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Batteries: Why are They so Important?

Author: Jason Ziglar, Duke University
Editor: Wei-Chung Chen
SCICOM MIT

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Batteries are essential components of most electrical devices. They exist in our cars, laptops, CD players, and other electronic appliances. A battery is essentially a can full of chemicals that produce electrons. The basic structure of battery includes two terminals. One terminal is marked positive, while the other is marked negative. In normal flashlight batteries, the ends of the battery are the terminals. In a large car battery, there are two heavy lead posts that act as the terminals.

Electrons collect on the negative terminal of the battery. When a wire is connected between the negative and positive terminals, the electrons will flow from the negative to the positive terminal as fast as they can. Normally, some type of load is connected to the battery using the wire. The load might be something like a light bulb, a motor or an electronic circuit like a radio.

Inside the battery itself, a chemical reaction produces the electrons. The speed of electron production by this chemical reaction (the battery's internal resistance) controls how many electrons can flow between the terminals. Electrons flow from the battery into a wire, and must travel from the negative to the positive terminal for the chemical reaction to take place. That is why a battery can sit on a shelf for a year and still have plenty of power. Unless electrons are flowing from the negative to the positive terminal, the chemical reaction does not take place. Once a wire connects both terminals, the reaction starts.

Historically, the first battery was created by Alessandro Volta in 1800. To create his battery, he made a stack by alternating layers of zinc, blotting paper soaked in salt water, and silver. This arrangement was known as a voltaic pile. The top and bottom layers of the pile must be different metals. By attaching a wire to the top and bottom of the pile, one can measure a voltage and a current from the pile. The pile can be stacked as high as possible, and each layer will increase the voltage by a fixed amount. The simplest battery that can be created is called a zinc/carbon battery. In a container filled with sulfuric acid, a zinc rod is placed in it. Immediately, the acid will start to eat away at the zinc. Hydrogen gas bubbles will be forming on the zinc rod, and the rod and acid will begin to heat up. Specifically, there are multiple reactions taking place. When a carbon rod is inserted in the acid, the acid does nothing to it. But by connecting a wire between the zinc rod and the carbon rod, two things change. First, the electrons flow through the wire and combine with hydrogen on the carbon rod, so hydrogen gas begins bubbling off the carbon rod. Second, there is less heat. You can power a light bulb or similar load using the electrons flowing through the wire, and you can measure a voltage and current in the wire. Some of the heat energy is turned into electron motion. The electrons go to the trouble to move to the carbon rod because they find it easier to combine with hydrogen there. Eventually, the zinc rod dissolves completely or the hydrogen ions in the acid get used up and the battery dies. Modern batteries use a variety of chemicals to power their reactions. Some of the batter chemicals include the following. As mentioned previously, there are zinc/carbon batteries. They are also known as a standard carbon battery, zinc/carbon chemistry is used in all inexpensive AA, C and D dry-cell batteries. The electrodes are zinc and carbon, with an acidic paste between them that serves as the electrolyte. Second type of battery is the alkaline battery that is used in common Duracell and Energizer batteries. In this type of battery, electrodes are zinc and manganese-oxide, with an alkaline electrolyte. Another commonly used battery is the lead-acid battery that is used in automobiles. The electrodes are made of lead and lead-oxide with a strong acidic electrolyte (rechargeable).

In almost any device that uses batteries, multiple batteries are used at the same time. Batteries are grouped together serially to form higher voltages, or in parallel to form higher currents. In a serial arrangement, the voltages add up. In a parallel arrangement, the currents add up. In a parallel arrangement, when each cell produces 2 volts, then four batteries in parallel will also produce 2 volts. However, the current supplied will be four times that of a single cell. In a serial arrangement, the four voltages add together to produce 8 volts.

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