MadSci Network: Chemistry

Re: Explain the concept of 'Resistance Polarization'

Date: Sun Apr 9 22:14:23 2000
Posted By: David Taylor, Staff, programming, Testronics
Area of science: Chemistry
ID: 954127361.Ch

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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