This applet is designed to allow you to explore the concept of the superposition of forces.
To get this applet to run faster on a slow computer, increase the "Speed" by upping that parameter with the button on the lower left.
To change the ratio of the charges (it is initially set to one), increase that ratio by hitting the corresponding button on the lower right.
To change the sign of one of the charges, hit the "reverse sign" button at the upper left. Red is positive charge, black is negative charge.
To move either change, click and drag on that charge.
The two blue vectors show the forces on a positive test charge. The green vector is the vector sum of those two individual forces, to show the total force. Click and drag in the vicinity of the tail of the green and blue vectors and you can explore the force on a test charge and see how it arises from adding up the two individual forces.