8B - 6_Resistor_11.ppt

# 8B - 6_Resistor_11.ppt - Electric Currents & Circuits Were...

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Electrodynamics - Moving Charge Previously E = 0 in conductors; charges did not move.(a) Now we have a closed circuit with a potential (voltage) across its ends. (b) There is an E field in the conductor and charges MOVE. The SI unit for current is the coulomb per second, or ampere (A, amp), which is an SI base unit:
Compare Water and Electricity Current Coulombs/sec = Ampere (A, amp) Gallons/sec Flow of stuff Charge [coulombs] Liter, gallon, kg Quantity of stuff Electricity Water Category

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Current At Junctions Because charge (like water) is conserved, the magnitudes of the currents in the branches must add to yield the magnitude of the current in the original conductor, so that
Directions of Currents A current arrow is drawn in the direction in which positive charge carriers would move, even if the actual charge carriers are negative and move in the opposite direction CHECKPOINT: The figure here shows a portion of a circuit. What are the magnitude and direction of the current i in the lower right-hand wire? 8A

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Compare Water to Electricity Potential or Voltage difference In [volts] Pressure difference Cause of flow Current Coulombs/sec = Ampere (A, amp) Gallons/sec Flow Charge [coulombs] Liter, gallon, kg Quantity of stuff Electricity Water Category
Resistance The SI unit for resistance that follows from the definition is [volt per ampere]. This combination occurs so often that we give it a special name, the ohm (symbol Ω ); that is, In a circuit diagram, we represent a resistor and a resistance with the symbol .

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Compare Electricity to Water Potential difference [Volts, V] Pressure difference Cause of flow Current [ampere, i] Gallons/sec Flow Charge [coulombs, q] Liter, gallon, kg Quantity of stuff Electricity Water Category Resistance [Ohms, Ω ] Clog Opposition to flow
Ohm’s Law The ratio of the potential difference to the current it generates is the RESISTANCE, R = V / I Materials for which R does not change with I or V are said to obey “Ohm’s Law”, or are “Ohmic” materials. I.e., for an Ohmic material the ratio V / I = R = constant, independent of V. Note that the ratio V/ I always defines a resistance. For Ohmic materials this ratio is independent of I or V.

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Ohm’s Law V = I R. For uniform V, E = V/L = I (R/L) Define current density, J = / A = current per unit area, E = I (R/L) = ( I /A)(RA/L) = ρ J where we define resistivity ρ = RA/L = resistance of a unit shape of material . Take A = 1m 2 and L = 1m. [ ρ ] = ohm-m. Its reciprocal, σ = 1/ ρ , is conductivity . J = σ E R = ρ L/A is an important formula, physically reasonable. The device is called a “resistor ”. (pipe “clogor”) ρ or σ (-ivity) are properties of the material . (pipe “clogivity”?) R (-ance) is a property of a particular object , depending on its dimensions and ρ . (pipe “clogance”)
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## This note was uploaded on 05/02/2011 for the course PHYS 8B taught by Professor Catherinebordel during the Spring '10 term at Berkeley.

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8B - 6_Resistor_11.ppt - Electric Currents & Circuits Were...

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