Basic Electronics Knowledge (Resistance)

Resistance

As its name implies, a resistor is an electronic component that resists the flow of electric current in a circuit. Electrical resistance is analogous to friction in a mechanical system. They both convert energy to heat and dissipate it to the surrounding environment, so electrical resistance can sometimes be thought of as a braking or damping mechanism in a circuit. 

Some Resistors

In metals such as silver or copper, which have high electrical conductivity and therefore low resistivity, electrons are able to skip freely from the conduction band of one atom to the next, encountering little resistance. However, in a material such as carbon, electrons encounter numerous collisions that make it more difficult for them to move through the material, according to Serif Uran, a professor of physics at Pittsburg State University. Insulators such as glass have extremely high resistivity, with virtually no spaces in their conduction bands that would allow electrons to move through them. 

Resistors are generally classified as either fixed or variable. Fixed-value resistors are simple passive components that always have the same resistance within their prescribed current and voltage limits. They are available in a wide range of resistance values from less than 1 ohm to several million ohms with tolerances ranging from plus or minus 0.1 percent to plus or minus 10 percent. Resistors are also classified by the maximum voltage they can tolerate as well as the maximum amount of power they can dissipate. 

The resistance of a simple resistor can be calculated as R = ρL/A, where R is resistance, L is its length, A is its cross-sectional area and ρ is resistivity, which is an inherent property of the material. Resistivity is the reciprocal of conductivity σ, i.e., ρ = 1/σ. All other things being equal, a resistor that is twice as long will have twice the resistance, and one with twice the cross-sectional areal will have half the resistance. Also, material with higher resistivity will result in proportionally greater resistance. 

Variable resistors are simple electro-mechanical devices, such as volume controls and dimmer switches, which increase the effective length of a resistor by turning a knob or moving a slide control. Strain gauges are resistors in which resistance changes with strain. Strain occurs when an object is stretched or compressed. A thermistor is a temperature sensor. It changes resistance when an increase in temperature excites electrons, making them available to conduct current, thus reducing the resistivity of the material. A piezoresistor changes its resistivity in response to a change in strain, which causes more or fewer electrons to be available to carry charge. 

The electrical resistance of a circuit component is defined as the ratio of the applied voltage to the electric current that flows through it, according to HyperPhysics. The standard unit for resistance is the ohm, which is named after German physicist Georg Simon Ohm. It is defined as the resistance in a circuit with a current of one ampere at one volt. Resistance can be calculated using Ohm's Law, which states that resistance equals voltage divided by current, or R = V/I, where R is resistance, V is voltage and I is current. Ohm's Law is more commonly written as the equivalent expression V = IR. One way to understand Ohm's Law is to hold one of these variables constant, change the value of another variable, and watch what happens to the third variable. For instance, if we keep voltage constant and increase the resistance, the current must decrease. If we keep the resistance constant and increase the voltage, the current must increase.