Add monopole electrostatic potential and field with unit tests

ffprime/electrostatics/multipole.py

@@ -0,0 +1,20 @@
+import numpy as np
+
+
+def monopole_potential(q: float, r_vec: np.ndarray) -> float:
+    r = np.linalg.norm(r_vec)
+
+    if r < 1e-12:
+        return 0.0
+
+    return q / r
+
+
+def monopole_field(q: float, r_vec: np.ndarray) -> np.ndarray:
+    r = np.linalg.norm(r_vec)
+
+    if r < 1e-12:
+        return np.zeros_like(r_vec)
+
+    return q * r_vec / r**3
+

tests/test_electrostatics.py

@@ -0,0 +1,28 @@
+import numpy as np
+from ffprime.electrostatics.multipole import (
+    monopole_potential,
+    monopole_field,
+)
+
+
+def test_monopole_potential_unit_distance():
+    q = 1.0
+    r_vec = np.array([1.0, 0.0, 0.0])
+    V = monopole_potential(q, r_vec)
+    assert np.isclose(V, 1.0)
+
+
+def test_monopole_field_direction():
+    q = 1.0
+    r_vec = np.array([1.0, 0.0, 0.0])
+    E = monopole_field(q, r_vec)
+    assert np.allclose(E, np.array([1.0, 0.0, 0.0]))
+
+
+def test_zero_distance_safe():
+    q = 1.0
+    r_vec = np.array([0.0, 0.0, 0.0])
+    V = monopole_potential(q, r_vec)
+    E = monopole_field(q, r_vec)
+    assert V == 0.0
+    assert np.allclose(E, np.zeros(3))