How Much Power is Dissipated on the Light Bulb?

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Discussion Overview

The discussion revolves around calculating the real power dissipated by a light bulb connected in series with a resistor when supplied with a standard AC voltage. Participants explore various methods and formulas to determine the power dissipation, resistance, and current in the circuit.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant states the need to determine the real power dissipated on the light bulb and questions if power is lost over the resistor.
  • Another participant asks for the resistance of the light bulb and the current to calculate the power dissipated by it.
  • It is noted that a 100W light bulb only dissipates 100W when connected directly to 120VAC.
  • One participant suggests using the formula P = V^2/R to find the resistance of the bulb, initially calculating it as 1440 ohms, which is later corrected to 144 ohms by another participant.
  • A participant proposes finding the current using Vrms/R and calculates it as 0.8333 A, leading to a power calculation of 99.99 W.
  • Another participant points out that the 120V is not fully across the light bulb, implying the need for voltage division to find the voltage across the bulb.
  • Participants discuss the validity of using voltage division to find Vrms over the bulb and subsequently finding Irms from that value.

Areas of Agreement / Disagreement

Participants express differing views on the calculations and methods to determine the power dissipated by the light bulb, indicating that no consensus has been reached regarding the correct approach or final values.

Contextual Notes

There are unresolved assumptions regarding the distribution of voltage across the components in the circuit and the implications of the light bulb's resistance on power dissipation calculations.

eatsleep
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1. A 25 ohm resistor is connected in series with a 100 W light bulb. The standard 120V/60Hz AC outlet voltage is applied to the series combination. Determine the real power dissipated on the light bulb. Assume the internal resistance of the light bulb is independent of the power dissipation. Round off your answer to two decimal places.



2. P=Vrms x Irms



3. I used 120 V as Vrms. To find Irms I did 120/25. Is power lost over the resistor? Thanks in advance
 
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What is the resistance of the light bulb? Then, what is the current? And finally, what is the power dissipated by the light bulb?
 
estsleep - A 100W light bulb only dissipates 100W when connected directly to 120VAC.
 
eatsleep said:
1. A 25 ohm resistor is connected in series with a 100 W light bulb. The standard 120V/60Hz AC outlet voltage is applied to the series combination. Determine the real power dissipated on the light bulb. Assume the internal resistance of the light bulb is independent of the power dissipation. Round off your answer to two decimal places.



2. P=Vrms x Irms



3. I used 120 V as Vrms. To find Irms I did 120/25. Is power lost over the resistor? Thanks in advance

120/25 would be the current in the resistor if the resistor were connected directly across the 120V. But it's not - there is a light bulb in series with it dropping some of the voltage ...
 
can i use the formula P = V^2/R to find the resistance of the bulb? so, 100 = 120^2/R. R=1440 ohms?
 
eatsleep said:
can i use the formula P = V^2/R to find the resistance of the bulb? so, 100 = 120^2/R. R=1440 ohms?

Almost. I compute 144 ohms.
 
ok, to find the current can I just Vrms/R? 120/144 = .8333. Then power is I^2 x R. Doing that gives me 99.99
 
No, because the 120V is not all across the light bulb, is it?
 
Will doing voltage division to find the Vrms over the bulb work? Then finding the Irms from Vrms/Rbulb?
 
  • #10
eatsleep said:
Will doing voltage division to find the Vrms over the bulb work? Then finding the Irms from Vrms/Rbulb?

Yes it would.
 
  • #11
rude man said:
Yes it would.

got it! thanks for the help
 

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