|MadSci Network: Chemistry|
Q: Explain the concept of 'Resistance Polarization'. When a electrochemical cell has an object (e.g. lightbulb) placed in the path between the anode and the cathode, the electrodes polarize due to resistance polarization...why does this happen? In other words, why does the anode become more positive, and why does the cathode become more negative in potential? A: I've never heard the exact phrase 'resistance polarization.' A more familiar word for electrochemical cell is "battery." Polarization refers to the tendency of the chemical reaction in the battery to push electrons in a certain direction. "Resistance" is just a measure of how difficult it is for electricity to flow. A piece of copper has a very low resistance. A piece of dry wood has a very high resistance. A light bulb has a resistance. So does a battery. The construction of the battery polarizes the terminals. A battery contains chemicals that want to react and release their stored energy. Inside the battery, one set of chemicals is electrically connected to one terminal. A different set of chemicals is electrically connected to the other terminal. An electrolyte separates the two sets of chemicals. The electrolyte conducts just enough chemical from one side to the other to allow a chemical reaction to take place. The battery is constructed so that the energy from the reaction can only be released as electricity at the terminals of the battery. If no electricity is drawn from the terminals, the reaction is almost completely halted. The chemical reaction pumps electric charge from one terminal of the battery to the other. The chemistry of the battery determines what direction the charge wants to flow and how strongly it is pushed. The construction of the battery polarizes the terminals. The anode and cathode start out with a charge. When the battery is totally "dead" the terminals no longer have a difference in charge. The battery is constantly trying to push electric charge from one side of the battery to the other. The battery can only "push" so hard and so fast. "Voltage" is a measure of how hard it can push. "Source resistance" is a measure of how fast it can push. When there isn't an electrical path between the terminals, the voltage is as high as that battery is capable of producing. The battery is "pushing" as hard as it can, but there isn't any current flowing. When you connect an electrical path between the terminals of the battery, current flows and the voltage drops. The voltage drops because some of the "push" capability is being used to push a current flow across that path. The faster you draw a current, the less hard that current can be pushed. When you put your thumb over the end of a garden hose, you build up pressure (voltage) in the hose. But there is no current. When you move your thumb, the water flows (current) but the pressure (voltage) drops. The size (source resistance) of your garden hose determines the amount of water that can flow (current). In a rechargeable battery, the chemical reaction can be forced to operate in reverse. You're forcing the chemical reaction to go against the direction it wants to react. In the process, you store energy.
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