1 / 19

Resistance

Resistance. and resistivity. Current. Current is sort of a vector Direction is constrained by conductor Restricted to forward or backward (+ or –). Resistance. Current does not flow unhindered Electrical resistance is analogous to friction or drag

dalit
Télécharger la présentation

Resistance

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Resistance and resistivity

  2. Current • Current is sort of a vector • Direction is constrained by conductor • Restricted to forward or backward (+ or –)

  3. Resistance • Current does not flow unhindered • Electrical resistance is analogous to friction or drag • Expressed as potential needed to maintain a current

  4. Ohm’s Law DV I = R I = current DV = voltage = electric potential drop R = resistance Unit of resistance : V / A = ohm (W)

  5. Voltage Causes Current Potential drop is the cause. Current is the effect. Resistance reduces the effect of potential.

  6. Does it Work? • Approximation of varying utility:R is independent of DV and I • When true, the material is ohmic

  7. Poll Question If you want to increase the current through a resistor, you need to • Increase the resistance or voltage. • Decrease the resistance or voltage. • Increase the resistance or decrease the voltage. • Decrease the resistance or increase the voltage.

  8. Ohm’s Law Rearranged If you know two, you can find the third. DV DV I = R = DV= IR R I I = current DV = potential R = resistance

  9. Example A 1.5-V battery powers a light bulb with a resistance of 9 W. What is the current through the bulb? Ohm’s Law I = V / R V = 1.5 V; R = 9 W I = (1.5 V ) / (9 V/A) = 1/6 A

  10. Resistivity For current through a cylinder: L A • Longer L greaterR. • Greater A smallerR. • More resistive material  bigger R.

  11. Resistivity • R = rL/A • ris Resistivity • Unit: ohm·meter = Wm • More or less constant depending on material, conditions

  12. Resistivity • Intensive quantity • Does not depend on the amount of material, only its conditions • Predictive value when mostly constant (ohmic)

  13. Resistivities vary widely

  14. Example The resistivity of copper is 1.710–8Wm. What is the resistance of a 100-km length of copper wire that is 1/4” in diameter?

  15. Classes of Conductors • How resistivity changes with temperature a = temperature coefficient of resistivity

  16. Classes of Conductors • How resistivity changes with temperature

  17. Power dissipated by a resistor

  18. Electric Power Potential is energy per charge:V = DE/ q Current is charge per time:I= q /Dt So, (potential times current) = (energy per time) = power Power = VI

  19. Group Work Power P = VI and V = IR. Using these, show that: • P = I2R • P = V2/R

More Related