Aconcave mirror with a radius of curvature of 20 cm has a focal length of

a. 40 cm
b. 20 cm
c. 10 cm
d. answer depends on the index of refraction of the air around the mirror

A. 40 cm

Explanation:

For a curved mirror, the focal length is twice the radius of curvature of the mirror. In formula:

where

f is the focal length

R is the radius of curvature

The concave mirror in this problem has a radius of curvature of

R = 20 cm

so, its focal length is

96.77 amp

explanation:

rl = 250/100 = 2.5 ohms.

egl = 250 + (100 × 0.1) + 2 = 262 v.

at 700 r.p.m., eg2 = 262 × 700/1000 = 183.4 v.

if ia is the new current, then eg2 - 2 - (ia × 0.1) = 2.5 ia

which means this will give ia = 96.77 amp.