1
\$\begingroup\$

We know that if a current is entered through the positive polarity of an element and exit through the negative polarity, then the power is positive which means that the power is consumed through the element. On the other hand, if the current is entered through the negative terminal then the power is negative which means that the element has generated power to any external element connected to it. My question is that I need to understand the physics behind this "rule" in an easy way. Thanks in advance.

Edit: I need to understand the physical reason behind this convention. For example, why specifically if the current enters the + terminal then the power is dissipating in the element why this is not the case if the current enters from the -ve terminal?

Edit 2: Ok Guys I think I have an explanation but I am not sure of it. When the current enters from a positive terminal and exits from the negative terminal, this means that the charges did enter from a point of high energy (high volt) and exit from a point of low energy (low volt) this means that the charges while they have passed through the element they have lost some energy and therefore this lost energy is absorbed by the element according to the law of conservation of energy. The opposite can be said if the element where generating energy. So is this explanation is right?

\$\endgroup\$
4
  • \$\begingroup\$ By convention, loads are positive and sources are negative with the sum =0 \$\endgroup\$ Feb 9 '20 at 20:56
  • \$\begingroup\$ Be careful not to conflate the sign of power with polarity of electron/hole flow. A 1 ohm resistor will dissipate 1 watt if you run 1 amp of current through it at 1V, regardless of the direction of the applied voltage/current. You appear to be thinking that reversing direction of current flow will cause a power using circuit to generate power and it will not. \$\endgroup\$
    – K H
    Feb 9 '20 at 21:52
  • \$\begingroup\$ what is element? \$\endgroup\$
    – jsotola
    Feb 9 '20 at 22:08
  • \$\begingroup\$ Does this answer your question? What's passive about the passive sign convention? \$\endgroup\$
    – winny
    Feb 10 '20 at 15:42
1
\$\begingroup\$

One way to define voltage (electrostatic potential) is the answer to the question "how much energy would it take to put an element of charge at this location?".

So if charge enters a circuit element with a certain amount of energy, and leaves with less energy, the energy of the charge must have gone somewhere in the element.

On the other hand, if charge enters a circuit element with a certain amount of energy, and leaves the element with more energy, the element must have done something to increase the energy of the charged particles involved.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.