When two waveforms go through their maximum and  minimum points at different times, a phase difference exists  between the two. In this case, the two waveforms are said  to be out of phase with each other. The terms lead and lag  are often used to describe the phase difference between  waveforms. The waveform that reaches its maximum or  minimum value first is said to lead the other waveform.  Figure 9-16B  shows this relationship. On the other hand, the  second waveform is said to be lagging the first source. When  a waveform is said to be leading or lagging, the difference in  degrees is usually stated. If the two waveforms differ by 360°,  they are said to be in phase with each other. If there is a 180°  difference between the two signals, then they are still out of  phase even though they are both reaching their minimum and  maximum values at the same time.  [Figure 9-16C] There are three factors that can create an opposition to the flow  of electrons (current) in an AC circuit. Resistance, similar  to resistance of DC circuits, is measured in ohms and has a  direct influence on AC regardless of frequency. Inductive  reactance and capacitive reactance, on the other hand, oppose  current flow only in AC circuits, not in DC circuits. Since  AC constantly changes direction and intensity, inductors and  capacitors may also create an opposition to current flow in  AC circuits. It should also be noted that inductive reactance  and capacitive reactance may create a phase shift between  the voltage and current in an AC circuit. Whenever analyzing  an AC circuit, it is very important to consider the resistance,  inductive reactance, and the capacitive reactance. All three  have an effect on the current of that circuit.  As mentioned, resistance creates an opposition to current  in an AC circuit similar to the resistance of a DC circuit.  The current through a resistive portion of an AC circuit  is inversely proportional to the resistance and directly  proportional to the voltage applied to that circuit or portion  of the circuit. The equations I = E / R & E = I × R show how  current is related to both voltage and resistance. It should be  noted that resistance in an AC circuit does not create a phase  shift between voltage and current.  Figure 9-17  shows how a circuit of 10 ohms allows 11.5 amps  of current flow through an AC resistive circuit of 115 volts. The physical factors that affect inductance are: The primary effect of a coil is its property to oppose  any change in current through it. This property is called  inductance. When current flows through any conductor, a  magnetic field starts to expand from the center of the wire.  As the lines of magnetic force grow outward through the  conductor, they induce an EMF in the conductor itself.  The induced voltage is always in the direction opposite  to the direction of the applied current flow. The effects  of this countering EMF are to oppose the applied current.  This effect is only a temporary condition. Once the current  reaches a steady value in the conductor, the lines of magnetic  force are no longer expanding and the countering EMF  is no longer present. Since AC is constantly changing in  value, the inductance repeats in a cycle always opposite the  applied voltage. It should be noted that the unit of measure  for inductance is the henry (H).  9-12 3.  Length of a coil—doubling the length of a coil, while  keeping the same number of turns, reduces inductance  by one-half.  2.  Cross-sectional area of the coil—the inductance of a  coil increases directly as the cross-sectional area of the  core increases. Doubling the radius of a coil increases  the inductance by a factor of four.  1.  Number of turns—doubling the number of turns in a  coil produces a field twice as strong if the same current  is used. As a general rule, the inductance varies with  the square of the number of turns.  Inductive Reactance Opposition to Current Flow of AC Resistance I =  E R I = 115V 10Ω I = 11.5 amps When moving a magnet through a coil of wire, a voltage is  induced across the coil. If a complete circuit is provided, then  a current will also be induced. The amount of induced voltage  is directly proportional to the rate of change of the magnetic  field with respect to the coil. Conversely, current flowing  through a coil of wire produces a magnetic field. When this  wire is formed into a coil, it then becomes a basic inductor. Figure 9-17.  Resistance. 115V AC  R = 10Ω Ammeter I = 11.5A A