A cathode is an electrode through which electrical current exits a polarized electrical device. Its opposite is the anode, through which electrical current enters the electrical device. Positively charged ions, called cations, move towards the cathode, while negatively charged ions, called anions, move towards the anode. In devices that provide power, like discharging batteries, the cathode is positively charged, but in power-consuming devices, including recharging batteries, it is negatively charged.
In chemistry, a cathode is the electrode of an electrochemical cell at which reduction, or the gain of electrons, occurs. If the electrochemical cell is electrolytic, meaning it is supplied with electrical energy in order to break down chemical compounds, its cathode is negative. In the opposite type of electrochemical cell, the galvanic cell, chemical reactions cause the release of electrical energy, and it becomes positive.
Negative polarity must be applied to the cathode of an electrolytic cell in order to drive the desired chemical reactions. In a galvanic cell, on the other hand, a positive pole must be connected in order to complete the circuit. The electrons emitted by the anode return to the cell through the cathode when the circuit is complete.
In physics and electronics, a cathode is the electrode responsible for emitting electrons into the device, causing it to run. The freely emitted electrons may be derived in a number of ways, either through heat or light energy, or through a strong external electromagnetic field. The latter method of driving electron emission is known as field electron emission, and the cathodes involved are called cold cathodes.
Though they may operate at high temperatures, cold cathodes are distinguished from the "hot" variety, because they are not directly heated in order to drive electron emission. Hot cathodes, also known as filaments, are much more common in modern electronics. Cathodes that derive their energy from light are powered by photoelectric emission, and are used in photovoltaic, or solar energy, applications. Different types of metal have electrodes suited to different methods of providing electrical power.