Electric Circuits

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Electric Circuits

• Electric Circuits
• Updated 2/26/07

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Electric Circuits

• Circuit Symbols
• Battery Cell
• (long is )
• Lamp
• Capacitor
• Ground

Circuit Symbols Resistor Voltmeter Ammeter O
hmmeter Diode
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Electric Circuits

• Definitions
• electrical circuit a complete conducting path.
  To be useful, a circuit must include a source of
  electromotive force (emf) and a load
  (resistance).
• A circuit that includes an emf source but no load
  is a short circuit.

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Electric Circuits

• Consider the circuit shown
• Imagine a quantity of positive charge, DQ, moving
  around the circuit from point A back to point A

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Electric Circuits

• Point A is grounded and its PE is zero
• As the charge moves through the battery from A to
  B, the PE of the system increases by DQDV
• The chemical energy of the battery decreases by
  the same amount

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Electric Circuits

• As the charge moves through the resistor, from C
  to D, it loses energy in collisions with the
  atoms of the resistor
• The energy is transferred to internal energy
• When the charge returns to A, the net result is
  that some chemical energy of the battery has been
  delivered to the resistor and caused its
  temperature to rise

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Electric Circuits

• Definitions
• electromotive force (emf) the total energy
  difference per unit charge (or work per unit
  charge) around a circuit.
• One emf source is a battery. (Another is a
  generator.) Some of a batteries energy is used
  up within the battery itself. This internal load
  is called the internal resistance of the battery.

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Electric Circuits

• A real battery has some internal resistance
• Therefore, the terminal voltage is not equal to
  the total emf of the battery

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Electric Circuits

• The schematic shows the internal resistance, r
• The terminal voltage is ?V Vb-Va
• ?V e Ir
• For the entire circuit, e IR Ir

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Electric Circuits

• Elements of a circuit can be connected in series
  or parallel.
• Ohms law applies both to an entire circuit and
  each of the elements in a circuit.
• A schematic diagram is a drawing of an electrical
  circuit.

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Electric Circuits

• Series Resistance
• Potentials add
• ?V IR1 IR2 I (R1R2)
• The equivalent resistance has the same effect on
  the circuit as the original combination of
  resistors

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Electric Circuits

• Resistance
• Series
• Rs R1 R2 Rn
• Parallel
• 1 1 1 1
• Rp R1 R2 Rn

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Electric Circuits

• Resistance Example

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Electric Circuits

• Parallel Resistance
• The potential difference across each resistor is
  the same because each is connected directly
  across the battery terminals.
• The current, I, that enters a point must be equal
  to the total current leaving that point.
• I I1 I2
• The currents are generally not the same.

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Electric Circuits

• Parallel Resistance
• Household circuits are wired so the electrical
  devices are connected in parallel

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Electric Circuits

• Parallel Resistance
• The inverse of the equivalent resistance of two
  or more resistors connected in parallel is the
  algebraic sum of the inverses of the individual
  resistance

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• Equivalent
• Resistance
• Req R1R2 R3

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Electric Circuits

• Capacitance
• Series
• 1 1 1 1
• Cs C1 C2 Cn
• Parallel
• Cp C1 C2 C3

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Electric Circuits

• Power
• P I V (units are watts)
• since V I R (Ohms Law)
• P I2 R (units are watts)
• P V2 / R (units are watts)
• (Assumes I is amps, V is volts and r is ohms)

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Electric Circuits

• Power
• The SI unit of power is Watt (W).
• The unit of energy used by electric companies is
  the kilowatt-hour
• This is defined in terms of the unit of power and
  the amount of time it is supplied
• 1 kWh 3.60 x 106 J

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Electric Circuits

• Electric Meters
• ammeter measures current. Must be connected in
  series at the point the current needs to be
  measured.
• voltmeter measures potential difference. Must
  be placed in parallel with the component to be
  measured. A perfect voltmeter has infinite
  resistance.

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Electric Circuits

• ammeter must be connected in series.
• voltmeter must be placed in parallel.

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Electric Circuits

• End of Electric Circuits