Some chemical changes result in the production of electrical energy. Dry cells used in our
cells. What chemicals are used in it?
Answers
Dry Cells
In dry cell batteries, no free liquid is present. Instead the electrolyte is a paste, just moist enough to allow current flow. This allows the dry cell battery to be operated in any position without worrying about spilling its contents. This is why dry cell batteries are commonly used in products which are frequently moved around and inverted, such as portable electronic devices. Dry cell batteries can be either primary or secondary batteries. The most common dry cell battery is the Leclanche cell.
Battery Performance
The capacity of a battery depends directly on the quantity of electrode and electrolyte material inside the cell. Primary batteries can lose around 8% to 20% of their charge over the course of a year without any use. This is caused by side chemical reactions that do not produce current. The rate of side reactions can be slowed by lowering temperature. Warmer temperatures can also lower the performance of the battery, by speeding up the side chemical reactions. Primary batteries become polarized with use. This is when hydrogen accumulates at the cathode, reducing the battery's effectiveness. Depolarizers can be used to remove this build up of hydrogen.
Secondary batteries self-discharge even more rapidly. They usually lose about 10% of their charge each month. Rechargeable batteries gradually lose capacity after every recharge cycle due to deterioration. This is caused by active materials falling off the electrodes or electrolytes moving away from the electrodes.
Peukert's law can be used to approximate relationships between current, capacity, and discharge time. This is represented by the equation
t=QpKk(Batteries.1)
where I is the current, k is a constant of about 1.3, t is the time the battery can sustain the current, and Qp is the capacity when discharged at a rate of 1 amp.
Current, Voltage, and Standard Reduction Potential
There is a significant correlation between a cell's current and voltage. Current, as the name implies, is the flow of electrical charge. Voltage is how much current can potentially flow through the system. Figure 4 illustrates the difference between current and voltage.
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Figure 4: The difference between voltage and current.
Water is flowing out of a hose and onto a waterwheel, turning it. Current can be thought of as the amount of water flowing through the hose. Voltage can be thought of as the pressure or strength of water flowing through the hose. The first hose does not have much water flowing through it and also lacks pressure, and is consequently unable to turn the waterwheel very effectively. The second hose has a significant amount of water flowing through it, so it has a large amount of current. The third hose does not have as much water flowing through it, but does have something blocking much of the hose. This increases the pressure of the water flowing out of the hose, giving it a large voltage and allowing the water to hit the waterwheel with more force than the first hose.
Standard reduction potential, Eo, is a measurement of voltage. Standard reduction potential can be calculated with the knowledge that it is the difference in energy potentials between the cathode and the anode: Eocell = Eocathode − Eoanode. For standard conditions, the electrode potentials for the half cells can be determined by using a table of standard electrode potentials.
For nonstandard conditions, determining the electrode potential for the cathode and the anode is not as simple as looking at a table. Instead, the Nernst equation must be used in to determine Eo for each half cell. The Nernst equation is represented by , where R is the universal gas constant (8.314 J K-1 mol-1), T is the temperature in Kelvin, n is the number of moles of electrons transferred in the half reaction, F is the Faraday constant (9.648 x 104 C mol-1), and Q is the reaction quotient.
Different Sizes of Batteries and Some Additional Facts
Batteries vary both in size and voltage due to the chemical properties and contents within the cell. However, batteries of different sizes may have the same voltage. The reason for this phenomenon is that the standard cell potential does not depend on the size of a battery but rather on its internal content. Therefore, batteries of different sizes can have the same voltage (Figure 5). Additionally, there are ways in which batteries can amplify their voltages and current. When batteries are lined up in a series of rows it increases their voltage, and when batteries are lined up in a series of columns it can increases their current.