Explicație:
25)
R=p*l/S
d=2r => r=d/2=0,4mm=0,4 *10^-3m
deci R=2.8*10^-8*36.5 / 0.4*10^-3
=255,5 * 10^-11
26) R=p*l/S => l=R*S/p
S=3mm^2= 3*10^-6m
l=3*3*10^-6/ 60*10^-8
l= 0.15*10^-14
27) l=125m
R=0.44W
p=1.72*10^-8
S=?
S=p*l/R
=1.72*10^-8*125/0.44
=488,63*10^-8
