Current Electricity and Electromagnetism

1
Current Electricity and Electromagnetism

• Dr. Don Franceschetti
• Physics 1010
• November 10-15, 2005

2
Electric Current is

• The flow of electric charge
• In a metal, the flow of conduction electrons
• Meassured in Amperes (amps, A)
• Remember
• Coulomb in the unit of charge
• Elementary charge is 1.6x10-19 C

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Voltage sources

• Create a difference in electrical potential
  across the ends of a conductor.
• 1 volt 1 joule/coulomb
• Battery converts chemical energy in to
  electricala source of d.c.
• Photocell converts light energy into electrical
  a source of d.c.
• Thermocouple converts heat energy into
  electricala source of d.c.
• Generator converts mechanical energy into
  electricity a source of a.c.

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Normal conductors

• Offer a resistance to electric currentturn
  electrical energy into heat
• Obey Ohms Law VIR
• But superconductors offer no resistance at all!
• R measured in ohms
• 1 ohm 1 volt/ampere

5
Electric Shock

• Skin resistance is
• High when dry
• Low when wet
• Current can kill you
• Especially through the heart

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Electric Power

• PIV, wattsamps x volts
• Using VIR, power loss in a resistance is
• PI2R
• PV2/R
• So the higher the voltage and the smaller the
  current, the less energy is lost getting from the
  source to the destination

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Direct vs. Alternating

• Direct is simple to understand, but hard to
  transform from low voltage to high to reduce
  power loss.
• Alternating is easy to transform, but hard to
  describe with equations.
• Edison loved DC, Westinghouse loved AC and
  Westinghouse won (fortunately).

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Simplest circuits

• Series current flows through each element in
  sequence. Break circuit anywhere and all current
  stops
• Parallel current flows through several parallel
  paths.
• house wiring is in parallel.
• fuses or circuit breakers used to prevent
  overload.

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Magnetic Forces

• Exist between moving charges in addition to
  electrostatic force.
• Permanent magnets exert magnetic forces because
  charge motion is synchronized on atomic level.
  Only a few elements (Fe, Co, Ni, Gd) and a few
  alloys, like Alnico can be made into permanent
  magnets

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Permanent magnets have poles

• North pole is really north-seeking pole
• South pole is really south-seeking pole
• So the Earths N pole is really a S pole and
  vice versa.
• Like poles repel each other
• Unlike poles attract each other.
• Dony confuse N and S with and -

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Magnetic Force acts on Currents or Moving Charges

• Parallel currents attract
• Anti parallel currents repel
• Torque on current loop makes it line up with
  magnetic field
• Basis for galvanometer