20. consider two stars a and b with temperatures ta and tb and radii ra and rb respectively. if ta
= 2 tb and ra= 12 rb, which statement about the stars' luminosities, la and lb, is true?
using this formula: l=(r^2)(t^4)
a) la =lb
b) la = 2 lb
la=4 lb
d) la = 12 lb
e) la = 14 lb

Athin nonconducting rod with a uniform distribution of positive charge q is bent into a circle of radius r. the central perpendicular axis through the ring is a z-axis, with the origin at the center of the ring. a) what is the magnitude of the electric field due to the rod at z = 0? n/c (b) what is the magnitude of the electric field due to the rod at z = infinity? n/c (c) in terms of r, at what positive value of z is that magnitude maximum? r (d) if r = 4.00 cm and q = 9.00 µc, what is the maximum magnitude? n/c