physics help me please?

Question

Three 60.0 W, 125 V lightbulbs are connected across a V = 125 V power source, as shown in Figure P18.48. (Assume that the resistance of each bulb is constant even though, in reality, the resistance increases markedly with current.)

Figure P18.48
http://www.webassign.net/sercp/p18-48alt.gif

(a) Find the total power delivered to the three bulbs.
W
(b) Find the potential difference across each light bulb. VR1 = V
VR2 = V
VR3 = V

Answer 1

First, the resistance of a 60W light bulb:

These are nominal ratings, implying that the power will be 60 W at the full voltage of 125 V. The nominal current can be found by:

P=I*V
60=I*125
I=60/125

V=I*R
R=V/I
R=125*125/60
R=260 ohms


That will be constant according to the problem statement.


First, lets calculate the current in the system.

Since R2 and R3 are in parallel:

the combined Resistance, Rc, is

1/Rc=1/R2 + 1/R3
or
Rc=(R2*R3)/(R2+R3)
Total Resistance
Rt=R1+Rc
=260+(260*260)/(2*260)
=260+260/2
=390 ohms

again, V=I*R
V/R=I

So power =V^2/R
I=125*125/390

= 40 W

and the current through R1 is
=.3205 A

That means the voltage is
=.32*260
=83.2 V

R2 and R3 will have equal voltage since they are in parallel. Also, since they are equal resistance, the currents will be the same.

The simplest way to compute the voltages is:
=125-83.2
=41.8 V

Check to see if the current is correct:
I=41.8/260
=.16 A

which checks since each bulb will conduct half the current.

j