Capacitors
Describe the physical properties of a parallel-plate capacitor.
- A parallel-plate capacitor consists of two separated parallel conducting surfaces that can hold equal amounts of charge with opposite signs.
- Capacitance relates the magnitude of the charge stored on each plate to the electric potential difference created by the separation of those charges. Relevant equation:
- The capacitance of a capacitor depends only on the physical properties of the capacitor, such as the capacitor’s shape and the material used to separate the plates.
- The capacitance of a parallel-plate capacitor is proportional to the area of one of its plates and inversely proportional to the distance between its plates. The constant of proportionality is the product of the dielectric constant, κ, of the material between the plates and the electric permittivity of free space, ε0. Relevant equation:
- The electric field between two charged parallel plates with uniformly distributed electric charge, such as in a parallel-plate capacitor, is constant in both magnitude and direction, except near the edges of the plates.
- The magnitude of the electric field between two charged parallel plates, where the plate separation is much smaller than the dimensions of the plates, can be described with the equation:
- A charged particle between two oppositely charged parallel plates undergoes constant acceleration and therefore its motion shares characteristics with the projectile motion of an object with mass in the gravitational field near Earth’s surface.
- The electric potential energy stored in a capacitor is equal to the work done by an external force to separate that amount of charge on the capacitor.
- The electric potential energy stored in a capacitor is described by the equation:
- Adding a dielectric between two plates of a capacitor changes the capacitance of the capacitor and induces an electric field in the dielectric in the opposite direction to the field between the plates.
While other shapes are also able to separate charges, only the analysis and descriptions of parallel-plate capacitors are required for AP Physics 2. Edge effects will be ignored unless explicitly stated otherwise.