4.1 Introduction. Ohm's Law in Steady State. Sinusoidal

4.1 Introduction. Ohm's Law in Steady State. Sinusoidal

Passive elements such as resistors, coils, and capacitors behave differently in direct current (DC) compared to alternating current (AC). The voltage sources that circuits can contain can be either direct or alternating sources (Fig. 4.1).

Fig. 4.1 Direct Current voltage source, and sinusoidal voltage source. Source Own elaboration

While in direct current the polarity remains constant in alternating current the polarity reverses at equal intervals of time. Because alternating current is variable, new phenomena will occur compared to direct current, particularly because of the use of coils and capacitors as receivers in circuits as we have seen in previous chapter. We can draw how a capacitor behaves in both alternating, and direct current (Fig. 4.2).

Fig. 4.2 Behavior of capacitors in DC and AC. Source Own elaboration

Question Why does a capacitor block DC but allow AC? We have to remember that the three passive elements (resistors, capacitors and coils) limit the current in a circuit, as described by Ohm's law for AC:

$$I=\frac{U}{Z}$$ (4.1)

Although Ohm's law for alternating current (Eq. 4.1) use rms values, we can also use instantaneous values. Then, we have that:

$$i=\frac{u}{z}$$ (4.2)

Next, we look at how passive elements, resistors, coils and capacitors behave in alternating current.