7. Consider three light bulbs with ratings of 50 W, 100 W and 150 W. (a) (5 poin

Question

7. Consider three light bulbs with ratings of 50 W, 100 W and 150 W. (a) (5 points) These three bulbs are connected to a standard household circuit with a rms voltage of 120 volts. What is the total rms current through the circuit if the bulbs are connected in parallel? What is the resistance of each bulb? (b) (5 points) What is the total current through the circuit if the bulbs are connected in series? Find the power consumed by each and rank them from brightest to dimmest. 8. (10 points) A cathode tube accelerates electrons through an electric field of 100 N/C. If the tube is 40.5 cm long, what is the final speed of the electrons, assuming they start from the rest? 9. (10 points) A power station delivers 50 MW of power at 50,000 volts. The power is transmitted along a line that has a resistance of 0.375 ohm. How much power is lost in the transmission line?

Explanation / Answer

7:

(a):

current through 50 W bulb = 50/120 = 5/12 A

current through 100 W bulb = 100/120 = 10/12 A

current through 150 W bulb = 150/120 = 5/4 A

current through 50 W bulb = 120/(50/120 )=120/( 5/12) = 288 ohm

current through 100 W bulb = 120(100/120) = 120/(10/12) = 144 ohm

R of 150 W bulb = 120/(150/120) =120/( 5/4 ) = 96 ohm

(b):

current when each R is connected in series = V/Rnet = 120 / (288+144+96) = 0.2273 A

the bulb with higher power will show more intensity of light

bulb(150W)>bulb(100W)>bulb(50W)

(8):

v^2 = u^2 + 2as

v^2 = 0 + 2*(1.6*10^(-19) * 100 / 9.1*10^(-31))*(40.5*106(-2))

v = 11.9339*106(3) m/s

(9):

current = Watt/ Voltage = 1000 A

Power lost = i^2* R = 1000^2 * 0.375 = 375000 watt

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