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Resistance can be calculated for any object. Good conductor – low resistance

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.

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Resistance can be calculated for any object. Good conductor – low resistance

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  1. 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.

  2. 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]

  3. 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.

  4. Length Area Resistivity: (units m) Resistance • The geometry of the resistor matters • Increase the Length, flow of electrons impeded • Increase the cross-sectional Area, flow enhanced

  5. For a 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.

  6. Increasing the cross-sectional area increases the number of available electrons.

  7. Resistivity Resistivity has units of Ohm· meters (· m) Therefore, the units of resistance are:

  8. Resistivity and 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.

  9. 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.

  10. Two wires, 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? dA = 4 dB dA = 2 dB dA = dB dA = ½ dB dA = ¼ dB

  11. A wire of resistance R is stretched uniformly (keeping its volume constant) until it is twice its original length. What happens to the resistance? it increases by a factor 4 it increases by a factor 2 it stays the same it decreases by a factor 2 it decreases by a factor 4

  12. Find the resistance of a piece of copper with a diameter of 1 mm and a length of 1 cm

  13. B 5 cm 20 cm A Bolbo the 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? =1.92 u don’t try to cash it in!

  14. A wire 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. What is the total resistance of the wire? What is the current flow through the wire?

  15. Summary

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