Below I copied and pasted an email letter I wrote to a friend who had been struggling with induction and the whole idea of "phase" as it applies to voltage and current. It is too long but I am too lazy to edit it. It kind of fits the question above at least as far as induction and phase goes, which is only a small part of what there is to say about phase in audio. A reference? If you are into tubes a pre 1960 ARRL Handbook is a decent place to start for basic electronics. What Book are you talking about Sean? The Art of Electronics?
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I am going to use some simple examples to help you visualize. Some might say they are too simple. They are right. You should discard them later as you know more. It is just very hard to visualize electronics if you are starting out and all the abstract statements on this topic like "induction opposes all changes in current" and mathematical formulas can seem somewhat befuddling at first. Lets take an over-simplified look at inductions opposition to current change and see how it relates to phase.
A.
Phase as a kind of Delayed Reaction that you already understand.
Phase in Mechanical systems.
An analogy to mechanical systems might help because we are more familiar with it. Induction is like inertia in mechanical systems. Example: if a ball (think of the ball as a little rubbery and not infinitely ridged) is moving South it has a certain kinetic force right? (1/2mass x velocity^2) Newton would say it is going to go forever unless opposed. If you apply a force against it (a new northerly force) it takes a certain force just to stop the ball going South before the ball stops and changes direction. The application of the force and the balls change in direction do not happen instantaneously. The force will be applied a short time before the ball changes direction and begins moving North. This has to be true because the ball has some stored force in it (inertia) and the new northerly force has to expend at least a small part of its energy overcoming this inertia. MAIN POINT: this takes a little time. A time gap is created between the application of the force and the balls change of direction. It might be only an msecond (1/1000 sec.) or a few u seconds (1/1,000,000 sec.), but this might turn out to be important if the ball is cycling back and forth thousands of times a second. This is easy to understand. In fact, youre probably thinking, how could the delayed response of the ball and the use of at least a little of the new northerly force in doing so be otherwise? Well, the same is true with alternating current.
You simply substitute Voltage for force and Current (a number of electrons moving in one direction) for the ball. First remember: in alternating current the voltage (pressure/polarity) AND current (electrons) direction are always changing right? Well, there is a slight lag before the current responds and there is an amount of force used up in just overcoming the inertia of the current. In electronics the gap in time is called a phase angle.
B.
Rubber balls, as you have guessed, are not at the bottom of electrical Inertia. It can be thought of, in part, as being caused by the exchange of energy between current and the magnetic field that is always associated with the current. This exchange sucks a little energy away from the current (to the Mfield) when the current tries to increase and returns the energy to the current (from the Mfield) when it tries to decrease. The sucking away opposes and delays increases in current and the giving back opposes and delays decreases in current in respect to the new voltage. That is ...it gets them out of phase with it. The oft used term electromagnetic force (emf) is the first clue that electricity (current) and magnetism are just versions of a common more fundamental force and in AC they spend a certain amount of time fighting each other. (note: Think of what follows as a model we can use to talk about it rather than an explanation of what is really happening.)
Inductance (and capacitance) does not use up/dissipate energy like resistance. You run current through a resistor and it uses up some power as heat. Not so with inductance which makes a magnetic field with some energy and then returns it to the circuit at some point in time (capacitance makes an electric field). How it does this and the timing of it are important to understanding phase.
Here is a more common way to think about what is happening with electrical inertia w/o resorting to rubber balls. Remember that magnetism is directional. You already know that because it is what makes a compass work. What is more directional than a compass? Magnetism is essential to induction (actually it might be more proper to say that it IS induction.) In addition, magnetism and current are kind of the same thing or are at least next of kin. For example: if you run a current through a wire a magnetic field is created around the wire. And, if you simply move a wire through a magnetic field (you have to move it in a particular direction) a current is created in the wire. You know this too because it is how generators work. Wed both be sitting in the dark now if it werent so. Current and magnetism are just two peas in a pod so to speak. The main thing to keep in mind for now is that, 1) these forces are intimately related to each other. They actually kind of miraculously morph into each other like a caterpillar to a butterfly (are they two bugs or is it one bug?), only faster, and each morphs into each, and quite frankly, Im not certain that anyone really understands how it happens any more than they understand life itself. So, if youre struck with a sense of mystery about all of this it is a good thing and does not mean you are stupid. Some smart folk have figured out how to measure and label its observable effects though! and 2) they are DIRECTIONAL.
Lets make sure we have a little grip on this directional issue. If you run a current through a wire in front of you, with wire and CURRENT running left to right, the current will produce (cause and effect here is a difficult thing. for now lets just say the current causes the field.) a magnetic field around the wire. The magnetic lines of force are usually thought of as little hoops surrounding the wire. The lines of force are directional moving away from you over the top of the wire and coming back toward you underneath the wire. If you ran the current in the opposite direction (right to left) the magnetic lines of force would be coming at you over the top of the wire and away from you under the bottom of the wire. This would be just the OPPOSITE as before. From this you can see if we are dealing with a current changing direction we are also dealing with OPPOSITE/OPPOSING magnetic forces too. Lets look at how these stack up in typical AC.
