Find the rms current in each bulb.

+3 votes

The figure below shows three lightbulbs connected to a 120-V AC (rms) household supply voltage. Bulbs 1 and 2 have a power rating of 45 W, and bulb 3 has a 75-W rating.

image
(a) Find the rms current in each bulb.
Rbulb 1  A
Rbulb 2  A
Rbulb 2  A

(b) Find the resistance of each bulb.
Rbulb 1  Ω
Rbulb 2  Ω
Rbulb 2  Ω

(c) What is the total resistance of the combination of the three lightbulbs?
 Ω

 

asked Mar 19, 2012 in Physics by Gummmys ~Expert~ (1,105 points)
    

1 Answer

+2 votes
 
Best answer

Part A:

Power = Voltage * Current

Current =  Power / Voltage

furthermore, we are using rms voltage, so we will also need rms current:

Rms Current =  Power / Rms Voltage

For example, Bulb one is 45 W, so

Rms Current = 45 W / 120 V =  0.375 A

 

Part B:

Power can also be written in terms of voltage:

Power = Current^2 * Resistance  

so

Resistance = Power / Current^2 

Use Rms Current from part A to calculate the Resistance for each bulb

 

Part C:

Total resistance is as follows for resistors in parallel:

R total = [1 / ( 1/R1 + 1/R2 + 1/R3) ]

*where R1, R2, and R3 are from Part B

answered Mar 23, 2012 by yoshi ~Expert~ (919 points)
selected Mar 24, 2012 by Gummmys

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