Transcript D-5 Notes
Science 9: Unit D: Electrical Science and Technology TOPIC 5 :PORTABLE POWER What is a Cell’s Function? To make a circuit work, energy must be generated so that electrons will flow through a resistor. The energy is supplied in many circuits by a battery or cell. A cell relies on chemicals to create its energy and is therefore called an electrochemical cell. The chemical reactions in the cell determine how much voltage can be produced. A typical alkaline cell can produce 1.5 V. Parts of an Electrochemical Cell Any cell contains two metal electrodes and an electrolyte. One electrode acts as an electron donor and electrons leave this electrode. This electrode is negatively charged and is called the anode. The other electrode acts as an electron acceptor and electrons move towards this electrode. This electrode is positively charged and is called the cathode. An electrolyte is a substance either a liquid or a paste that can conduct electricity. The Electrochemical Cell in Action As the anode donates its electrons its atoms become positive ions (cations) and dissolve into the electrolyte. As the electrons continue to move from anode to cathode the anode gradually disappears into the electrolyte. This is how an electrochemical cell eventually dies. How do you keep the current flowing in an EC Cell? If there was no electrolyte then the cell would work for an instant and then ‘turn off.’ Why? Electrons move from negative to positive charges. However electrons moving to the cathode would make it negative and the current would stop. How do we keep the copper cathode neutral and the electrons moving? By putting the cathode in a solution of its own ions. The positive copper ions move to the copper metal as it becomes negative, keeping it neutral. You actually the copper strip getting bigger. The ions need to be able to move back and forth so that the electrolytes stay neutral and the current keeps moving. Types of Batteries Primary batteries – batteries that cannot be recharged. Once the reactants are used up (electrodes and electrolytes) the battery is ‘dead.’ Secondary batteries – batteries that can be recharged. An electric current is forced through the battery which reverses the chemical reaction and builds back up the reactants. Over time, however, the electrode metals dissolve completely into the electrolyte. In an automobile, while the car is moving, the alternator turns a generator which forces an electric current back into the lead acid car battery. This process allows a car battery for many years. Wet cell – A cell that uses a liquid (usually an acid) electrolyte. Dry Cell – A cell that uses a solid (usually a paste) electrolyte. Voltaic Pile – The first kind of battery invented in the 1700s by Alessandro Volta. It consists of alternating pieces of metal (the electrodes) separated by salt water. Animal Electricity Animal Electricity – Luigi Galvani discovered that there is electricity in the tissue of animals. Attaching a battery/cell to the nerves of an animal can cause them to twitch. (This was the idea behind Frankenstein). Commonly Used Cells and Batteries