College of Engineering & Science living with the lab Introduction to Electricity What is electricity? electricity is the flow of electric charge... for us, this amounts to the movement of electrons or protons e proton (two up quarks & a down quark) electron (believed to be a fundamental particle) + - Wikipedia © 2012 David Hall

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three types of materials: conductors – electricity can flow (or conduct) easily though good conductors insulators – poor conductors semiconductors – these modern materials are designed to conduct only under certain conditions living with the lab 3 electricity can move through some materials better than others

living with the lab 4 identify the type of materials conductors insulators semiconductors glass gold aluminum copper plastic integrated circuits LEDs transistors rubber

Why do some materials conduct better than others? living with the lab 5 atomic structure – the ability of a material to conduct electricity depends on the arrangement of electrons around the nucleus Cu atomic number 29 protons + 35 neutrons 64 # protons + average # neutrons ≈ atomic weight (electrons are very light relative to protons and neutrons) The Bohr Model of an atom predicts that electrons will configure themselves into shells around the nucleus of an atom. It’s easy for copper to let go of the 4s 1 electron. Valence electrons in the outer shell influence conductivity. Insulators tend to have filled outer electron shells and consequently hold onto their electrons more tightly; it takes much more energy to induce electron flow in an insulator (a higher voltage).

thought experiment living with the lab 6 If you were talking on the phone with a friend in Beijing, China, your voice would cause the diaphragm in the microphone to move which would induce electron motion. Would an electron that you just caused to move in the microphone somehow get to China in a fraction of a second? Assume that your phone is directly wired to your friends phone in China. 6,900 miles or 11,100 km maps.google.com electron introduced due to chemical reaction in battery When one electron leaves the negative terminal of the battery, another one enters the positive terminal, so that the battery remains electrically neutral overall.

how many electrons are we talking about????? living with the lab to get some idea, let’s compute the number of valence electrons in a 1cm x 1cm x 1cm cube of copper 1 cm This is more than the number of grains of sand on the entire earth, assuming grains of sand 1mm in diameter, packed at 68% efficiency, 10cm deep, over the earth’s 200 million square mile surface. 7

electric current living with the lab 8 While electrons migrate from the supply to the shortage, the actual path that electrons take is scattered mostly due to atom oscillations. electron supply - electron shortage + + conventional current - Conventional current is actually opposite of the actual flow of electrons and goes from + to –. We determine the conventional current I when we measure or compute currents. + net flow of electrons - the net flow of electrons through a material is called CURRENT

Consider the interaction of electrons with ions as they pass through a material. living with the lab 9 electric resistance the conduction of electrons through a material is interrupted due to destructive interactions with oscillating atoms atoms oscillate about their equilibrium positions due to thermal energy this scattering and destructive interaction cause electrical resistance as well as heat generation Electrical resistance is a measure of a materials opposition to the passage of electric current. the amplitude of oscillation increases with temperature

living with the lab electron supply - electron shortage + voltage The potential difference between the strength of the electron supply and the electron shortage defines the voltage. Voltage is a measure of how bad electrons want to cover a distance. “Cloud to ground lightening begins within the cloud when the localized electrical potential gradient exceeds 3 million volts per meter along a path perhaps 50m long.” Voltage is a measure of how bad electrons want to cover a distance. “Cloud to ground lightening begins within the cloud when the localized electrical potential gradient exceeds 3 million volts per meter along a path perhaps 50m long.” Meteorology Today: An Introduction to Weather, Climate, and the Environment by C. Donald Ahrens 10