Resistance – Starter #2 Most wires are cylindrical in shape with a radius r and a length L. How would you calculate the cross sectional area (A) of the wire? If the radius is doubled, what happens to the cross sectional area A?

Resistance - Starter A copper wire connected across a battery carries a current. How would the current change if the wire was….. twice as long? the same length but with twice the diameter?

Current and Resistance –Essay A. Research the following and write an essay answering each. 1. How is electric current defined? What is the formula? 2. What are the S.I. units for current? 3. How does the resistance of a metal wire depend on the type of metal, the length of the wire, and the cross section area of the wire? ( Hyperphysics has the formula you need and PhET has a good simulation “ Resistance in a Wire”. ) 4. Why are the wires that connect your car battery to the starter thicker than the wires that connect your headlights to the battery? 5. Which has more resistance? A thick or a thin wire? A long or a short wire? The same size copper wire or aluminum wire?

Sample Resistance Calculation Find the resistance of a 50m long wire ( radius = 1mm) made of iron. Use : R = rL/A Look Up the resistivity : riron = 9.71 x 10-8 W . m Get the cross sectional area : A = pr2 = p ( 1 x 10-3)2 R = rL/A = (9.71 x 10-8) (50) / (p ( 1 x 10-3)2) = 1.55 W