Current and Charge

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Current and Charge

  • Charge is a property of protons and electrons
  • Current is the flow of charge
  • I = Q/T
  • I = Current (A), Q = Charge (c), T = Time (s)
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Conventional Current

  • The usual charge carrier is the electron
  • Electrons carry a negative charge
  • The convention is to always label current in the positive direction
  • The conventional current is in the opposite direction to the flow of electrons
  • +ve to -ve is conventional current
  • -ve to +ve is electron flow
  • In a semiconductor, the number of charge carriers changes with temperature. As temperature increases, it liberates electrons so the resistance is reduced. This property is crucial in the design of compuer silicon chips.
  • Batteries use chemicals to store charge. This is released when the battery is used until it is depleted
  • Rechargeable batteries contain chemicals where the reactions can be reversed if a potential difference is applied.
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Rechargeable Batteries

  • A car battery is a 12V rechargeable battery designed to supply a very large current to start the engine. The batter is recharged when the car engine is running. Smaller rechargeable batteries are used in portable electronic equipment (e.g. mobile phones). Such a battery supplies a much smaller current than a car battery. Disposible batteries cant be recharged. Once they have run down, it is no longer of any use. These are not as environmentally friendly as a rechargeable battery.
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Physics and the Human Genome Project

  • To map the human genome, fragments of DNA are tagged with amino acid bases. Each tagged fragment carries a neagtive charge. A voltage is applied across a ***** of gel with a spot of liquid on it containing tagged fragments. The fragments are attracted to the positive electrode. The smaller the fragment, the faster it moves, so the fragments seperate out according to sze as they move to the positive electrode. The fragments pass through a spot of laser light, which causes a dye attatched to each tag to flouresce as it passes through the laser spot. Light sensors linked to a computer detect the glow from each tag. The computer is programmed to work out and display the sequence of bases in the DNA fragments.
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