensive_quantity(test_args.m, lb)
+    with raises(TypeError):
+        law.calculate_extensive_quantity(test_args.m, 100)