{"id":93,"date":"2019-02-10T17:54:38","date_gmt":"2019-02-10T22:54:38","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/basicelectricity\/?post_type=chapter&p=93"},"modified":"2020-01-13T16:12:27","modified_gmt":"2020-01-13T21:12:27","slug":"methods-for-producing-voltage","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/basicelectricity\/chapter\/methods-for-producing-voltage\/","title":{"raw":"Methods for Producing Voltage","rendered":"Methods for Producing Voltage"},"content":{"raw":"
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TEST YOUR KNOWLEDGE!<\/span><\/h2>\r\n

Before starting the chapter, let's see how much you know!<\/span><\/p>\r\n\r\n<\/header>\r\n

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Electromotive Force (EMF) and Potential Difference<\/strong><\/h1>\r\nIf there is a surplus of electrons at one end of a conductor and a deficiency at the other end, a current flows.\u00a0Certain devices create this difference in charge so current flows.\u00a0These devices are sources of electromotive force.\r\n\r\nEMF<\/strong> is defined as:\r\n\r\nThe energy transferred per unit when one type of energy is converted into electrical energy.<\/em>\r\n\r\nPotential difference is another term that is almost the same but has a subtle difference.\r\n\r\nPotential difference<\/strong> is defined as:\r\n\r\nThe energy transferred per unit when electrical energy is converted into one other type of energy.<\/em>\r\n\r\nWe will discuss this in more depth in a bit.\r\n\r\nThe six most common types of EMF are:\r\n
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  1. Friction<\/li>\r\n \t
  2. Chemical<\/li>\r\n \t
  3. Pressure<\/li>\r\n \t
  4. Heat<\/li>\r\n \t
  5. Light<\/li>\r\n \t
  6. Magnetism<\/li>\r\n<\/ol>\r\nFriction EMF<\/strong>\r\n\r\nWhen two dissimilar materials are rubbed together, one material can transfer some of its electrons to the other.\r\n\r\nThis is the triboelectric effect, which is similar to what we discussed in the atomic structure chapter when electrons transfer, causing one object to become negatively charged (a surplus of electrons) and the other to become positively charged (a deficiency of electrons). This situation can result in an electrostatic discharge when the force of attraction becomes so great that the electrons are attracted to the positively charged object.\r\n\r\nThis attraction creates an arc most commonly seen in lightning.\u00a0The clouds build up a charge as the water droplets move around. The charge is then attracted to the positively charged earth and ZAP!\r\n\r\n[caption id=\"attachment_97\" align=\"aligncenter\" width=\"201\"]\"\" Figure 4. Lightning image by Griffenstorm. Shared under the Creative Commons Attribution-Share Alike 4.0 International license.[\/caption]\r\n\r\nOf course, it is much more fun to put your cat in a box full of styrofoam chips and watch them stick.\r\n\r\n[caption id=\"attachment_98\" align=\"aligncenter\" width=\"300\"]\"\" Figure 5. Cat Static electricity image by Sean McGrath. Shared under the Creative Commons CC-BY 2.0 License.[\/caption]\r\n\r\nChemical EMF<\/strong>\r\n\r\nThis is the principle upon which batteries work.\r\n\r\n[caption id=\"attachment_99\" align=\"alignright\" width=\"300\"]\"\" Figure 6. Batteries[\/caption]\r\n\r\nWithout getting too deeply into the chemistry, basically, batteries work on the process of ionization.\r\n\r\nWhat is an ion? It is a particle that has an electron added or removed (positive or negative).\r\n\r\nIn ionization, chemicals are added that have particles with a negative charge and particles with a positive charge. Metal plates that take on these charges are added. This allows a difference and creates an EMF.\r\n\r\nPressure EMF<\/strong>\r\n\r\nThis process is also known as piezoelectricity.\r\n\r\n[caption id=\"attachment_100\" align=\"alignleft\" width=\"300\"]\"\" Figure 7. Ignitor[\/caption]\r\n\r\nPiezoelectricity is the electric charge that accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity<\/em> means electricity resulting from pressure.\r\n\r\nWhen pressure is applied to some objects, the pressure displaces the positive and negative charges in an otherwise neutral object.\r\n\r\nThe best-known application is the electric cigarette lighter: pressing the button causes a spring-loaded hammer to hit a piezoelectric crystal, producing a sufficiently high voltage electric current that flows across a small spark gap, thus heating and igniting the gas. The portable sparkers used to ignite gas stoves work the same way, and many types of gas burners now have built-in piezo-based ignition systems.\r\n\r\nAnother use is a microphone or guitar pickup. Sound hits the crystal and generates a voltage.\r\n\r\nHeat EMF<\/strong>\r\n\r\nThis process is known as the thermoelectric effect.\r\n\r\n[caption id=\"attachment_168\" align=\"alignright\" width=\"200\"]\"\" Figure 8. Thermocouple[\/caption]\r\n\r\nTo summarize, when two dissimilar metals are at different temperatures and are touching, they produce an EMF.\r\n\r\nThis is because the electrons from the hot side (negative) want to jump to the cold side (positive).\r\n\r\nA very common use of this principle is the thermocouple in your furnace\r\n\r\nWhen the pilot light is lit, it generates a voltage on the thermocouple. This voltage allows a relay to switch on and allows gas to flow when the furnace calls for it. If the pilot light is out, there is no voltage. Therefore the relay won\u2019t be on and the furnace can call for the gas but it won\u2019t get it.\r\n\r\nLight EMF<\/strong>\r\n\r\nThis is the photovoltaic effect.\u00a0Photovoltaic cells (PV cells) are used as sources of EMF.\r\n\r\n[caption id=\"attachment_101\" align=\"alignleft\" width=\"300\"]\"\" Figure 9. Solar panels[\/caption]\r\n\r\nPhotovoltaic cells are made of special materials called semiconductors, such as silicon, which is currently used most commonly.\r\n\r\nHow many valence electrons does a semiconductor have? Four.\r\n\r\nAn impurity is added to give a free electron.\r\n\r\nBasically, when light strikes the cell, a certain portion of it is absorbed within the semiconductor material.\r\n\r\nThis means that the energy of the absorbed light is transferred to the semiconductor.\r\n\r\nThe energy knocks electrons loose, allowing them to flow freely.\r\n\r\nWe are seeing this technology used everywhere these days as it does not require fossil fuels to get energy.\r\n\r\nMagnetism EMF<\/strong>\r\n\r\nThis is where most of our energy comes from.\u00a0Magnets create lines of flux.\u00a0When these lines of flux are cut by a conductor, an EMF is generated.\u00a0Steam turbines, co-gen plants, windmills, and hydroelectric dams all use this technology.\r\n\r\nBelow is an example of a hydroelectric dam.\r\n
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    Image Hotspot<\/span><\/h2>\r\n

    Click on the \"+\" to find out more.<\/span><\/p>\r\n\r\n<\/header>\r\n

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    [h5p id=\"2\"]<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\nAll six of these sources of EMF achieve the same thing:\r\n