Electric Circuits

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Electric Circuits

• Electric Current
• The amount of charge passing through a surface
  per second.
• Unit ampere or amp (A) coulombs/seconds
• The conventional current (I) is defined by the
  motion of positive charges ? Benjamin Franklin

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Electric Circuits

• Drift Speed
• The average speed of charges that make a current
• Usually 0.1 mm/s
• The electric field is created throughout the
  wire so that all charges respond almost
  instantaneously
• The charges collide with atoms in the wire and
  proceed with a random walk
• The atoms get hotter due to these collisions ?
  light !

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Electric Circuits

• Ohms Law and Resistance
• Unit ohms (O) volts/amps

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Electric Circuits

• Resistivity
• What determines the value of R for a resistor?
• Type of material, length and thickness
• Temperature?
• Conductors, insulators, heating units, lights,
  etc.

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Electric Circuits
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Electric Circuits

• Energy and Electric Potential in a Circuit
• The PE of a charge is unchanged after traveling
  around a circuit
• PE is gained at the voltage source (ie. battery)
  and lost at the resistor (ie. light bulb)
• The sum of the ?V around the circuit equals zero
• Voltage drops across a resistor

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Electric Circuits

• Series circuits

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Electric Circuits

• Series circuits
• 1. The current is the same everywhere in the
  circuit.
• Itotal I1 I2
• 2. The total resistance of the circuit is
• Rtotal R1 R2
• 3. The sum of the voltages across each device
  equals the source voltage
• ?Vsource ?V1 ?V2

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Electric Circuits

• Parallel circuits

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Electric Circuits

• Parallel circuits
• 1. The total current in the circuit equals the
  sum of the current in each device.
• Itotal I1 I2
• 2. The total resistance of the circuit is
• 3. The voltage across each device is the same.
• ?Vsource ?V1 ?V2

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Electric Circuits

• Electric Power
• Energy transfer per unit time
• Unit watts or joules/second
• Since it takes work (W) to move a charge (q)
  through a potential difference (?V) then we can
  say that

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Electric Circuits

• Electric Power
• Overloading occurs when the circuit resistance
  drops to where the circuit carries too much
  current.
• Fuses or breakers are used to control the amount
  of current in a circuit