Ohm Potentiometer
Ohm Potentiometer
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Resistance of Materials.
The opposition to the flow of electric current in an electrical circuit is called Resistance. However, resistance is not the same for all types of materials as some will have a very high value of resistance making them insulators, while others will have a very low value of resistance making them conductors. The resistance of any material is determined by the following factors:
- The type of material from which it is made - wood, plastic, steel, copper etc
- The cross-sectional area of the material - the larger its diameter the lower the resistance
- The length of the material - the longer it is the higher is its resistance
- The temperature of the material - with a positive temperature coefficient, the resistance will increase as the temperature increases
The amount of resistance, (R) that a material has to the flow of electric current is called resistivity and this is measured in Ohms per metre, (Ω/m). The inverse of resistivity is called conductance. The resistance of a material is directly proportional to its length, and inversely proportional to its cross-sectional area. So if the materials length is doubled its resistance doubles and if the materials diameter doubles its resistance halves.
A good conductor such as copper, aluminum or silver contains an abundance of free electrons within its lattice structure so current flows freely through them as these materials offer very little resistance, while other materials that oppose current flow have a higher resistance. Either way, the number of free electrons present in a materials crystalline structure determines the amount of opposition to current flow and therefore its electrical resistance.
Electrical Resistance and Resistors.
Electrical resistance in a D.C. circuit is measured in OHMS (Ω) and a material will have a value of one ohm when a potential of one volt produces a current of one ampere. Then Ohms can also be considered as volts/amps. The more resistance a material or circuit possesses the less current will flow. Opposition to the flow of current in the form of resistance can sometimes be a bad thing as wires, cables and components become hot. This is because the current flowing through the resistive material produces a form of friction which in turn causes its energy to be dissipated in the form of heat.
Electrical components that use their resistivity to control the amount of electric current that flows through them are called Resistors. An ideal resistor in an electrical circuit is a device which produces a linear relationship between the voltage applied across it and the current flowing through it. These types of devices are classed as Ohmic as they produce a proportional relationship according to Ohm's law.
Ohm's law defines the fundamental relationship between voltage, current and resistance and this is given as: V = I x R
As well as limiting the flow of current through a circuit, there are many different uses for the humble resistor. Resistors can be used to set accurate operating current conditions or provide a voltage drop across themselves when a current flows through them. Also, they can be used to act as voltage dividers or to provide feedback networks in amplifier circuits. Resistors are very flexible devices that can be either of a fixed resistive value or designed to have a variable resistive value such as potentiometers.
Basically an electrical resistor consists of two metal end caps with the resistive material placed in between them. There are many different types of resistors available in the marketplace but they all do the same thing, limit current flow. The three most common substances used for the resistive element of a resistor are: Carbon Composition, Metal Oxide and Wire-wound.
Carbon Resistors are extremely cheap to produce but they have very large tolerance ratings and their resistance value changes with temperature and time.
Metal Oxide Resistors are more expensive but are more accurate and stable than the carbon types.
Wire-wound Resistors are the most expensive as they are made from coils of fine non-inductive wire. They are used to dissipate large amounts of power and so are designed to be mounted onto heat sinks.
Individual resistors can also be connected together in series combination, a parallel combination or both series and parallel together. A series combination will have a common current flowing through them all, while a parallel combination will have a common voltage across them. Resistors are rated in both terms of the resistive value in Ohm's and their ability to dissipate heat energy in watts.
To learn more about resistance, resistivity and connecting resistors together plus lots of other good basic electronics tutorials go to http://www.electronics-tutorials.ws.
