Electricity: Resistance

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Electricity: Resistance

  1. 1. Resistance
  2. 2. Recap: Last time we have tackled about electricity . Again… A type of energy fueled by the transfer of electrons from positive and negative points within a conductor. Electricity is widely used for providing power to buildings, electric devices, and even some automobiles… Electricity is very harmful to humans because of high voltage it contains but electricity can be lessened by the use of Resistors with different resistance…
  3. 3. Resistance Resistance of an object to the flow of electrical current. R = V/I Resistance equals the ratio of voltage to current. Unit: Ohm ( Ω ) An object of uniform cross section has a resistance proportional to its resistivity and length and inversely proportional to its cross-sectional area.
  4. 4. Georg Ohm andOhm’s LawGeorg Simon Ohm (1789 - 1854) first investigated therelationship between the current in an electric circuitelement and the electrical potential difference, oftencalled voltage, across the circuit element.Ohms LawFor a circuit or circuit element, Ohms law mathematicallyrelates the current, the voltage, and the electricalresistance. The voltage is more properly called theelectrical potential difference, but voltage is often usedbecause it is less of a mouthful.Ohm found that in a circuit the voltage and current aredirectly proportional to each other. That means if thecurrent, I, in a circuit is plotted along the horizontal, or x,axis and the voltage, V, is plotted along the vertical, or y,axis, then the result will be a straight line. The slope ofthis line is the resistance, R, of the circuit.
  5. 5. ResistorsA resistor is a two-terminal electroniccomponent that produces a voltage across itsterminals that is proportional to the electriccurrent through it in accordance with Ohmslaw: V = IRResistors are elements of electrical networksand are ubiquitous in most electronicequipment. Practical resistors can be made ofvarious compounds and films, as well asresistance wire. The primary characteristics ofa resistor are the resistance, the tolerance, themaximum working voltage and the powerrating.
  6. 6. ConductorsA conductor is a material which containsmovable electric charges. In metallic conductors, suchas copper or aluminum, the movable charged particlesare electrons. Positive charges may also be mobile inthe form of atoms in a lattice that are missingelectrons (known as holes), or in the formof ions, such as in the electrolyte of a battery.Insulators are non-conducting materials with fewermobile charges, which resist the flow of electriccurrent.All conductors contain electric charges which willmove when an electric potential difference (measuredin volts) is applied across separate points on thematerial. This flow of charge (measured in amperes)is what is meant by electric current. In mostmaterials, the direct current is proportional to thevoltage (as determined by Ohms law), provided thetemperature remains constant and the materialremains in the same shape and state.
  7. 7. In Metals A metal consists of a lattice of atoms, eachwith a shell of electrons. This is also known as apositive ionic lattice. The outer electrons arefree to dissociate from their parent atoms andtravel through the lattice, creating a sea ofelectrons, making the metal a conductor. Whenan electrical potential difference (a voltage) isapplied across the metal, the electrons driftfrom one end of the conductor to the other under
  8. 8. In Insulators In metals, the Fermi level lies in theconduction band giving rise to free conductionelectrons. However, in semiconductors theposition of the Fermi level is within the bandgap, approximately half-way between theconduction band minimum and valence bandmaximum for intrinsic (semiconductors. Thismeans that at 0 kelvins, there are no freeconduction electrons and the resistance isinfinite. However, the resistance will continue
  9. 9. In liquids/electrolytes In electrolytes, electrical conductionhappens not by band electrons or holes, but byfull atomic species (ions) traveling, eachcarrying an electrical charge. The resistivity ofionic liquids varies tremendously by theconcentration - while distilled water is almostan insulator, salt water is a very efficientelectrical conductor. In biological membranes,currents are carried by ionic salts. Small holes
  10. 10. Difference between an AlternatingCurrent and Direct CurrentAlternating Current Direct Current In alternating Direct current (DC) is current (AC) the the undirectional flow movement of electric of electric charge. charge periodically reverses The electric charge flows in direction. AC is the form in a constant direction, which electric power is distinguishing it delivered to businesses and from alternating current residences. The usual (AC). waveform of an AC power circuit is a sine wave.
  11. 11. The First Direct Current Electricity is a very different energy source than heat or light. In nature, electricity only rarely occurs, in some animals or with lightning. In the search to create electrical energy, scientists discovered that electrical and magnetic fields are related. A magnetic field near a wire causes electrons to flow in a single direction along the wire because they are repelled and attracted by the north or south poles. Thus, DC power from a battery was born, primarily attributed to Thomas Edisons work and promotion.
  12. 12. Thank You!!! 

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