A “cathode” is one of the two main terminals found in electronic devices that involve the flow of electric current. The specific definition of a cathode can vary based on the type of device and the context in which it’s being described. Here’s an overview:
General Definition:
- The cathode is typically the terminal where current flows out of a device, returning to the external circuit.
Batteries:
- Primary (Non-rechargeable) Batteries: During discharge, the cathode is the positive terminal where reduction occurs.
- Secondary (Rechargeable) Batteries: During charging of batteries like lithium-ion, the cathode becomes the negative terminal. However, during discharging, it reverts to being the positive terminal.
Electrolytic Cells:
- In an electrolytic cell, the cathode is the electrode where reduction takes place. It can be either a positive or negative electrode, depending on the specific type of cell.
Diodes:
- In semiconductor diodes, the cathode is typically marked with a stripe and is the negative terminal (or the “N” side).
Electroplating:
- During electroplating, the cathode is the item that is to be plated. Metal ions from the solution are reduced at the cathode and deposit onto the item.
Electron Tubes (like CRTs):
- In electron tubes, the cathode is a heated element that emits electrons. These electrons then flow toward the positively charged anode, sometimes passing through a control grid that regulates the flow of electrons.
Fuel Cells:
- In fuel cells, the cathode is where reduction happens, typically involving oxygen to produce water.
X-ray Tubes:
- In X-ray tubes, electrons emitted from the cathode collide with the anode, resulting in X-ray production.
In terms of symbolism:
- On a circuit diagram, cathodes are typically marked with a “minus” (−) sign if the polarity is essential in the context of the diagram.
The understanding of “cathode” is often paired with its counterpart, the “anode.” The specific roles and labels of cathodes and anodes can vary depending on the device and its operational context.