Electrical load types fall into four categories: resistive, capacitive, inductive or a combination of these. Few loads are purely resistive, capacitive or inductive. The imperfect nature of how electrical and electronic devices are built causes inductance, capacitance and resistance to be an inherent part of many devices.
A resistor is a device that resists the flow of electricity. In doing so, some of the electrical energy is dissipated as heat. Two common resistive loads are incandescent light bulbs and electric heaters. Resistance (R) is measured in ohms.
An incandescent light bulb produces light by passing an electric current through a filament in a vacuum. The resistance of the filament causes it to heat up and the electrical energy is converted to light energy. Electric heaters work in the same way except they produce little, if any, light.
The electrical current and the voltage in a resistive load are said to be "in phase" with each other. As voltage rises or falls, the current also rises and falls with it.
A capacitor stores electrical energy. Two conductive surfaces are separated by a non-conductive insulator. When an electrical current is applied to a capacitor, electrons from the current gather on the plate attached to the terminal to which the electric current is applied. When the current is removed, the electrons will to flow back through the circuit to reach the other terminal of the capacitor.
Capacitors are used in electric motors, radio circuits, power supplies and many other circuits. The capability of a capacitor to store electrical energy is called capacitance (C). The main unit of measure is the farad, but most capacitors are measured in microfarads.
The current leads the voltage of a capacitor. The voltage across the terminals starts out at zero volts while the current is at its maximum. As the charge builds on the capacitors plate, the voltage rises and the current falls. As a capacitor discharges, the current rises as the voltage falls.
An inductor may be any conductive material. When a changing current passes through an inductor, it induces a magnetic field around itself. Turning the inductor into a coil increases the magnetic field. A similar principal occurs when a conductor is placed within a changing magnetic field. The magnetic field induces an electrical current within the conductor.
Examples of inductive loads include transformers, electric motors and coils. Two sets of magnetic fields in an electric motor oppose each other, forcing the motor's shaft to spin.
A transformer has two inductors, a primary and a secondary. The magnetic field in the primary winding induces an electric current in the secondary winding.
A coil stores energy in the magnetic field it induces when a changing current passes through it and releases the energy when the current is removed.
Inductance (L) is measured in henries. The changing voltage and current in an inductor are out of phase. As current rises to a maximum, the voltage falls.
All conductors have some resistance under normal conditions and also exhibit inductive and capacitive influences, but these small influences are generally dismissed for practical purposes. Other loads make use of various combinations of inductors, capacitors and resistors to perform specific functions.
The tuning circuit of a radio uses variable inductors or capacitors in combination with a resistor to filter out a range of frequencies while allowing just one narrow band to pass through to the rest of the circuit.
A cathode ray tube in a monitor or television makes use of inductors, resistors and the inherent capacitance of the tube to control and display a picture on the phosphor coatings of the tube.
Single phase motors often use capacitors to aid the motor during starting and running. The start capacitor provides an additional phase of voltage to the motor since it shifts the current and voltage out of phase with each other.
- "Introductory Circuit Analysis"; Robert L. Boylestad; 1981
- Photo Credit Hemera Technologies/PhotoObjects.net/Getty Images
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