resistance

(noun)

The opposition to the passage of an electric current through that element.

Related Terms

Examples of resistance in the following topics:

  • Resistance and Resistivity

    • Resistance and resistivity describe the extent to which an object or material impedes the flow of electric current.
    • Conductance and resistance are reciprocals .
    • What determines resistivity?
    • Its resistance to the flow of current is similar to the resistance posed by a pipe to fluid flow.
    • Identify properties of the material that are described by the resistance and resistivity

  • Dependence of Resistance on Temperature

    • Resistivity and resistance depend on temperature with the dependence being linear for small temperature changes and nonlinear for large.
    • The resistivity of all materials depends on temperature.
    • where ρ0 is the original resistivity and α is the temperature coefficient of resistivity.
    • is the temperature dependence of the resistance of an object, where R0 is the original resistance and R is the resistance after a temperature change T.
    • Compare temperature dependence of resistivity and resistance for large and small temperature changes

  • Resisitors in Series

    • The total resistance in the circuit with resistors connected in series is equal to the sum of the individual resistances.
    • A measure of this limit on charge flow is called resistance.
    • The total resistance in the circuit is equal to the sum of the individual resistances, since the current has to pass through each resistor in sequence through the circuit.
    • This implies that the total resistance in a series is equal to the sum of the individual resistances.
    • Since all of the current must pass through each resistor, it experiences the resistance of each, and resistances in series simply add up.

  • Combination Circuits

    • This is commonly encountered, especially when wire resistances is considered.
    • In that case, wire resistance is in series with other resistances that are in parallel.
    • Essentially, wire resistance is a series with the resistor.
    • It thus increases the total resistance and decreases the current.
    • Each is identified and reduced to an equivalent resistance, and these are further reduced until a single equivalent resistance is reached.

  • Current and Voltage Measurements in Circuits

    • The electrical current is directly proportional to the voltage applied and inversely related to the resistance in a circuit.
    • The electric property that impedes current (crudely similar to friction and air resistance) is called resistance R.
    • Resistance is inversely proportional to current.
    • Using this equation, we can calculate the current, voltage, or resistance in a given circuit.
    • Describe the relationship between the electrical current, voltage, and resistance in a circuit

  • Resistors in Parallel

    • The total resistance in a parallel circuit is equal to the sum of the inverse of each individual resistances.
    • This implies that the total resistance in a parallel circuit is equal to the sum of the inverse of each individual resistances.
    • This relationship results in a total resistance that is less than the smallest of the individual resistances.
    • Three resistors connected in parallel to a battery and the equivalent single or parallel resistance.
    • Calculate the total resistance in the circuit with resistors connected in parallel

  • Charging a Battery: EMFs in Series and Parallel

    • When voltage sources are connected in series, their emfs and internal resistances are additive; in parallel, they stay the same.
    • The disadvantage of series connections of cells in this manner, though, is that their internal resistances add.
    • But the total internal resistance is reduced, since the internal resistances are in parallel.
    • Current flows in the direction of the greater emf and is limited by the sum of the internal resistances.
    • This schematic represents a flashlight with two cells (voltage sources) and a single bulb (load resistance) in series.

  • Ohm's Law

    • Example: Calculating Resistance: An Automobile Headlight What is the resistance of an automobile headlight through which 2.50 A flows when 12.0 V is applied to it?
    • Discussion: This is a relatively small resistance, but it is larger than the cold resistance of the headlight.
    • As we shall see in Resistance and Resistivity, resistance usually increases with temperature, and so the bulb has a lower resistance when it is first switched on and will draw considerably more current during its brief warm-up period.
    • The unit for resistance is the ohm where 1Ω = 1 V/A.
    • An object that has simple resistance is called a resistor, even if its resistance is small .

  • Null Measurements

    • Many so-called ohmmeters measure resistance.
    • Most common ohmmeters apply a voltage to a resistance, measure the current, and calculate the resistance using Ohm's law.
    • Their readout is this calculated resistance.
    • The Wheatstone bridge is used to calculate unknown resistances.
    • The unknown EMF is thus proportional to the resistance of the wire segment.

  • Conductors and Insulators

    • All materials can be categorized as either insulators or conductors based on a physical property known as resistivity.
    • An insulator is a material in which, when exposed to an electric field, the electric charges do not flow freely—it has a high resistivity.
    • Conversely, a conductor is a material that permits the flow of electric charges in one or more directions—its resistivity is low.
    • This usually is the current at which the heat released due to resistance melts the material.
    • While there is no perfect insulator with infinite resistivity, materials like glass, paper and Teflon have very high resistivity and can effectively serve as insulators in most instances.