Using conventional current (not electron current), the following is always true.
Given a black box that has two terminals, if current is flowing into the box at the terminal that has the higher of the two terminal voltages, then power is flowing into the box and it is acting like a load. If current is flowing out of the terminal that has the higher of the two terminal voltages, the power is flowing out of the box and it is acting like a source.
Given a circuit that you are going to analyze, you are free to assign the polarity of the voltages and currents however you want, as long as you then interpret them with the above statement in mind.
To make this easier and less error prone, there is what is known as the 'passive sign convention' which basically says the following:
For each component, decide whether you will consider it to be a source or a load. If you are unsure, then just pick one (I generally call it a load if I'm not sure). Then go through and assign the voltages and currents as follows: For each load, take your best guess at either the direction of the current or the polarity of the voltage (sometimes one will be more apparent than the other). Once you have chosen one of those, then assign the polarity of the other so that current flows into the positive side, through the device, and out the negative side. Then do the same, except in reverse, for all of the components that you declared to be a source -- namely take your best guess at one quantity (voltage or current) and then assign the polarity of the other so that it enters the device at the negative side, goes through the device, and comes out the positive side. Just remember, current enters the positive side of a load and exits the positive side of a source.
Now analyze the circuit. When you are done, one of the following will be the case for each component: Both the voltage and the current for that device are positive -- congrats, you picked both the type of device (source or load) and the polarities correctly. Both the voltage and the current for that device are negative -- you guess the type of device correctly, but got the polarities wrong, so simply swap the directions on both the voltage and the current. One is positive and the other is negative -- you got the type of device wrong, so swap the type of device and the polarity of whichever quantity is negative.
Regardless of whether you choose to make the corrections mentioned above or just live with the negative quantities, you interpret them as follows:
For something you called a load: If the voltage times the current yields a positive power, then it is acting as a load and is absorbing power, otherwise it is acting as a source and supplying power.
For something you called a source: If the voltage times the current yields a positive power, then it is acting as a source and is supplying power, otherwise it is acting as a load and is absorbing power.
Regardless of whether you picked things right or wrong, the following will be true:
The total power summed over all the things you called sources will equal the total power summed over all the things you called loads. This will be true even if you just blindly called everything a source or everything a load or if you randomly assigned the source/load designations.
Given a black box that has two terminals, if current is flowing into the box at the terminal that has the higher of the two terminal voltages, then power is flowing into the box and it is acting like a load. If current is flowing out of the terminal that has the higher of the two terminal voltages, the power is flowing out of the box and it is acting like a source.
Given a circuit that you are going to analyze, you are free to assign the polarity of the voltages and currents however you want, as long as you then interpret them with the above statement in mind.
To make this easier and less error prone, there is what is known as the 'passive sign convention' which basically says the following:
For each component, decide whether you will consider it to be a source or a load. If you are unsure, then just pick one (I generally call it a load if I'm not sure). Then go through and assign the voltages and currents as follows: For each load, take your best guess at either the direction of the current or the polarity of the voltage (sometimes one will be more apparent than the other). Once you have chosen one of those, then assign the polarity of the other so that current flows into the positive side, through the device, and out the negative side. Then do the same, except in reverse, for all of the components that you declared to be a source -- namely take your best guess at one quantity (voltage or current) and then assign the polarity of the other so that it enters the device at the negative side, goes through the device, and comes out the positive side. Just remember, current enters the positive side of a load and exits the positive side of a source.
Now analyze the circuit. When you are done, one of the following will be the case for each component: Both the voltage and the current for that device are positive -- congrats, you picked both the type of device (source or load) and the polarities correctly. Both the voltage and the current for that device are negative -- you guess the type of device correctly, but got the polarities wrong, so simply swap the directions on both the voltage and the current. One is positive and the other is negative -- you got the type of device wrong, so swap the type of device and the polarity of whichever quantity is negative.
Regardless of whether you choose to make the corrections mentioned above or just live with the negative quantities, you interpret them as follows:
For something you called a load: If the voltage times the current yields a positive power, then it is acting as a load and is absorbing power, otherwise it is acting as a source and supplying power.
For something you called a source: If the voltage times the current yields a positive power, then it is acting as a source and is supplying power, otherwise it is acting as a load and is absorbing power.
Regardless of whether you picked things right or wrong, the following will be true:
The total power summed over all the things you called sources will equal the total power summed over all the things you called loads. This will be true even if you just blindly called everything a source or everything a load or if you randomly assigned the source/load designations.
