Physics 18: Electric Currents

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Physics 18 Electric Currents

• Christopher Chui

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The Electric Battery

• Volta originated the battery, which transforms
  chemical energy into electrical energy
• All batteries have electrodes and electrolytes
• An electric cell is a device that has electrodes
  and electrolytes. Several cells connected
  together is a battery
• In a single cell, electrons are drawn from the
  positive terminal. When the electrodes are
  connected with a conductor, then the electrons
  complete the circuit. Thus, the current flows. By
  convention, current flows from positive terminal
  to negative terminal

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Electric Current and Ohms Law

• A flow of charge from one terminal to another is
  called an electric current, I DQ/Dt
• The unit for current is Ampere (1775-1836), 1 A1
  C/s
• Conventional current starts from ve terminal to
  the ve terminal electron flow is opposite
• Ohms law V IR, R is in ohms
• Resistor color code black 0, brown 1, red 2,
  orange 3, yellow 4, green 5, blue 6, violet 7,
  gray 8, white 9
• Tolerance gold 5, silver 10, no color 20
• Resistance R of a metal wire is proportional to L
  and inversely proportional to cross-sectional
  area A
• R r L/A r is resistivity, which depends on the
  material
• Effect of temperature rT ro 1 a(T-To)

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Electric Power

• P power energy transformed/time QV/t IV
• P I(IR) I2R V2/R
• A short circuit means that 2 wires have crossed
  so that the path of the current is shortened
• Fuses and circuit breakers are safety devices to
  prevent shorts in the household
• When current flows steadily in one direction, it
  is DC
• AC currents alternate from ve to ve
  continuously
• In AC, V Vo sin 2pft Vopeak voltage, IIo sin
  2pft
• Average power ½ Io2R ½ Vo2/R Irms2R
• Irms Io/sqrt(2) 0.707Io and Vrms
  Vo/sqrt20.707Vo
• I DQ/Dt neAvd