Lets say you have the wire hooked to a battery and the current as described above flowing left to right and the inescapable directional magnetic field around the wire. (That would mean that the Neg. terminal of the battery is hooked to the left and the Pos. to the right side of the circuit. Current flows from Neg. to Pos.) You have some energy in the fieldcreated by the current. You flip a switch that reverses the battery connections. What happens? The current begins slowing down and tries to stop going left to right (the original direction) because the battery terminals, and therefore the voltage polarity (the force pushing the current), have been reversed. The battery is now trying to make the current go right to left. The new battery driven voltage phase is changed to the opposite. What about the current and its phase? Well, as soon as you flip the switch something else happens too. The current begins decreasing and as soon as it does the magnetic field created by the current going in the Original Direction collapses and, in a since, returns its energy to the wire and this energy is still pushing the current in the direction of the ORIGINAL current (left to right) because it is directional! You have flipped the switch and tried to change the direction of the current and the collapsing magnetic energy is opposing it. This is a simple example of what they mean when they say induction opposes changes in current. In this example we manually changed the polarity of the DC battery circuit. With AC change in direction of current/polarity is frequency dependent. (It is easy to see that an inductor will tend to impede higher fast changing frequencies from flowing while allowing low frequencies to pass and why inductors are natural low freq. pass filters.
If, instead of decreasing the current, you wanted to increase (and thereby change) the current going left to right you would find that induction sucks up an amount of the current energy and morphs it into a butterfly, no sorry, I mean a magnetic field. This has the effect of delaying/opposing the increase in current because some of the energy has been well .
kidnapped by magnetism. If you put current through an inductor/choke, which is just a coil of wire, the first thing it does, before it allows much current to pass, is take a lions share of the energy from the current and it immediately drops/creates a voltage across itself. The voltage is developed first and the increase in current lags behind. And of course, if you try to decrease the current again, there will be even more energy coming from the collapsing magnetic field to fight/oppose the decrease in current. In all of the above examples induction has opposed a change in current and caused a time gap between the change of the applied Voltage and that of the current. That is, it has caused E (voltage) and I (current) to be out of phase.
Summary
It is as if a current, when starting out, chooses to store some of itself in the form of a Mfield for a rainy day. When its going good and current is increasing the current saves some of itself in the form of a Mfield and it does this as soon as it changes to a new level. Then, when it starts going down, it draws on the account from the Mfield. (Actually, both exchanges are usually going on at the same time.) So, if you are changing a current by increasing it, there is an opposition to the increased current because some of the energy is immediately redirected from the current to the field. If you try to decrease the same current there is an opposition as the field returns this energy. Its sort of an attempt at self-preservation or a penchant for the steady state. From here it is easy to see that if you had a current that you wanted flattened out (think of a sine wave that you want to turn it into a straight line.) because you are trying to rectify/make direct current or something, you could do so by running it through an inductor, because it is going to oppose all those ups and downs and flatten the sine wave.
Anyway, the result is that in Alternating Current (which is always changing) you have a constant storing up and releasing of field energy and this energy exchange works against the change in the current. This exchange of energy causes a time gap between changes in the applied voltage and the response to such changes by the current. In circuits where there is inductance they are doing a little dance that is always out of step. Of course this analogy is an oversimplification too. At least it is a starting point to understand phase. For example, the word directional, as I have used it, has glossed over more than it has explained.
C. Power and Phase
In any event, phase is important for a number of reasons. One of them is power. Generally, when you use electricity you want it to do some work. You are sending it to a light bulb or a speaker and have a particular use for it (the Load). Well, if the force is used fighting electrical inertia it cant be used for the load can it? If you had a pure inductive circuit (available in theory only) the voltage is so out of phase with the current
that it is spending all its force fighting itself! You can have a current flowing and no power delivered to your load. The power used overcoming the inertia is not used by the load and is called wattless power not real power. Phase angle and the real power are thus directly related. That is why the simple equation for power (P=IE, power equals current times voltage) is true for dc circuits and ac circuits with resistance only. With ac circuits having inductance and/or capacitance you must multiply the P=IE result by the power factor which is derived from the phase angle.
A last note. I think sine waves can cause real confusion about this stuff. Sine waves often carry the implication of motion. However, voltages do NOT move. When you look at a representation of a voltage using a sine wave it helps to remember this. The sine wave is telling you about QUANTITY, not a motion or movement of the voltage. Similarly, current does not have a polarity a like voltage. It has a direction and movement. To often it is implied that current below the reference line on a sine wave is negative and above positive. This is not true. Anyway if you look at a representation of voltage and current in sine wave form for purposes of examining phase and start thinking about voltage in motion and current with polarities I think you will only confuse yourself.
Cheers,
CR
