8B - 6_Resistor_11.ppt

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

Info iconThis preview shows pages 1–11. Sign up to view the full content.

View Full Document Right Arrow Icon
We’re on the move.
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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:
Background image of page 2
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
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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
Background image of page 4
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
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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
Background image of page 6
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 .
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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
Background image of page 8
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.
Background image of page 9

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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”)
Background image of page 10
Image of page 11
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 05/02/2011 for the course PHYS 8B taught by Professor Catherinebordel during the Spring '10 term at Berkeley.

Page1 / 37

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

This preview shows document pages 1 - 11. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online