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HyperPhysics - Capacitors

PY106 Notes - Capacitors and batteries
Clear, concise introduction to the concept of capacitance, capacitors, and dielectric materials.

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Special points of emphasis


Newton's Laws


When you study capacitors, remember that the parallel plate capacitor is a favorite device of MCAT writers. This is because it is an excellent model system for basic physics questions.

If the plates are large relative to the distance between them, the region between capacitor plates contains an electric field that is almost nearly uniform. No matter where you place a test-charge in this region, the magnitude and direction of the electric force produced will be the same. This means that the force acting on a charged particle between the plates will be constant.

The region between capacitor plates is a constant acceleration environment for charged particles. The movement of a charged particle between capacitor plates is similar in many ways to the projectile motion of an object in a uniform gravitational field.

Yes, I know I have mentioned that a few times already, but you will be glad to have a good picture of how things work within a uniform electric field, both because it is a great place to practice conceptualizing concepts like electric potential and because it shows up on the MCAT frequently.


DC Current

AC Current

Before we move on from capacitors, let's just mention that we will have much more to say about them in the discussion of AC circuits. Along with inductors, capacitors have a prominent role as an important kind of circuit element. Capacitors are especially important in AC circuits.

A key concept to be thinking about capacitors to remember that in a circuit a charged capacitor is its own voltage source. As a capacitor takes on charge, it stores energy. A charged capacitor becomes polarized in opposition to the voltage source that is charging it.

In AC current, one of the biggest hurdles is coming to the understanding of the cycle of charging and discharging that occurs with a capacitor, as current washes back and forth between the plates.

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