Capacitor

A capacitor is an  electronic component which stores and releases electrical energy.  It can store charge for a certain period and release it when needed.  

Structure of a Capacitor

A capacitor consists of:

• Two conductive plates that so not touch
• A dielectric (insulator) between the plates. In a simple parallels plate capacitor, the dielectric is often air, but material like ceramic can enhance performance
• Electrical connections that allow the capacitor to be connected to an external voltage source

How does a capacitor work?

1.         Applying voltage:

•  When a voltage source is connected, electrons redistribute between the plates  
•  Electrons move from the plate connected to the positive terminal of the source, making it positively charged
• Electrons accumulate on the plate connected to the negative terminal, making it negatively charged

2.         Charge building:

• The two plated hold equal and opposite charges (magnitude Q)
• The electric field created by the separated charges opposes the applied voltage

3.         Reaching equilibrium:

• As charge accumulated, the voltage across the capacitor increases
• The charging process stops when the capacitor‘s voltage equals the source voltage

4.         Energy storage:

• Even when the voltage source is removed, the charges remain stored in the capacitor
• The stored energy can be released later by connecting a circuit element

Capacitance and charge storage

The charge Q stored in a capacitor is directly proportional to the voltage V:

Q=C x V

C is the capacity with the unit Farads (F).

A capacitor has a capacitance if 1F if it stored 1 Coulomb (C) of charge when charged to 1 Volt (V).

The capacity tells us how much charge a capacitor can store at a given voltage. It determines the capacitor‘s  ability to release energy in a circuit.

Thermionic Emission (Edison Effect)

Thermionic emission, also know as the Edison effect, is the phenomenon where electrons are emitted from a heated metal surface. Thomas Edison discovered this effect and improved the light bulb with his discovery.

In a metal, electrons are attracted by the Coulomb force from atomic nuclei. At normal temperatures, electrons lack the energy to escape the metal surface.

As a current heats up the metal, electrons gain kinetic energy. When they reach the critical energy threshold, they can overcome the attraction of the nuclei and escape the metal.

You can compare the electron emission to the evaporation of water:

Just as water molecules need heat energy to escape the  surface tension of water (due to the forces from neighboring  molecules), electrons need energy to escape from metal.

Escaping electrons form a negative charge cloud around the hot metal. If a positively charges electrode is placed nearby, the electrons are attracted toward it, allowing current to flow.