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L04 - Chapter 4 Electricity Principles Motivation 4.1...

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Chapter 4: Electricity Principles – Motivation Electric energy is ubiquitous in our society as it is subsequently transformed into other useful forms: mechanical, light, heat, sound, etc. 4.1 2009 In 2009 the net electric energy output of the U.S. was about 3.9 trillion kWh – double the value of the energy delivered at the beginning of the 80’s The dominating sources are fossil fuels coal and natural gas – followed by In order to understand how we can convert these primary energy sources into electric energy, we have to introduce some principles of electromagnetism We’ll take a look at some basic topics such as: a) the electric charges carrying energy bundled electric currents along power dissipating wires, b) the principles of electromagnetism driving the generators converting various energy sources into electric energy via mechanical energy, c) the idea behind minimizing the loss of power in electric transmission lines, etc. nuclear and hydroelectric sources
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Electric Charges and Forces – Measuring the presence of electricity Electric energy is transported by moving electric charges. What are these? Let’s look first at a static case: an electric charge can be “loaded” on an object such as a piece of plastic or glass by simply rubbing the object Simple experiments can show that there are two types of electric charges, which are conventionally called positive and negative Like charges repel and unlike charges attract each other: in other words they interact via forces which can be associated with the mechanics described in Chapter 2 in order to explain the dynamics of electrically charged objects 4.2 So, the presence of electric charges can be demonstrated by their interaction. How can we quantify the electrostatic forces? Let’s consider that the interacting charged objects are very small; in this case the strength of the interaction is given by Coulomb Law : If two small objects with charges q and Q are placed at a distance r from each other they will repel or attract each other with a force F = k qQ r 2 Def : The electric charge q of an object is a property describing the ability of the object to interact electrically with other charged objects [ ] Coulomb C = ( ) 9 2 2 8.99 10 Nm C k = ×
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Atomic Structure – What are in fact electric charges? So, objects can be loaded or unloaded electrically. But how does this happen? The explanation is at the microscopic level, in the atomic structure of matter Atoms can be rather complex structures, but a simplified model can be useful in order to figure out the nature of electric charges The so called planetary model (or Bohr model ) of atom suggests that in an atom particles called electrons carrying a negative e = –1.6 10 -19 C orbit about a much heavier nucleus containing electrically neutral neutrons, and protons carrying a positive charge +1.6 10 -19 C 4.3 The number of protons in an atom is called atomic number Z and different chemical elements have different atomic numbers
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