Nov. 6th. 2012
MECHANICS 7.0: VOLTAIC WET CELLS: BATTERIES
Almost any two different solid conductors, the electrodes, immersed in a variety of active solutions, known as electrolytes, function more or less as a battery. Chemical energy in the interactive bonds is converted to electrical potential energy as the solution, and one or both of the electrodes undergo an oxidation- reduction reaction. In the voltaic wet cell, commonly known as the lead/acid battery, the electrodes are alternating plates of lead and lead (IV) oxide plugs; the electrolyte is dilute sulfuric acid. Each pair of plates is a cell, the plate that undergoes oxidation is designated the anode, the plate that is reduced is considered the cathode. Each cell produces approximately 2 volts of electrical potential. The cells are connected in series; hence, a 6-volt battery has 3 cells and a 12 volt battery, 6 cells. Discharge mode: Electrons are produced at the lead anode by the oxidation of lead to lead (II) ions which immediately precipitate on the plate as lead (II) sulfate. This electrode, which "pumps" electrons into the external circuit is designated the negative pole. Electrons moving through the circuit perform useful work on resistance wires, switches, capacitors, light bulbs, and starter motor. The electrons pass from the external circuit to the cathode reducing lead (IV) oxide to lead (II) ions which also immediately precipitate on that plate as lead (II) sulfate. This electrode, which "pulls" electrons from the external circuit is considered the positive pole of the cell. Deposits of lead (II) sulfate slowly build up on the plates, partially covering and replacing the lead and lead (IV) oxide. As the cell discharges, the concentration of sulfuric acid decreases. For every mole of lead reacting, two moles sulfuric acid are replaced by two moles of water. The "state of charge" of a battery can be checked by measuring the density of the electrolyte. A low density indicates a low sulfuric acid concentration, and hence, a partially discharged cell. Two advantages of lead "storage" battery are the ability to deliver large amounts of energy for a short time (starter motors) and the ability to be recharged. A disadvantage is its high weight-to-energy ratio. Recharge mode: A lead acid battery can be restored to its original condition by passing a direct current through it in the reverse direction. The electrical energy required to bring about renewal may be furnished by a direct current generator or by an alternator equipped with a direct current rectifier. In different current-carrying materials, the charges of the moving particles may be positive or negative. In metals, the moving charges are always (negative) electrons, while in an ionized gas (plasma) or ionic solution (electrolyte), the moving charges may include both electrons and positively charged ions. In semi-conductor material, such as germanium or silicon, conduction is achieved partly by electrons and partly by the motion of vacancies, also known as "holes" These are sites of missing electrons and act like positive charges.
Conventional Current Positive charges moving in the direction of the electric field produce the same current as the same number of negative charges of the same magnitude moving at the same speed in the direction opposite to the field. A flow of negative charges to the left is equivalent to an equal flow of positive charge to the right. Thus, currents are described as though they consisted entirely of positive charge flow, even in cases in which one knows the actual current is due to electrons. This is really a matter of convention. Why is this? Ben Franklin envisioned current to be mobile positive charge carriers. In fact, he was the first to designate a charge as being either negative or positive. Therefore, the direction of flow of positive charge is traditionally taken to be the direction of current, regardless of the actual sign of the participating carriers.