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Advanced Physics

Advanced Physics. Chapter 18 Electric Currents. Chapter 18 Electric Currents. 18.1 The Electric Battery 18.2 Electric Current 18.3 Ohm’s Law 18.4 Resistivity 18-5 Superconductivity 18.6 Electric Power 18.7 Power in Household Circuits 18.8 Alternating Current

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Advanced Physics

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  1. Advanced Physics Chapter 18 Electric Currents

  2. Chapter 18Electric Currents • 18.1 The Electric Battery • 18.2 Electric Current • 18.3 Ohm’s Law • 18.4 Resistivity • 18-5 Superconductivity • 18.6 Electric Power • 18.7 Power in Household Circuits • 18.8 Alternating Current • 18.9 Microscopic View of Electric Current • 18.10 The Nervous System and Nerve Conduction

  3. 18.1 The Electric Battery • Alessandro Volta (1800’s) invented the electric battery, the first source of a steady flow of electric charge • Parts of a simple battery: • Electrodes-plates or rods of dissimilar metals (carbon) • Electrolyte-solution through which charged material (ions) flow

  4. 18.1 The Electric Battery Electric cell • Two electrodes and an electrolyte Battery • Several cells connected together • Symbol Terminal • Part of electrode that extends outside the electrolyte

  5. _ + 18.1 The Electric Battery e- How a simple cell works: • Acid attacks Zinc electrode • Zinc ionizes (Zn2+) and 2 e- leaves at negative electrode • Zn2+ enters solution • Zn2+ pulls e- off carbon electrode it becomes positive • If terminals are not connected then only a small amount of zinc reacts • If terminals are connected then flow of electrons Zinc Carbon Zn2+ H2SO4

  6. 18.1 The Electric Battery Carbon terminal Conventional current • For positive to negative (the flow of positive charges) Dry cell • Use of an electrolyte paste • Connect batteries in series to increase voltage insulation Electrolyte paste Casing Zinc terminal

  7. 18.2 Electric Current Electric circuit • Continuous conducting path between terminals of a battery Electric current • The flow of charges through a conductor (I) • I = Q/t • Current is the net charge (Q) that flows through a conductor per unit time (t) at any point. • Unit: ampere, amp, A 1A = 1C/1s • Current is the same at any point in a conductor between two terminals.

  8. 18.3 Ohm’s Law • To produce an electric current in a circuit a difference in (electric) potential is required. Simon Ohm (1787-1854) • Experimentally determined that I  V • Exactly how much current flows depends on voltage and resistance to the flow of electrons Resistance • How much a conductor impedes the flow of electrons • Unit: ohm ()

  9. 18.3 Ohm’s Law Ohm’s Law • V = IR • Only good when there is no change in temperature due to current flow Resistor • A device of known resistance • Color codes • symbol

  10. 18.4 Resistivity • Resistance is greater for a thin wire and for a long wire (why?) • R = (L/A) where: • R = resistance •  = resistivity (p.535) and depends on material, temperature and other factors • Silver < copper < aluminum • L = length of wire • A = cross sectional area of wire

  11. 18.4 Resistivity • Since resistivity depends on temperature • As temperature   resistance  (why?) • T = o [1 + (T – To)] where: • T = resistivity at any temperature (T) • o = resistivity at reference temperature (To) •  = temperature coefficient of resistivity • Equation holds true for “small” T’s

  12. 18-5 Superconductivity • When a compound (metal alloy) has a resistivity of zero • Occurs at low temperatures (below transition temperature-Tc) • Loses all resistance to the flow of electrons • Costly and brittle • Uses: electromagnets

  13. 18.6 Electric Power Power • The rate at which electrical energy is transferred or transformed into another form of energy (thermal, kinetic, light, etc) • P = QV/t since I = Q/t then P =IV

  14. 18.6 Electric Power Power • The rate at which electrical energy is transferred or transformed into another form of energy (thermal, kinetic, light, etc) • If this energy transfer is due to resistance then it can be calculated by…… • P=IV + V=IR  P = I2R = V2/R

  15. 18.6 Electric Power Power • When you purchase electricity from the power company you are buying energy not power. • You purchase kilowatt-hours (energy) not kilowatts (power) • P = E/t so…E = Pt (or kWh = kWhr)

  16. 18.7 Power in Household Circuits • In a household circuit the current in the wiring can cause an increase in temperature that can lead to fires (why?) • Short circuits also can cause overheating To prevent this electricians use: • Fuses • Circuit breaks

  17. 18.7 Power in Household Circuits • Household circuits are constructed in parallel so that….. • Each device used gets the same voltage • Total current in circuit is equal to the current through each device • But this can lead to extreme heating of wires (why?)—Chapter 19!

  18. 18.8 Alternating Current • A battery produces a direct current (DC)—current flows only in one direction (which way?)

  19. 18.8 Alternating Current • An electric generator produces an alternating current (AC)—current flows in two directions

  20. 18.8 Alternating Current • A battery produces a direct current (DC)—current flows only in one direction (which way?) • An electric generator produces an alternating current (AC)—current flows in two directions • Frequency of an alternating current is number of times the current changes direction per second (in US 60 Hz)

  21. 18.8 Alternating Current • A graph of the current versus time produces a sinusoidal curve. • Voltage can be written as a function of time: • V = Vosin2ftwhere • V = average voltage • Vo = peak voltage • f = frequency • t = time

  22. 18.8 Alternating Current • V = Vosin2ft • Using Ohm’s Law we can find peak current (Io) • I = Iosin2ft • And average power (P) • P = Io2Rsin2ft • P = ½ Io2R = ½ (Vo2/R)

  23. 18.8 Alternating Current • The average value for the square of the current or voltage is important for calculating average power • The square root of these values (root mean square-rms) is the average voltage/current • Irms = 0.707Io • Vrms = 0.707Vo • These rms values are called the “effective values” • Io and Vo are peak current and voltage!

  24. 18.8 Alternating Current • These rms values are called the “effective values” • These values can be directly used in the power equations • P = I2rmsR = V2rms/R

  25. 18.9 Microscopic View of Electric Current • As electrons travel through a conductor they bounce off the atoms that make up the conductor • This causes the electrons to speed up and slow down and determine the speed at which they flow through a conductor. Drift speed-the average speed that electrons move through a conductor

  26. 18.9 Microscopic View of Electric Current Drift speed-the average speed that electrons move through a conductor (vd) • So current in a wire is… • I = Q/t = neAvd where: • n = number of free electrons • e = charge of an electron (1.6 x 10-19 C) • A = cross-sectional area

  27. 18.9 Microscopic View of Electric Current Drift speed for electrons through a wire is very slow (0.05mm/s) but electricity travels at close to the speed of light (3 x 108 m/s)—how can this be true???

  28. 18.10 The Nervous System and Nerve Conduction • Read it and know it • Summary due at end of class • Do your homework for this Chapter!

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