Introduction to Electricity

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What is electricity? Flow of electric charge Movement of electrons or protons Protons: Positively charged Consist of two up quarks and one down quark Electrons: Negatively charged Believed to be a fundamental particle Electricity moves through some materials better than others. Three types of materials Conductors – electricity can flow (or conduct) easily Insulators – electricity is prohibited from flowing easily Semiconductors – modern materials designed to conduct only under certain conditions

Identify the type of material: Insulator (I), Conductor (C), or Semiconductor (S) glass copper LEDs gold transistors I S C I plastic Integrated circuits rubber aluminum

Why do some materials conduct better than others? Atomic Structure – the ability of a material to conduct electricity depends on the arrangement of electrons around the nucleus Cu 29 63.546 atomic number atomic weight 𝑔𝑟𝑎𝑚𝑠 𝑚𝑜𝑙 # protons + average # neutrons ≈ atomic weight (electrons are very light relative to protons and neutrons) 29 protons + 35 neutrons 64 The Bohr Model of an atom predicts that electrons will configure themselves into shells around the nucleus of an atom. 𝐶𝑢=1 𝑠 2 2 𝑠 2 2 𝑝 6 3 𝑠 2 3 𝑝 6 3 𝑑 10 4 𝑠 1 It is easy for the copper to let go of the 4s1 electron. Valence electrons in the outer shell influence conductivity. How does this relate to insulators?

Class Problem: Find the number of valence electrons in a cubic centimeter block of copper. 1 cm Things you may need to know: Atomic Weight of Cu = 63.55 𝑔 𝑚𝑜𝑙 , Density of Cu = 8.94 𝑔 𝑐𝑚 3 , Avogadro's Number = 6.022× 10 23 𝑎𝑡𝑜𝑚𝑠 𝑚𝑜𝑙 Getting Started: Are the number of valence electrons equal to the number of atoms? Why or why not? How will this help us? # 𝑉𝑎𝑙𝑒𝑛𝑐𝑒 𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠=# 𝑎𝑡𝑜𝑚𝑠∙ 1 𝑒 − 𝑎𝑡𝑜𝑚 =𝑉𝑜𝑙𝑢𝑚𝑒 ∙𝐷𝑒𝑛𝑠𝑖𝑡𝑦 ∙ 1 𝐴𝑊 ∙𝐴𝑣𝑜𝑔𝑎𝑑𝑟 𝑜 ′ 𝑠 𝑁𝑢𝑚𝑏𝑒𝑟∙ 1 𝑒 − 𝑎𝑡𝑜𝑚 ∙ 𝑚𝑜𝑙 𝑔 # 𝑉𝑎𝑙𝑒𝑛𝑐𝑒 𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠=#𝑎𝑡𝑜𝑚𝑠∙ 1 𝑒 − 𝑎𝑡𝑜𝑚 = ∙ 𝑔 𝑐𝑚 3 ∙ 𝑎𝑡𝑜𝑚𝑠 𝑚𝑜𝑙 ∙ 1 𝑒 − 𝑎𝑡𝑜𝑚 Look at units: 𝑐𝑚 3 ∙8.94 𝑔 𝑐𝑚 3 ∙ 𝑚𝑜𝑙 63.55𝑔 ∙ 6.022× 10 23 𝑎𝑡𝑜𝑚 𝑚𝑜𝑙 ∙ 1 𝑒 − 𝑎𝑡𝑜𝑚 # 𝑉𝑎𝑙𝑒𝑛𝑐𝑒 𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠= 1𝑐𝑚 3 =8.5× 10 22 𝑣𝑎𝑙𝑒𝑛𝑐𝑒 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠 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 6

How do electrons move? Scenario: You are talking on the phone with a friend in Beijing, China. Your voice causes 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. 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. 6,900 miles or 11,100 km maps.google.com

+ net flow of electrons - Electric Current Electrons migrate from the supply to the shortage. electron supply - electron shortage + The actual path that electrons take is scattered mostly due to atom oscillations. + net flow of electrons - the net flow of electrons through a material is called electron current + conventional current - 𝑐𝑜𝑛𝑣𝑒𝑛𝑡𝑖𝑜𝑛𝑎𝑙 𝑐𝑢𝑟𝑟𝑒𝑛𝑡=𝐼 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.

Electrical Resistance Consider the interaction of electrons with ions as they pass through a material. Atoms oscillate about their equilibrium positions due to thermal energy The amplitude of oscillation increases with temperature The conduction of electrons through a material is interrupted due to destructive interactions with oscillating atoms This scattering and destructive interaction cause electrical resistance as well as heat generation Electrical resistance is a measure of a material’s opposition to the passage of electric current. 𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒=𝑅

- + Voltage Electrons migrate from the supply to the shortage. electron shortage electron supply - + The potential difference between the strength of the electron supply and the electron shortage defines the voltage. 𝑣𝑜𝑙𝑡𝑎𝑔𝑒=𝑉 Voltage is a measure of how badly 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