6. (a) J = i/A; assume circular cross section (b) v_d = J/ne ; Cu has one conduction electron per atom so n can be found from Avagadro's number x density / molar mass. This has already been done in sample problem 28-3

9. Current equals sum of electron current and proton current.

28. Resistance is directly proportional to length divided by area, with the proportionality constant of the restivity of the material. Area is of course directly proportional to the square of the diameter. (R' = 2R)

32. (a) use eqn 28-15, look up resistivity of aluminum. (1.3 mohm) (b) use same eqn, but Cu restivity, solve for d from area. (4.6 mm)

48. (a) use eqn 28-22; (1.0 kW) (b) power in kilowatts x time in hours gives kW hrs ($ 0.25)

52. (a) Ohm's law and resistivity combined (1.74 A) (b) J = i/A (2.15 x 10⁶ A/m²) (c) E = V/L (36.3 mV/m) (d) P = Vi (2.09 W)