Cathode

A cathode [ kato ː də ] (also cathode, from Ancient Greek κάθοδος káthodos " way back ", literally " way down " ) is an electrode which are supplied to the electron a system. A system can be a vacuum (see the electron tube ), a gas phase (see the gas discharge tube ), a plasma or an electrolyte. According to the definition, for example, positive ions (cations) can take place at this electrode reduction reactions are discharged.

The cathode is the counter electrode to the anode. Between these electrodes, ions or free electrons. The cations migrate to the cathode and anions to the anode.

(-) According to the present electrical polarity between the electrodes of a cathode either a positive ( ) or a minus is assigned. In voluntary running redox reactions such as the discharging of batteries, the cathode is the positive electrode. In a forced by applied voltage the redox reaction, such as the electrolysis, the cathode is negatively polarized electrode.

Rechargeable batteries (accumulators ) is the same electrode operates as either anode or cathode depending on whether the battery is charged or discharged.

Chemistry

In the chemical industry, a cathode is the electrode at which reduction reaction takes place. Electrons are supplied through the electrical conductors and delivered to the chemical reactant through the electrode. The electrochemical reaction takes place always in the phase boundary between the electrode and the electrolyte solution (or an ion-conducting solid electrolyte, or a melt).

Examples of cathode reactions:

Deposition of elemental copper as a partial reaction in a Daniell cell:

Release of gaseous hydrogen as a partial reaction of water electrolysis:

Electrical Engineering

In electrical engineering, the cathode is an electrode of a cathode ray tube, fluorescent lamp, diode, fuel cell, lead-acid battery and so on.

In this case, the cathode is the electrode, the electrons pass into the surrounding medium (electrolyte, vacuum, silicon). Electrons move in the observed component of the cathode to the anode, and then to flow through the external electrical circuit from the anode to the cathode. As the reference direction for the current flow in a positive charge carrier and is thus in the opposite direction related to the direction of movement of electrons, so the current in the external circuit from the cathode to the anode passes. Within the component in the current flowing from the anode to the cathode; the circuit is closed.

This statement has nothing to do with whether the potential of the anode is higher or lower than the potential of the cathode; or in other words if the voltage from anode to cathode is positive or negative. There are the terms positive and negative poles. In this case, the potential of the positive pole is always greater than the potential of the negative terminal. Therefore, the voltage from the positive pole to the negative pole always exceeds zero. To give positive pole and negative pole to the stress direction, while the anode and cathode associated with the current direction.

On components, where the anode has a positive voltage relative to the cathode, electric energy into another form of energy (heat, chemical energy, ...) is changed, such as a diode, a cathode ray tube or to an accumulator which is charged.

On components, where the anode has a negative voltage relative to the cathode, electric power is delivered at the expense of another form of energy (e.g., chemical energy ) to the external electric circuit, such as a fuel cell or a storage battery, the discharge will.

• When an applied voltage exceeds the material-specific work function of the negative electrode, from the contact material electrons ( field emission ); The material at this point to the cathode.
• The material- specific work function is applied more easily when we apply additional energy - for example, by increasing temperature. An important application of this effect is the hot cathode ( thermionic emission ) at a tube amplifier.
• When supplying additional energy by irradiation with light, it is called a photocathode (see photoelectric effect ).

Lighting technology

In flat panel so-called cold cathode lamps for display backlighting can be used. It is to fluorescent lamps, which are often much thinner than conventional tubes. Normal fluorescent lamps have a heated filament, the temperature of the exit of the electrons from the wire favored ( thermionic emission ). In cold cathode tubes ( CFL tube ), however, the voltage between both ends of the tube is increased to achieve a comparable effect. This, however, an electronic ballast is required in every case, which provides the high voltage.

The advantage of CFL tubes is on one hand a lower heat emission, on the other hand, in the longer life as the cathode not ' wear out ' and finally in the small size.

The thin fluorescent lamps can be used in many ways and are often used by both furniture designers as well as the PC modding, often where the interior of equipment is illuminated. As with normal fluorescent lamps producing the tubes in many different colors is possible.

Other applications

Very time-consuming cathode is found in plants for high-voltage direct -current transmission, for example in the form of buried in the sea giant copper rings; as in electroplating or in batteries (eg car batteries ) and the cathodic corrosion protection.