# lecture14 - September 27, 2011 Physics for Scientists &...

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September 27, 2011 Physics for Scientists & Engineers 2, Chapter 25 1

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September 27, 2011 Physics for Scientists & Engineers 2, Chapter 25 2 Some materials conduct electricity better than others If we apply a given voltage across a conductor, we get a large current If we apply the same voltage across an insulator, we get very little current e property of a material that describes its ability to conduct electric currents is called the resistivity , ρ e property of a particular device or object that describes its ability to conduct electric currents is called the resistance , R Resistivity is a property of the material Resistance is a property of a particular object made from that material
September 27, 2011 Physics for Scientists & Engineers 2, Chapter 25 3 If we apply an electric potential diFerence Δ V across a conductor and measure the resulting current i in the conductor, we de±ne the resistance R of that conductor as e unit of resistance is volt per ampere In honor of Georg Simon Ohm (1789-1854), resistance has been given the unit ohm, Ω Rearrange the equation to get Ohm’s Law R = Δ V i 1 Ω = 1 V 1 A i = Δ V R or Δ V = iR

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September 27, 2011 Physics for Scientists & Engineers 2, Chapter 25 4 We will assume that the resistance of the device is uniform for all directions of the current; e.g., uniform metals e resistance R of a device depends on the material from which the device is constructed as well as the geometry of the device e conducting properties of a material are characterized in terms of its resistivity We de±ne the resistivity of a material by the ratio e units of resistivity are ρ = E J [ ] = E [ ] J [ ] = V/m A/m 2 = V m A = Ω m
September 27, 2011 Physics for Scientists & Engineers 2, Chapter 25 5 e resistivities of some representative conductors at 20°C are listed below (more in Table 25.1) Typical values for the resistivity of metals used in wires are on the order of 10 -8 Ω m.

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