[FEAT] Add Coulomb's Law for electrostatics (#7017)

Co-authored-by: Priyanshu1303d <priyanshu130d@gmail.com>

src/test/java/com/thealgorithms/physics/CoulombsLawTest.java

@@ -0,0 +1,100 @@
+package com.thealgorithms.physics;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Unit tests for the CoulombsLaw utility class.
+ */
+final class CoulombsLawTest {
+
+    // A small tolerance (delta) for comparing floating-point numbers
+    private static final double DELTA = 1e-9;
+    private static final double K = CoulombsLaw.COULOMBS_CONSTANT;
+
+    @Test
+    @DisplayName("Test repulsive force between two charges on the x-axis")
+    void testSimpleRepulsiveForce() {
+        // Two positive 1C charges, 1 meter apart.
+        // Force on q2 should be F = K*1*1 / 1^2 = K, directed away from q1 (positive x)
+        double[] forceOnB = CoulombsLaw.calculateForceVector(1.0, 0, 0, 1.0, 1, 0);
+        assertArrayEquals(new double[] {K, 0.0}, forceOnB, DELTA);
+
+        // Force on q1 should be equal and opposite (negative x)
+        double[] forceOnA = CoulombsLaw.calculateForceVector(1.0, 1, 0, 1.0, 0, 0);
+        assertArrayEquals(new double[] {-K, 0.0}, forceOnA, DELTA);
+    }
+
+    @Test
+    @DisplayName("Test attractive force between two charges on the x-axis")
+    void testSimpleAttractiveForce() {
+        // One positive 1C, one negative -1C, 1 meter apart.
+        // Force on q2 should be F = K*1*(-1) / 1^2 = -K, directed toward q1 (negative x)
+        double[] forceOnB = CoulombsLaw.calculateForceVector(1.0, 0, 0, -1.0, 1, 0);
+        assertArrayEquals(new double[] {-K, 0.0}, forceOnB, DELTA);
+    }
+
+    @Test
+    @DisplayName("Test electrostatic force in a 2D plane (repulsive)")
+    void test2DRepulsiveForce() {
+        // q1 at (0,0) with charge +2C
+        // q2 at (3,4) with charge +1C
+        // Distance is 5 meters.
+        double magnitude = K * 2.0 * 1.0 / 25.0; // 2K/25
+        // Unit vector from 1 to 2 is (3/5, 4/5)
+        double expectedFx = magnitude * (3.0 / 5.0); // 6K / 125
+        double expectedFy = magnitude * (4.0 / 5.0); // 8K / 125
+
+        double[] forceOnB = CoulombsLaw.calculateForceVector(2.0, 0, 0, 1.0, 3, 4);
+        assertArrayEquals(new double[] {expectedFx, expectedFy}, forceOnB, DELTA);
+    }
+
+    @Test
+    @DisplayName("Test overlapping charges should result in zero force")
+    void testOverlappingCharges() {
+        double[] force = CoulombsLaw.calculateForceVector(1.0, 1.5, -2.5, -1.0, 1.5, -2.5);
+        assertArrayEquals(new double[] {0.0, 0.0}, force, DELTA);
+    }
+
+    @Test
+    @DisplayName("Test circular orbit velocity with simple values")
+    void testCircularOrbitVelocity() {
+        // v = sqrt( (K*1*1 / 1^2) * 1 / 1 ) = sqrt(K)
+        double velocity = CoulombsLaw.calculateCircularOrbitVelocity(1.0, 1.0, 1.0, 1.0);
+        assertEquals(Math.sqrt(K), velocity, DELTA);
+    }
+
+    @Test
+    @DisplayName("Test orbital velocity for a Hydrogen atom (Bohr model)")
+    void testHydrogenAtomVelocity() {
+        // Charge of a proton
+        double protonCharge = 1.602176634e-19;
+        // Charge of an electron
+        double electronCharge = -1.602176634e-19;
+        // Mass of an electron
+        double electronMass = 9.1093837e-31;
+        // Bohr radius (avg distance)
+        double bohrRadius = 5.29177e-11;
+
+        double expectedVelocity = 2.1876917e6;
+
+        double velocity = CoulombsLaw.calculateCircularOrbitVelocity(protonCharge, electronCharge, electronMass, bohrRadius);
+        // Use a wider delta for this real-world calculation
+        assertEquals(expectedVelocity, velocity, 1.0);
+    }
+
+    @Test
+    @DisplayName("Test invalid inputs for orbital velocity throw exception")
+    void testInvalidOrbitalVelocityInputs() {
+        // Non-positive mass
+        assertThrows(IllegalArgumentException.class, () -> CoulombsLaw.calculateCircularOrbitVelocity(1, 1, 0, 100));
+        assertThrows(IllegalArgumentException.class, () -> CoulombsLaw.calculateCircularOrbitVelocity(1, 1, -1, 100));
+        // Non-positive radius
+        assertThrows(IllegalArgumentException.class, () -> CoulombsLaw.calculateCircularOrbitVelocity(1, 1, 1, 0));
+        assertThrows(IllegalArgumentException.class, () -> CoulombsLaw.calculateCircularOrbitVelocity(1, 1, 1, -100));
+    }
+}