The magnitude of each of the charges in Figure 18.21 is 9.40 10-12 C. The lengths of the sides of the rectangle are 3.00 cm and 5.00 cm. Find the magnitude of the electric field at the center of the rectangle in Figures 18.21a and 18.21b. I know figure a is 198.22, but I can't seem to figure out b. Also I'm struggling with how to tell acceleration: A uniform electric field has a magnitude of 2.6 103 N/C. In a vacuum, a proton begins with a speed of 2.1 104 m/s and moves in the direction of this field. Find the speed of the proton after it has moved a distance of 2.0 mm. I can't seem how to find the acceleration. The closest I can figure is that F=ma for the formula E= f/q, but there is no mass, and that formula does not include a distance so I believe it is not the one I should be using. Any thoughts on how to do these?