FAQs in EG 1103 Electrical Engineering

• Q: Does the arrow associated with a voltage source always point at the + (high potential) terminal?
A: No. The arrow itself is meaningless. As re-iterated in the class, any voltage or current is actually characterized by two things: its direction and its value. The arrow of the voltage symbol for a voltage source could point at the negative terminal (in this case, the value of the voltage will be negative) or at the positive terminal (in this case, its value will be positive).
• Q: What is the current of a voltage source?
A: The current of a voltage source is depended on the other part of circuit connected to it.
• Q: Does a voltage source always supply power to other components in a circuit?
A: NO. A voltage source might be consuming power if it is connected to a circuit which has other more powerful sources. Thus, it is a bad idea to pre-determine whether a source is consuming power or supplying power. The best way to determine it is to follow the definition in our text and compute the power. If the value turns out to be positive, then the source will be consuming power. Otherwise, it is supplying power to the other part of the circuit.
• Q: Is the current of a voltage source always flowing from + to - terminals?
A: NO. The current of a voltage source is not necessarily flowing from the positive terminal to the negative terminal.
• Q: What is the voltage cross over a current source?
A: It depends on the circuit connected to it.
• Q: Is the reference power (or voltage, or current) in the definition of the relative power (or voltage, or current) unique?
A: No. The reference power  (voltage or current) can be any value. Remember that whenever you deal with the relative power (voltage or current), you should keep in your mind that there are a reference power (voltage or current) and an actual power (voltage or current) associated with it.

•  Q: How to determine whether a device is consuming or supplying power?
A: Follow the definition we use in the textbook. First, make sure that the current and voltage of the device you consider are in the opposite directions (if not, change one of them so that they are in the opposite directions. Of course, you will have to change the sign of its value as well). Next, compute the power of the device, say X, as the product of the values of the current and voltage. Then, you can conclude that the device is consuming a power of X, which could a positive or negative scalar. If X is positive, then the device is actually consuming a power of X. On the other hand, if X is negative, then the device is actually supplying a power of -X to the other components in the circuit.

By the way, if you add up the total powers consumed by devices in a circuit and the total power supplied by devices in the same circuit, they must be equal. This is one way to check whether your answers are correct or not.

•  Q: What is the definition of independent loops in a circuit?
A: A set of loops is said to be independent if any loop in the set has at least one branch, which is not a part of all the other loops.
•  Q: Can we apply KVL to the magnitudes or the rms values of voltages in AC circuits?
A: No. In an AC circuit, the KVL is valid for the phasors of the voltages in a closed-loop, i.e., the sum of the phasors of voltages in a closed-loop is equal to 0 provided that they are all assigned to the same direction. KVL cannot be applied to the magnitudes or rms values of the voltages alone. For example, a closed-loop circuit containing a series of an AC source, a resistor and a capacitor could have the following situation: The source has a voltage with a rms value of 20V, while the resistor and the capacitor have their voltages with the rms values of 9V and 15V, respectively. All in all, if you want to apply KVL in AC circuits, apply it to the phasors of its voltages.

By the way, KVL is valid as well when the voltages are specified as functions of time. This is true for any type of circuits.

•  Q: Does a frequency response has a unit?
A: Yes. To be precise, it does have a unit. Frequency response is defined as the ratio of the phasor of the output, which can be a voltage signal or a current signal, to the phasor of the input, which again can be a voltage signal or a current signal. If both the input and output are voltage signals as those examples in the text, then its frequency response carries a unit V/V (or unitless). If both are current signals, the corresponding frequency response has a unit A/A (unitless). Similarly, for the case when the input is a current (voltage) and the output is a voltage (current), its frequency response should have a unit V/A (A/V).