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Electricity --- Current and voltage

Electricity --- Current and voltage. Summary of conductors and insulators. Conductors – need free (mobile) charges In metals there are free electrons Insulators have all charges firmly bound (glass, most plastics) air? Human body?. Faraday’s Cage. Electric current. Q0.

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Electricity --- Current and voltage

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  1. Electricity--- Current and voltage

  2. Summary of conductors and insulators • Conductors – need free (mobile) charges In metals there are free electrons • Insulators have all charges firmly bound (glass, most plastics) • air? • Human body? Faraday’s Cage

  3. Electric current

  4. Q0 The electric current in this case, • increases • decreases • remains unchanged as a function of time.

  5. Voltage Before plates are connected, certain electric energy is stored. These energy stored is characterized electric potential difference or Voltage.

  6. conduction electrons in a wire No connected to charged plates When connected to plates

  7. Maintain a constant electric current

  8. A Battery keeps current flowing !! Voltage of a battery

  9. Electric potential Gravitational Potential Energy mgh Gravitational Potential gh Electrical potential Energy qV Electrical potential V

  10. Some Typical Voltages Voltage Source (approx.) Thundercloud to ground 108 V High-voltage power line 106 V Power supply for TV tube 104 V Automobile ignition 104 V Household outlet 120 V Automobile battery 12 V Flashlight battery 1.5 V Resting potential across nerve membrane 10-1 V Potential changes on skin (EKG and EEG) 10-4 V

  11. Sources of electric potential • Batteries • Van de Graaff generator • Power stations • Alternators • Wind generators • Solar panels • Piezoelectric materials

  12. Electric current The bigger the voltage, The bigger the current.

  13. Current: the flow of charges. • I = Q/t • SI Unit ampere A • What kind of charges? electrons, ions, holes

  14. Q1 A and B copper wires of same lengths connected to two identical batteries. A with bigger cross-section than the other. A should carry a • Bigger current; • Smaller current; • Same current; • None of above .

  15. Q2 A and B copper wires of same cross sections. A is longer than B. A should carry a • Bigger current; • Smaller current; • Same current; • None of above.

  16. Ohm’s Law I = σA V/d = V / R where A is a cross section area of a wire, d is length. V is the voltage across the wire. We call the quantity characterizing the material conductivity σ. Inverse of σ is called resistivity ρ, ρ = 1/σ R= ρ d/A

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