The terms anode and cathode describe the flow of electrical energy. Correctly identifying these two terminals is a foundational skill in electrical work and electrochemistry, as their misapplication can lead to component failure or system malfunction. These labels define the orientation required for components like batteries, diodes, and capacitors to function. Understanding the physical markings and testing procedures associated with the anode and cathode ensures devices are connected safely and operate as intended.
Understanding the Core Definitions
The most consistent way to distinguish the anode from the cathode is by the chemical reaction that occurs at the electrode surface. The anode is universally defined as the electrode where oxidation takes place, involving the loss of electrons by a chemical species. Conversely, the cathode is the electrode where reduction occurs, meaning the gain of electrons from the circuit. A simple mnemonic to remember this relationship is “An Ox, Red Cat,” standing for Anode-Oxidation and Reduction-Cathode.
This chemical definition directly links to the direction of electron movement and conventional current flow, which can be a source of confusion. Electrons, which carry a negative charge, flow out of the anode and into the cathode through the external circuit of an electrical power source. Conventional current, defined as the flow of positive charge, travels in the opposite direction, entering the cathode and exiting the anode within the external circuit. Therefore, while the chemical reactions remain fixed, the charge polarity of the anode and cathode can vary depending on whether the device is generating power (like a discharging battery) or consuming power (like an electrolytic cell).
Identifying Polarity in DC Power Sources
In standard direct current (DC) power sources like household batteries, the physical terminals are permanently labeled to indicate the correct external connection. During discharge, a battery functions as a galvanic cell where the chemical reaction spontaneously generates electricity. The anode is the negative terminal because it is the source of electrons, and the cathode is the positive terminal because it receives electrons from the external circuit.
Manufacturers use specific markings to designate these poles. The positive terminal, which acts as the cathode during discharge, is marked with a plus sign (+). The negative terminal, the anode during discharge, is marked with a minus sign (-). For devices that use cable connections, a standardized color code is often utilized, with red indicating the positive terminal (cathode) and black indicating the negative terminal (anode).
In cylindrical batteries, the positive terminal is usually slightly raised or features a small bump, while the negative terminal is flat. This physical difference assists in correct orientation when inserting the battery into a device. While the internal chemical role switches when a rechargeable battery is being charged, the external positive and negative markings remain fixed to ensure the charger is connected correctly.
Identifying Polarity in Electronic Components
Many common electronic devices, such as diodes, Light Emitting Diodes (LEDs), and electrolytic capacitors, require a specific orientation to function. For standard cylindrical diodes, a stripe or band is painted around the casing near one of the leads. This stripe serves as the marker for the cathode (the negative side), which allows conventional current to flow from the anode side toward the band when the component is properly biased.
LEDs, which are a type of diode, offer several physical cues for identification. The longer of the two connecting leads is the anode, which must be connected to the more positive potential. The shorter lead is the cathode, which connects to the more negative potential. If the leads have been cut to equal length, the plastic casing often features a small flat edge or notch corresponding to the cathode lead.
Looking inside the transparent casing of an LED reveals two internal metal structures: the larger metal flag connects to the cathode, and the smaller structure connects to the anode. Polarized aluminum electrolytic capacitors also have distinct markings. They feature a vertical stripe or series of minus signs (-) along the side of the canister that designates the negative terminal (cathode). The lead on the opposite side of this stripe is the positive terminal (anode).
Practical Testing Methods
When physical markings are absent or damaged, a multimeter provides a reliable method for determining polarity. For unknown power sources, the multimeter is set to measure DC voltage, and the red and black probes are touched to the terminals. If the display shows a positive voltage reading, the red probe is correctly touching the positive terminal (cathode) and the black probe is touching the negative terminal (anode). A negative voltage reading indicates that the probes are reversed.
To test polarized components like diodes or LEDs, the multimeter must be switched to the continuity or diode test mode. The multimeter’s red probe is internally connected to a positive voltage source and the black probe to a negative source, treating the meter as a small power supply. A diode will only allow current to flow when it is forward-biased, which occurs when the positive voltage is applied to the anode and the negative voltage is applied to the cathode.
When the red probe touches the anode and the black probe touches the cathode, the multimeter displays a voltage drop reading (usually 0.2 to 0.7 volts for silicon diodes), indicating successful conduction. If the probes are reversed, the diode is reverse-biased, and the meter will display an open circuit (often shown as “OL” or “1”), indicating no continuity. The terminal connected to the red probe when the meter shows a reading is the anode, and the terminal connected to the black probe is the cathode.