L12 Electrical properties

# L12 Electrical properties - Electrical Properties Lecture...

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Electrical Properties Lecture 12 MTE 208

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Ohm’s law Δ V = I R http://en.wikipedia.org/wiki/File:3_Resistors.jpg
Material of interest Ammeter: measures current, I V: applied voltage Ground (V=0) Cross-sectional area, A l Length, l I

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Compare R 2 to R 1 1 2 Direction of current flow Which material has a higher resistance? Can you estimate the ratio of resistances?
Compare R 2 to R 1 1 2 Direction of current flow The longer the sample, the greater its resistance

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Compare R 2 to R 1 1 2 Direction of current flow R 1 R 1 R 2 R 2 = 2R 1 Resistors in series: l 2 = 2 l 1
Compare R 3 to R 1 1 3 Direction of current flow Which material has a higher resistance? Can you estimate the ratio of resistances?

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Compare R 3 to R 1 1 3 Direction of current flow The greater the cross-sectional area of the sample, the lower its resistance
Compare R 3 to R 1 1 3 Direction of current flow R 1 R 1 R 3 Resistors in parallel: R 3 = ½ R 1 A 3 =2A 1

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Extensive vs. Intensive Properties Resistance, R, depends on the shape of the sample, it is an extensive property Normalize R by a geometric factor to find a property that does not depend on the shape of the sample. Resistivity, ρ , is an intensive property that does not depend on the shape of the sample.
How would you normalize R 1 , R 2 and R 3 ? 1 2 Direction of current flow 3 A l

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A l R ρ = = l A R resistivity Resistance Resistivity is a materials property. For a sample made from a particular material, the resistivity will be the same regardless of the shape of the sample. Ex) All pure copper wires will have the same resistivity.
ρ σ 1 = Conductivity Conductivity is another materials property that is used to characterize materials. It is the inverse of the resistivity. Calculate in your group: What are the units of conductivity?

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What are the units of conductivity? Α) Ω m Β) Ω / m C) 1 / Ω D) m/ Ω E) 1 / ( Ω m)
I have a spool of 31 AWG copper wire. The diameter of this wire is approximately 10 mils (1 mil = 1/1000 inch) (1 inch = 2.54 cm) The resistivity of copper is around Question: What simple experiment could I do that would let me calculate the length of the wire without unrolling the spool? Calculate the length of the wire.

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L12 Electrical properties - Electrical Properties Lecture...

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