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why is anode negatively charged in an electrochemical cell?

why is anode negatively charged in an electrochemical cell?


1 Answers

Suraj Prasad IIT Patna
askIITians Faculty 286 Points
6 years ago
This charge convention screws people up all of the time. The problem is that the sign convention for a discharging battery (like we deal with in circuits) is different from the plate charges in electrophoresis (which is a charged capacitor).

For a battery in a circuit (according to physics convention), we are told that the anode must be negative, because current is flowing to it. We are likewise told the cathode must be positive, because current is flowing from it. The reality is that it is a chemical reaction that generates the emf and there is not actual charge on either electrode. But by convention in physics, the cathode terminal of a battery is assigned a (+) and the anode terminal of a battery is assigned a (-).

For electrophoresis, the plates of a capacitor are charged as the result of an applied voltage. The electrons flow from the anode (site of oxidation: loss of electrons) to the cathode (site of reduction: gain of electrons), so the anode builds up a (+) charge over time and the cathode builds up a (-) charge over time. The electric field that is created can be filled with a viscous gel so that charged macromolecules can be separated according to their migration rate and/or migration direction through the resistive gel. Cations migrate to the cathode and anions migrate to the anode.

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