electrical resistance

Examples of electrical resistance in the following topics:

• Overview of Electric Current

  • Electric current is the flow of electric charge and resistance is the opposition to that flow.
  • The opposite of conductance is resistance - a quantity that describes how strongly a material opposes the flow of electric current.
  • An object or medium that has high electrical resistance is called a resistor.
  • The SI unit for resistance is the ohm (symbol: ).
  • A useful and practical way to learn about electric current and resistance is to study circuits .

• Resistance and Resistivity

  • Resistance and resistivity describe the extent to which an object or material impedes the flow of electric current.
  • Resistance is the electric property that impedes a current.
  • As you might expect, the cylinder's electric resistance R is directly proportional to its length L, similar to the resistance of a pipe to fluid flow.
  • What determines resistivity?
  • Identify properties of the material that are described by the resistance and resistivity

• 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.
  • A simple electric circuit made up of a voltage source and a resistor
  • Describe the relationship between the electrical current, voltage, and resistance in a circuit

• Conductors and Insulators

  • 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 flow of charge is electric current.
  • 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.

• Energy Usage

  • The energy used is the time integral of the electric power.
  • For example, we (or the electric power utility) may want to calculate the amount of money owed for electricity consumed.
  • The electric power in watts produced by an electric current I consisting of a charge of Q coulombs every t seconds passing through an electric potential (voltage) difference of V is $P = \frac{QV}{t} = IV$, where Q is electric charge in coulombs, t is time in seconds, I is electric current in amperes, and V is electric potential or voltage in volts.
  • In resistive circuits where Ohm's Law applies, the power can be expressed as $P = I^2R = \frac{V^2}{R}$, where R is the electrical resistance.
  • The general expression for electric power is then

• Different Types of Currents

  • An electrical circuit is an interconnection of electrical elements that has a closed loop giving a return path for the current.
  • A resistive circuit is a circuit containing only resistors and ideal current and voltage sources.
  • Analysis of resistive circuits is less complicated than analysis of circuits containing capacitors and inductors.
  • In circuit diagrams such as this, electrical elements are represented by symbols and usually labeled with appropriate characteristics, such as the resistance r of a resistor.
  • A number of electrical laws apply to all electrical networks.

• Ideal Conductors

  • Conductivity (σ) is the inverse of resistance, measured in units of current per potential difference.
  • In an ideal conductor, the material's conductivity is infinite, and its resistance approaches 0.
  • The principle of near-zero resistance is akin to that of frictionless surfaces: tTheoretically, with the slightest force (voltage), an object (current) on a frictionless surface (zero-resistance conductor) can proceed without restriction.
  • The electric field (Etan) and electric flux density (Dtan) tangential to the surface of a conductor must be equal to 0.
  • This means that the electric field inside a perfect conductor is 0.

• Sources of EMF

  • By separating positive and negative charges, electric potential difference is produced, generating an electric field.
  • When current flows, however, the voltage across the terminals of the source of EMF is no longer the open-circuit value, due to voltage drops inside the device due to its internal resistance.
  • In the case of an electrical generator, a time-varying magnetic field inside the generator creates an electric field via electromagnetic induction, which in turn creates an energy difference between generator terminals.
  • Again the EMF is countered by the electrical voltage due to charge separation.
  • The general principle governing the EMF in such electrical machines is Faraday's law of Induction.

• Humans and Electric Hazards

  • The hazards from electricity can be categorized into thermal and shock hazards.
  • A shock hazard occurs when electric current passes through a person.
  • A short circuit is a low-resistance path between terminals of a voltage source.
  • The lethality of an electric shock is dependent on several variables:
  • A short circuit is an undesired low-resistance path across a voltage source.

• Ohm's Law

  • 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.
  • When a voltage source is connected to a conductor, it applies a potential difference V that creates an electric field.
  • The electric field, in turn, exerts force on charges, causing current.
  • An object that has simple resistance is called a resistor, even if its resistance is small .
  • The voltage source supplies energy (causing an electric field and a current), and the resistor converts it to another form (such as thermal energy).