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![Resistance and Resistivity
A conductor’s Resistance is defined as the ratio of the Applied Voltage to
the Current produced.
R = V/I
Scalar
Units: Ohm [Ω] = [V/A]](https://image.slidesharecdn.com/resistivity-230227001118-27aa4b5f/85/Resistivity-ppt-3-320.jpg)
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Resistivity.ppt
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- 2. Resistance and Resistivity Resistivity,what is it? Resistivity is the opposite of conductivity, it’s a measure of how effectively a material slows down the flow of electricity. Insulators have a high resistivity rating. Materials such as metals and other conductors have a low resistivity rating.
- 3. Resistance and Resistivity A conductor’s Resistance is defined as the ratio of the Applied Voltage to the Current produced. R = V/I Scalar Units: Ohm [Ω] = [V/A]
- 4. Resistance and Resistivity Resistance can be calculated for any object. Good conductor – low resistance Poor conductor –high resistance All circuit elements have some resistance. The resistance of connecting wire leads is considered negligible.
- 5. Resistance and Resistivity Resistance The geometry of the resistor matters L R=ρ A Increase the Length, flow of electrons impeded Increase the cross-sectional Area, flow enhanced Resistivity: (units m) Length Area
- 6. Resistance and Resistivity Fora wire of length l and cross-sectional area A the resistance R: Is proportional to l And inversely proportional to A The constant ρ (rho) Is known as the resistivity. l R A
- 7. Resistance and Resistivity Increasingthe cross-sectional area increases the number of available electrons.
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- 9. Resistance and Resistivity Resistivity 2 () m R m Ohms m l R A Resistivity has units of Ohm· meters ( · m) Therefore, the units of resistance are:
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- 12. Resistance and Resistivity Resistivityand temperature. The resistance and resistivity changes with temperature, therefore resistivity at quoted at a specific temperature The resistance is proportional to the length l and inversely proportional to the cross-sectional area A, the material the conductor is made from, and the temperature.
- 13. Resistance and Resistivity Superconductivity Below a certain critical temperature, Resistance becomes ZERO, allowing current to exist without energy wasted. Superconductivity, although predicted and discovered a century ago has only become useful recently due to the development of rare-earth conductive ceramics. Superconductive wires are used in the electromagnets of MRI machines.
- 14. Resistance and Resistivity Twowires, A and B, are made of the same metal and have equal length, but the resistance of wire A is four times the resistance of wire B. How do their diameters compare? a. dA = 4 dB b. dA = 2 dB c. dA = dB d. dA = ½ dB e. dA = ¼ dB l R A
- 15. Resistance and Resistivity Awire of resistance R is stretched uniformly (keeping its volume constant) until it is twice its original length. What happens to the resistance? a. it increases by a factor 4 b. it increases by a factor 2 c. it stays the same d. it decreases by a factor 2 e. it decreases by a factor 4
- 16. Resistance and Resistivity Findthe resistance of a piece of copper with a diameter of 1 mm and a length of 1 cm -8 -2 -4 -3 2 (3 x 10 )(1 x 10 ) 12 = = x 10 ( (.5 x 10 ) l R A
- 17. Resistance and Resistivity Bolbothe gold broker from Baghdad has just received a gold bullion bar for his birthday from his brother Bob. The face of the bar is 5 cm on both sides, and the length is 20 cm. The resistance between faces A and B is measured to be 0.8 micro-ohms. Bolbo the broker assumes that the bullion is gold if the measured resistance is within +/- 10% of the theoretical value. Is the material gold or is it bogus? B 8 2 2 6 / (2.4 10 )(.2 ) /.05 ) 1.92 10 R L A x m m m x =1.92 u don’t try to cash it in!
- 18. Resistance and Resistivity Awire 10 m long consists of 5 m of copper followed by 5 m of aluminum of equal diameter of 1 mm. A voltage difference of 80 V is placed across the composite wire. a. What is the total resistance of the wire? b. What is the current flow through the wire?
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