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A.C. Circuits

Background of a frequency feneration and transmission Background of a frequency generation and transmission These Images show many knobs which are nothing but the combinations of variable capacitors and resistances in LCR circuits, employed to output a particular frequency, eliminating the noise. Such frequencies are generated and transmitted from the radio transmitting stations using antenna, which are further received at the receiver's end using a radio. Let's learn more about these circuits powered by alternating currents.

Learning objectives

After completing the topic, the student will be able to:

  • Discuss the importance of AC circuits and determine the time period of alternating voltages with root mean square values of voltages and currents.
  • Investigate how to represent an AC signal in a phasor model and its applications in dealing with sinusoidally varying signals.
  • Probe how to measure the resistance of a resistor in any given AC circuit using Ohm's law.
  • Discover the instantaneous current through the capacitor and explore its capacitive reactance for any given AC circuit.
  • Determine the inductance of an inductor present in any given AC circuit and understand the behavior of current with respect to voltage in the circuit.
  • Calculate the current, voltage, impedance of an LCR circuit and investigate the phasor diagrams to know the behavior of current and voltages in the given circuit.
  • Explore about the resonance of LCR circuits and its applications in the AC circuits.
  • Survey the power absorbed by inductor, capacitor and resistor in a given LCR circuit.
Direct Current Direct Current Charging an Ipod with Direct Current Source(Batteries).

Till now we have explored the basic physics of direct current. This is the type of electricity that is produced by batteries, by rubbing certain types of materials against each other, lightning etc. A voltage is created, and possibly stored, until a circuit is completed. When the circuit is complete, the current flows directly, in one direction at a specific, constant voltage. When we use a flashlight, pocket radio, portable CD player, Ipod or virtually any other type of portable or battery–powered device, we are using the direct current. Most DC circuits are relatively low in voltage.

But most present day household and industrial power distribution systems operate with alternating current. Alternating current is a current that varies continuously in magnitude and periodically reverses its direction. The electricity is not provided as a single, constant voltage, but rather as a sinusoidal (sine) wave that over time starts at zero, increases to a maximum value, then decreases to a minimum value, and repeats. A representation of an alternating current's voltage over time is shown in the diagram. Any appliance that we plug into a wall outlet uses alternating current. Circuits in modern communication equipment, including computers and televisions make use of alternating currents (AC) extensively.

Alternating Current Alternating current Graph showing voltage across a sinusoidal AC source
AC (Alternating current)

One might wonder why anyone would bother with such a thing as AC. The use of direct currents is limited to a few applications like charging of batteries, electroplating, etc. Most of electrical energy is generated and used in the form of alternating current due to many reasons. Alternating voltages can be changed in value very easily by means of a transformer. It's cheaper and easier to make devices for AC power. Probably the biggest advantage of AC is that we can use high voltages with small currents to reduce losses (I2 R) when we transmit the energy stored in it. This is the best method of electric power distribution.

Up to now we have discussed circuits that contain combinations of resistor, capacitor, and inductor when they are connected to a dc source of emf. Now we shall discuss these circuit elements when they are connected to a source of alternating voltage that produces an alternating current (AC).

To supply an alternating current to a circuit, a source of alternating emf or voltage is required. A coil of wire rotating with constant angular velocity in a magnetic field or a rotating magnetic field around a set of stationary coils of wire is an example of such a source. This is the basic operating principle of an AC generator, also known as an alternator and it develops a sinusoidal alternating emf. We use the term AC source for any device that supplies a sinusoidally varying voltage (potential difference) 'V' or current 'I'.

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