Coulomb's law describes the force between two point charges, q-1 and q-2. What is the force experienced by a third point charge, Q? q-1 and q-2 are called source charges, and Q is called the test charge. Let Q experience force F-1 due to q-1, and F-2 due to q-2. Experiments show that the net force experienced by the test charge is the vector sum of F-1 and F-2. This is called the principle of superposition. If there are more than two source charges, the force on the test charge can be calculated similarly. Imagine the test charge is an electron at the origin; q-1 equals +8-e, placed on the x-axis at a distance 2-d from the origin, and q-2 equals +18-e, placed on the y-axis at 3-d from the origin. Then, F-1 and F-2 turn out to be equal in magnitude, directed along the x and y axes, respectively. Hence, the net force on Q is about 1.4 times this magnitude and is directed at 45 degrees to the x-axis.