A downward electric force of 6.4 N is exerted on a $-7.3 \; \mu \textrm{C}$ charge. Find the magnitude and direction of the electric field at the position of this charge.

VIDEO TRANSCRIPT

This is Giancoli Answers with Mr. Dychko. The direction of the electric field is the easiest thing to figure out first of all. Since the negative charge experiences a force downwards, electric field must be pointing upwards because electric field points in the same direction as force that would be exerted on a positive charge, and so it points in the opposite direction to the direction of force on the negative charge. If the force on this negative charge is down, electric field must be going upwards. For the size of the electric field, we can take this formula, force is the charge multiplied by the field strength. And rearrange it for E, divide both sides by q. And the force is 6.4 Newtons divided by the charge of 7.3 times 10 to the minus six Coulombs, this gives us 8.8 times 10 to the five Newtons per Coulomb directed upwards for the electric field.