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Electricity & Magnetism Part I Static Electricity Unit 10 Concept Map Electricity and Magnetism Static Electricity Current Electricity Magnetism & Electromagnetism The Electrical Charge Carrier • The Electron – Negatively charged – 9.11x10-31 kg • The Proton – Positively charged – 1.67x10-27 kg Why is the electron principally responsible in determining electric charge? Charge Basics Opposite charges attract Like charges repel Electrons and protons have equal and opposite charges Lowest possible charge is that of electron or proton (no fractional charges…yet) Benjamin Franklin 1706-1790 • Famous kite experiment • First to give charges the names positive and negative • First to realize Law of Conservation of Charge • Invented the lightning rod The SI Unit of Charge The Coulomb (C) 6.25x1018 electrons (or protons) are needed to make one Coulomb The charge on one single electron/proton 1.6x10-19 C See reference table for values Charles-Augustin de Coulomb 1736-1806 Questions? • How does an object become negatively charged? • How does an object become positively charged? • What is the net charge in Coulombs of 5 electrons? • What is the number of protons that make up a charge of 10 nanoCoulombs? Law of Conservation of Charge • Electric charge can neither be created nor destroyed -5 -8 -8 -11 Transferring Charge The Electroscope • A device used to detect electric charge Charging by Induction • Isolating charge without physical contact between objects • Once charged rod is removed, the electroscope stays neutrally charged Charging by Conduction • Isolating electric charge by physical contact • Once rod is removed, the electroscope remains charged Permanently Charging by Induction Coulomb’s Law There exists an electrostatic force between charged objects that is directly proportional to the magnitude of the charges and indirectly proportional to the square of the distance between them. This is another Inverse-Square law This force could be attractive or repulsive Coulomb’s Law F21 indicates force on q1 due to q2 F12 indicates force on q2 due to q1 Coulomb’s Law kq1q2 Fe 2 r Coulomb’s Constant kq1q2 Fe 2 r N m k 9.0 x10 2 C 9 2 Let’s Try a Problem F21 indicates force on q1 due to q2 The Van de Graaff Generator • Generates large static charges that build up on dome • Some generators can charge to thousands of volts • They are generally safe, but caution should be heeded Electric Fields • An electric field is an invisible force field that affects electric charges • The field is defined based on how it affects a positive test-charge • All charged objects are assumed to have an electric field associated with them Electric Field Mapping Electric Field Around a Positively Charged Object Electric Field Around Charged Pairs Electric Field of a Set of Parallel Plates Electric Field Mapping Rules • E-Field lines must never touch or cross themselves or other field lines • E-Field lines run out of a positive charge and into a negative charge • E-Field lines must intersect charged object at right angle (90o) • Number and density of lines indicate field strength or intensity Electric Field Intensity Fe E q q represents charge on test-charge Units of electric field intensity are Newtons per Coulomb Electric Field Equation for Point Charges kQ E 2 r Capital Q indicates charge on the object Electric Potential Energy • Electric potential energy is the energy stored within an electric field • The work done in moving a charge within an electric field is equal to the potential energy stored • The kinetic energy gained by a charged particle within an electric field is equal to the potential energy lost • What are the units for EPE? Work-Energy Theorem Review W KE W EPE Ei E f Electric Potential • Electric potential is the ratio of the work done on a charged particle within an electric field divided by the particle’s charge • Units are Joules per Coulomb • Other terms used for electric potential are electric potential difference and voltage Electric Potential W V q Units of Electric Potential are Joules per Coulomb or Volts Electric Fields Within a Charged Object Potential Inside a Conductor Electric Field of a Set of Parallel Plates The Capacitor • Capacitor Uses – – – – Frequency tuner Power supplies Filter Block DC current Capacitance of Parallel Plates C 0 A d 0 8.85 x10 12 2 C 2 N m Capacitance (in terms of Charge and Voltage) Q C V Millikan Oil Drop Experiment E & M Part 2 Current Electricity • • • • • • Current Resistance Voltage Ohm’s Law Power Circuits – Series – Parallel Electric Current • Time rate of flow of electric charge • I represents electric current • Units of current are Coulombs per second (C ) ( Ampere) ( s) q I t Electrical Resistance • The opposition to steady electric current flow • The unit of resistance is the Ohm (W) • Similar to friction there is no such thing as zero resistance Carbon Resistor Resistance in Electrical Conductors i.e. wires • Resistance depends on: – Length of wire – Thickness of wire – Type of material used (metal) – Temperature – How do you think each one of these parameters effects resistance? Resistance in Electrical Conductors L R A R is resistance L is wire length A is cross-sectional area is resistivity of conductor Resistance of Electrical Conductors L R A Area Wire Gauges Resistivity Table The Simple Circuit • Needed for circuit – Voltage source – Complete path – Load Electrical Circuit Analogy Battery Light Bulb Schematic Symbols Electron-Flow Current V R I P IV 2 V P R PI R 2 Series Circuit • Circuit that has only one path for current to flow. Series Circuit Analogy Battery Series Circuit Schematic Series Circuit Equations RTotal R1 R2 ... Rn I Total I1 I 2 ... I n VTotal V1 V2 ... Vn P IV Parallel Circuit • Circuit that has more than one path for current to flow. Parallel Circuit Analogy Battery Parallel Circuit Schematic Parallel Circuit Equations 1 RTotal 1 1 1 ... R1 R2 Rn I Total I1 I 2 ... I n VTotal V1 V2 ... Vn P IV Kirchoff’s Law of Current • The current going into a junction (node) is equal to the current coming out ???????? Which one of these is correct? Electrical Safety • Never Intentionally take a shock! Electrical Safety • Remove Jewelry, watches, rings, etc. Electrical Safety • Use one hand (if possible) on live circuit Electrical Safety • Strictly follow lab instructions Electrical Safety • Absolutely NO HORSING AROUND!!! Electrical Safety • Remember: Current KILLS, not Voltage – 5 mA = 0.005 A – 15 mA = 0.015 A – 20 mA = 0.020 A – 100 mA = 0.100 A – 200 mA = 0.200 A Sensation felt Painful Loss of muscle control Heart fibrillation Severe burns, Death Resistor Color Code