CAPACITIVE AC CIRCUITS

Capacitors are key components of AC circuits. Capacitors combined with resistors and inductors form useful electronic networks.

CAPACITORS IN AC CIRCUITS

When an AC voltage is applied to a capacitor, it gives the appearance that electrons are flowing in the circuit. However, electrons do not pass through the dielectric of the capacitor. As the applied AC voltage increases and decreases in amplitude, the capacitor charges and discharges. The resulting movement of electrons from one plate of the capacitor to the other represents current flow.

The current and applied voltage in a capacitive AC circuit differs from those in a pure resistive circuit. In a pure resistive circuit, the current flows in phase with the applied voltage. Current and voltage in a capacitive AC circuit do not flow in phase with each other.

                                               Note the  out-of-phase relationship between the current and the voltage in a capacitive AC circuit.                         The current leads the applied voltage. 

 INDUCTIVE AC CIRCUITS

Inductors, like capacitors, oppose current flow in AC circuits. They may also introduce a phase shift between the voltage and the current in AC circuits. A large number of electronic circuits are composed of inductors and resistors.

INDUCTORS IN AC CIRCUITS

Inductors in AC circuits offer opposition to current flow. When an AC voltage is placed across an inductor, it creates a magnetic field. As the AC voltage changes polarity, it causes the magnetic field to expand and collapse. It also induces a voltage in the inductor coil. This induced voltage is called a counter electromotive force (cemf); the greater the inductance, the greater the cemf. The cemf is out of phase with the applied voltage by 180° and opposes the applied voltage. This opposition is as effective in reducing current flow as a resistor.

The applied voltage and the induced voltage are 180° out of phase with each other in an inductive circuit.

 

The amount of voltage induced in the inductor depends on the rate of change of the magnetic field. The faster the magnetic field expands and collapses, the greater the induced voltage. The total effective voltage across the inductor is the difference between the applied voltage and the induced voltage. The induced voltage is always less than the applied voltage.

Figure below shows the relationship of the current to the applied voltage. In a purely inductive circuit, the current lags behind the applied voltage by 90°. 

The current lags the applied voltage in an AC inductive circuit.

BASIC AC RESISTIVE CIRCUITS

The relationship of current, voltage, and  resistance  is similar in DC and AC circuits. The simple AC circuit must be understood before moving on to more complex circuits containing capacitance andinductance.

 BASIC AC RESISTIVE CIRCUITS

A basic AC circuit consists of an AC source, conductors, and a resistive load. The AC source can be an AC generator or a circuit that generates an AC voltage. The resistiveload can be a resistor, a heater, a lamp, or any similar device.

When an AC voltage is applied to the resistive load, the AC current’s amplitude and direction vary in the same manner as those of the applied voltage. When  the applied voltage changes polarity, the current also  changes. They are said to be in phase. 

AC Circuits

The usual waveform of alternating current in most electric power circuits is a sine wave, whose positive half-period corresponds with positive direction of the current and vice versa. In certain applications, like guitar amplifiers, different waveforms are used, such as triangular waves or square waves. Audio and radio signals carried on electrical wires are also examples of alternating current. These types of alternating current carry information such as sound (audio) or images (video) sometimes carried by modulation of an AC carrier signal. These currents typically alternate at higher frequencies than those used in power transmission.