Circuits – Capacitors, Part 2 | MCAT Physics Prep

Need help preparing for the MCAT physics section? This video is Part 2 of a two-part series where MedSchoolCoach expert, Ken Tao, teaches you what you need to know about capacitors- within circuits. Watch this video to learn how to do well on the physics section of the MCAT exam!

When a capacitor is charged, one plate takes on a positive charge and one plate takes on a negative charge. As it turns out, there is a uniform electric field produced between these capacitor plates. A uniform electric field has the same magnitude and direction at all points within it. If you placed an electric charge at any point in the electric field, it would be affected by a consistent electric force. The strength of the electric field within a capacitor is shown by the relationship below, where V is the voltage across the capacitor plates and d is the distance between the plates.

V = Ed

Dielectrics

Recall that between the parallel plates of a capacitor, there can be either a vacuum or an insulating material. Dielectrics are insulating materials that are polarizable, such that their molecules have molecular dipoles that can be oriented in a particular direction. Adding a dielectric material to a capacitor can increase its capacitance. The molecules of the dielectric have positive and negative ends, so when its added to the middle of a capacitor, the positive ends will be attracted to the negatively charged capacitor plate and the negative ends will be attracted to the positively charged capacitor plate. The separation of charges created within the dielectric is responsible for the formation of an electric field. Electric fields are additive, and so the total electric field within the capacitor is then equal to the electric field of the dielectric material plus the electric field of the capacitor itself. However, if you imagine the orientation of positive and negative charges in the capacitor and dielectric, the electric fields must be in opposite directions. The positive charges of the dielectric are oriented toward the negatively charged plate, and vice versa. Therefore, when you add the two electric fields together, they partially cancel each other out. The final result is that the electric field of the capacitor is overall reduced.

The equation V = Ed tells us that if the electric field of a capacitor is reduced, the voltage across the capacitor plates is reduced. Current flows across a capacitor until the voltage of the capacitor is equal that of the battery. Therefore, more charge will be able to accumulate on the plates of the capacitor, because the net voltage they generate will be reduced by the dielectric. Due to their ability to increase the amount of stored charge on parallel plates, a dielectric is said to increase the capacitance of a capacitor.

Watch Circuit Capacitors Part 1:
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