f.5 phy

by P = V^2 / R

先計一計每個燈泡的resistance.

putting P = 4, V = 2

we have,
4 = 2^2 / R

R = 4 / 4 = 1 ohm


then,
因3個燈泡是並聯, 故它們的equivalent resistance

= 1/ [1/1 + 1/1 + 1/1] (by equivalent R = 1/ [1/R + 1/R' + 1/R''] )

= 1 / 3 ohm

然後, 流通每一個燈泡的電流是

= 4 / 2 (by P = V x I)

= 2 A

因3個燈泡是並聯,so

main current 是2 + 2 + 2 = 6 A

so, consider the main power supply and the main current,
by V = I x R

6 = 6 x R
R = 1 ohm

所以main resistance is 1 ohm

因電阻是與因3個燈泡串聯,

故main R = R + equivalent R of 3個燈泡

1 = R + 1/3

R = 2/3 ohm

R=2^2/4=1
So R of 3 bulb is 3 ohm
6=R+3
R=3 ohm