8.5.P20

1. Write Ohm's law in symbols, carefully defining each of the variables.

2. Write Ohm's law as a "word equation" expressing a relationship among physical measurements.

3. Discuss what you found in your laboratory about whether all materials obey Ohm's law and show a graph illustrating your results.

4. Discuss the microscopic mechanism responsible for Ohm's law working in a resistor and identify what characteristics of the material the resistor is made of are relevant for determining the current flow in the resistor.

5. If we have a uniform block of material, its mass is dependent on the geometrical properties of the block (its volume) and a property of the material that is independent of the size of the particular bloc (the density). We can then write

(mass) = (density)(volume)

in order to separate those factors.

Consider a cylindrical resistor of cross sectional area, A, and length, L, carrying an electrical current perpendicular to A and parallel to L. Write a decomposition of the resistance of the cylinder on the geometrical properties of the resistor (A and L) and on other properties of the material that are independent of the size and shape of the resistor.

Joe Redish 4/26/10