Passive sign convention (negative watts, and negative current confusion)

In summary, the current flowing into a positive terminal of a rechargeable battery is absorbed by the battery. The battery has to do work to carry the current through to the negative terminal.
  • #1
FocusedWolf
81
0
This is my (error full?) understanding from my readings:

1. positive current flowing into a positive terminal of let's say a rechargeable battery is power being absorbed by the battery...so positive watts

2. negative current flowing into a positive terminal encounters the higher electrical potential at the positive terminal and the battery has to do work to carry it through to the negative terminal, so the battery is discharging...so negative watts

3. positive current flowing into negative terminal... charges the battery?

4. negative current flowing into the negative terminal... discharges some watts?

so I'm a bit unsure with the last two and want to know what, if anything, is wrong with my observations.
 
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  • #2
ok, sign convention can be a pain but as long as you are consistent, it doesn't really matter whether it is +/-... after all it is just a convention. Now, in calculations, usually common sense can help you out a lot in determining polarities and/or whether a device is active or passive.

In cases where that is not so clear, ie. just a black box, then extra care should be taken. Some general rules:-

negative current is just positive current flowing in the opposite direction

Assuming a 2-terminal device
so -10A into -ve terminal which is the same as 10A out of -ve terminal; which is same as 10A into +ve terminal; which is same as -10A out of +ve terminal!

defining +/- terminals? you may say what if one delibrately defines the polarity so that +ve terminal actually has a lower potential than the -ve terminal (relative to the same ground/ref point). In that case, the voltage across the given device is simply negative .

In the examples you listed, since you know that it is a battery, so your know that when the battery is giving out energy your +ve terminal is the end with higher potential. Of course if it is just a black box, you will probably need more info from the rest of the circuit to be sure. But once everything is worked out (conservation of charge/energy etc...) , if the polarity/direction was set incorrectly, then the voltage/current will simply get a -ve sign. when working on power remember you have two conventions:

P=VI for passive convention (+ve current into +ve terminal)

P=-VI for active device (+ve current into -ve terminal)

NB +/-ve here are AS DEFINED by you in the diagram.

if you are consistent, everything will work out in the end. :smile:
 
  • #3
I think i figured it out if the attached picture shows it correctly :P
 

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  • #4
Hmm no, still confused.

In attached picture, problem (b), if i flip the current arrow so its 1.75A entering the plus side... isn't that passive sign convention satisfied then? and p = (-3.8V) * (1.75A) = -6.65W? book says answer is 6.65W.
 

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  • #5
can't view your picture, so can't help you. If you are sure that you have been consistent, then you will prevail over the book :smile:
 
  • #7
ok, i think you have just read the question wrong :smile:

Q 2.7 asks for power being generated... you probably have assumed the wording for 2.6 when answering it ..but the questions clearly refer to which diagram they are talking about...
 
  • #8
DOOOOH! AAAAHAHAHAHA! :rofl:
 
Last edited:

1. What is the passive sign convention in electricity?

The passive sign convention is a convention used in electrical circuit analysis that defines the direction of current and voltage in a circuit. It states that current flows from higher potential to lower potential, and voltage is positive when a circuit element absorbs power and negative when it delivers power.

2. What is the difference between positive and negative watts?

In the passive sign convention, positive watts indicate that a circuit element is absorbing power, while negative watts indicate that it is delivering power. Positive watts are associated with a voltage drop, while negative watts are associated with a voltage rise. This convention helps to clarify the direction of power flow in a circuit.

3. How do negative watts and negative current cause confusion?

The confusion arises when the direction of current and voltage do not align with the passive sign convention. In some cases, negative current may flow in the opposite direction of the positive voltage drop, leading to negative watts. This can be counterintuitive and may cause confusion when analyzing a circuit.

4. Why is the passive sign convention important in circuit analysis?

The passive sign convention is important because it helps to maintain consistency and clarity in circuit analysis. It allows for a standard method of representing power flow and simplifies the calculations involved in analyzing a circuit. Without this convention, it would be difficult to determine the direction of power flow and accurately analyze a circuit.

5. How can one avoid confusion with negative watts and current?

To avoid confusion, it is important to always follow the passive sign convention when analyzing a circuit. This means that the direction of current should be from higher potential to lower potential, and the voltage should be positive when a circuit element absorbs power and negative when it delivers power. It may also be helpful to double-check calculations and make sure they align with the passive sign convention to avoid any errors or confusion.

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