Yes, Determine Current From Watts

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Discussion Overview

The discussion revolves around whether it is possible to determine the current flowing through a device given only its power rating in watts. Participants explore this question in the context of electrical circuits, specifically using a lightbulb as an example.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants assert that it is not possible to determine current from power alone, emphasizing the need for two of the four electrical parameters: voltage, current, resistance, or power.
  • One participant clarifies that if the voltage is known (as in the case of a 120V lightbulb), the current can be calculated using the formula I = P/V, resulting in 1 Amp for a 120W bulb on a 120V circuit.
  • Another participant notes that a bulb rated at 120W will not draw that power unless supplied with the expected voltage, indicating that the relationship between power, voltage, and current is conditional on the specific circuit conditions.

Areas of Agreement / Disagreement

Participants generally agree that voltage is necessary to determine current from power, but there is some disagreement on the implications of this relationship and the conditions under which it holds true.

Contextual Notes

Participants mention the importance of voltage in determining current, highlighting that assumptions about circuit conditions (such as voltage) are crucial for accurate calculations. There is also a note on the dependency of power ratings on the voltage supplied to the device.

gkangelexa
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Can you determine current given only the watts?

Ex) if you have a lightbulb with 120 Watts, can you determine the current (charge) that flows through it in a second, for example?
 
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Nope.
In a low voltage circuit, you need a lot of current to produce the same power as for a high voltage circuit.
Remember - Power (Watts) = Volts (Volts) X Current (Amps)
 
From the way the question is phrased, no was the correct answer.
You need any 2 (Voltage/Current/Resistance/Power) to determine an unknown in basic circuits.

However, if you MEANT (since you referenced a 120W light bulb) how much current will the bulb draw on a standard 120 VAC circuit, then that is simply P=V*I ---> I = P/V = 120W/120V = 1 Amp.

A 120 W rated bulb will not actually draw/dissipate 120 W of power unless it is powered by the expected voltage (ie a 12V battery will not push 10 amps through it).
 
gkangelexa said:
Can you determine current given only the watts?

Ex) if you have a lightbulb with 120 Watts, can you determine the current (charge) that flows through it in a second, for example?

In general no, you need also voltage:
Power = voltage times current.

In the lightbulb example you probably know the voltage implicitly i.e. it's a lightbulb used with 120V AC house lights. In that case you would solve to find the current is 1 amp (1 coulomb per second).

current = power / voltage
 

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