Which to use to calculate I: P=VI or V=IR

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Discussion Overview

The discussion revolves around the calculation of current in a power transmission cable given specific values for power, voltage, and resistance. Participants explore the implications of using different formulas, specifically P=VI and V=IR, and the potential inconsistencies in the provided values.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Exploratory

Main Points Raised

  • One participant questions whether to use P=VI or V=IR to calculate current, noting that both seem to yield different answers.
  • Another participant asserts that the situation is fundamentally impossible if the values are correct, suggesting that both formulas should provide the same result.
  • A third participant identifies potential inconsistencies in the provided values, calculating different resistances based on the given power, voltage, and resistance.
  • One participant suggests that a load of 2 ohms may be missing from the problem, which would adjust the total resistance and clarify the situation.
  • Another participant emphasizes the importance of context, questioning whether the power and voltage refer to the source or the load, indicating multiple interpretations of the problem.
  • It is noted that the question lacks clarity, leading to various possible answers depending on assumptions made about the system.

Areas of Agreement / Disagreement

Participants express disagreement regarding the interpretation of the problem and the application of formulas. There is no consensus on the correct approach or the validity of the provided values.

Contextual Notes

The discussion highlights limitations in the problem statement, including ambiguity about whether the power is transmitted or dissipated, and whether the voltage is measured at the source or the load. These factors contribute to the uncertainty in deriving a definitive answer.

liangtp
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Hi,

In a power transmission cable, the power, P=24W, Voltage=12V, the resistance of the cable, R=4 ohm.

What is the current that flows in this cable? Shall I use P=VI or V=IR to calculate the current?
Both seems to give different answers.

Thanks
 
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I'm fairly certain that this situation is impossible fundamentally. You should get the same answer for both.
 
One of your given values seems to be incorrect.

If the voltage and resistance are correct, then:
P = V2/R = 12V2/4Ω = 36W

If power and voltage are correct:
R = V2/P = 122/24W = 6Ω

If power and resistance are correct:
V = √(P*R) = √(24W*4Ω) = 9.8V
 
Last edited:
Thanks all for your response.

Agreed with both of you.
Somehow my gut instinct told me that the question I had was somehow flawed.

Thanks again.
 
liangtp said:
Hi,

In a power transmission cable, the power, P=24W, Voltage=12V, the resistance of the cable, R=4 ohm.

What is the current that flows in this cable? Shall I use P=VI or V=IR to calculate the current?
Both seems to give different answers.

Thanks

In this type of situation, or power system, it is generally meant,
the power source is a 12V source and it is delivering 24W power.
In such case, for current flowing from the source, use
P = VI
Then I = 2A.



Probably a load of 2 ohms is missing from the problem, which makes total resistance 6 ohm.
You shouldn't use V=IR unless the problem clearly indicates 12V is dropped in the resistance R.
 
It's also useful to remember that P = V2/R (alternatively P = I2R)

You can of course derive both expressions.
You can see immediately that 12*12/4 is not 24 so the question is nonsense.
 
Neandethal00 said:
Probably a load of 2 ohms is missing from the problem, which makes total resistance 6 ohm.
I think that is the solution to the issue.
A power transmission cable is supposed to transmit something - using it as short circuit (with just the cable resistance) would be a bit pointless anyway.

=> Current I=24W/12V=2A

The cable will get P=I^2*R = 4A^2*4Ω=16W of heat, and the load will receive 4A^2*2Ω=8W.
 
The question is not clear enough to be sure you can get the right answer.

For example..

The power is 24W but is that the transmitted power or the power dissipated in the load?
The voltage is 12V but is that the voltage at the source or the load?

So that's at least four possible right answers!

In most situations a transmission system has to be designed to suit the load not the source. For example in a PC the motherboard needs defined voltages (with a tollerance) measured at the motherboard not at the power supply end of the wiring loom.

So one valid answer would be to assume the load needs 12V delivered and will draw 24W.

That would suggest the current through the load is 24W/12V = 2A.
If the the cable resistance is 4 Ohms the source voltage needs to be 12+2*4 = 20V.
The power delivered by the source would be 20V * 2A = 40W.

but as I said that's just one of a number of possible answers that could be argued are correct given the problem statement is so loose.
 

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