Voltage and Resistance

  • Resistance is a material's opposition to the flow of electric current; measured in ohms.
  • Voltage, also known as potential difference. is a measure of the energy provided to the charge carriers. It can be defined as the amount of work done per unit charge
  • V =W/Q
  • R = V/I
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Ohm's Law

  • Ohm's Law: "The current in an ohmic calculator is proportional to the voltage across it, provided that the temperature and other physical conditions are kept constant".
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The effect of temperature on resistance

  • Electrical resistance is similar to friction in that it is a resistance to movement. Electrons drift slowly through a conductor when a voltage is put across the ends. The metal's atoms intefere with the motion of the electrons, causing resistance.
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  • Resitivity is a measure of the resisting power of a specified material to the flow of an electric current
  • Resistivity = (Resistance x Cross-Sectional Area)/Length
  • Resistivity = (RA)/L
  • Resistivity is measured in ohm metres
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Current and Drift Velocity

  • I = nAve
  • I = Current (amps)
  • n = Charge Particles per unit volume
  • A = Cross-Sectional Area (m^2)
  • v = drift velocity (ms^-1)
  • e = Charge of an electron (1.6x10^-19C)
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Critical Temperature

  • The temperature at which the electrical resitivity drops to zero is called the critical temperature
  • The transition to superconduction is abrupt and complete. The resitivity of the superconducting material is at least 10^12 times less than that of the material at room temperature
  • The materials are cooled using liquid nitrogen (77K) or liquid helium (4K)
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  • A superconductor is defined as a material with zero electrical resistance
  • At critical temperatures, free electrons couple to form Cooper pairs. These are more stable than a single electron.
  • A passing electron attracts the lattice of the superconducting structure, drawing the positive nuclein inwards and causing a positive ripple in its path. Another electron is attracted to that area of increasing positive charge,
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Superconducting Materials

  • There are two types of superconductors: Type I and Type II
  • Type I are majoritively metals whilst Type II are inorganic, ceramic solids
  • Type II superconductors have critical temperatures of around 120K and are therefore called high temperature superconductors
  • Superconductors are very expensive and have to be kept very cold
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Uses of Superconductors

  • Power transmission in overhead cables creates heating in the cables and hence a loss of energ. Superconductive cables would eliminate this problem.
  • Superconductors are needed to create extremely strong, stable magnetic fields.
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