Electric Current and Circuits

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Electric Current and Circuits

• Physics

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Introduction to Charge Flow

• With using just one wire, one light bulb, and one
  battery get the bulb to light up.
• What features are important about the light bulb?
• What features are important about the battery?
• Now using 3 D Batteries in a battery pack, two
  light bulb in a socket and three wires, form a
  single, continuous closed circuit (series) in
  which the bulbs both light up.
• When completing the circuit by connecting the
  wire to the () end of the battery, which bulb do
  you observe lighting up first?
• Which bulb do you believe lights up first?
• Are the answers above dependant on which part of
  the circuit is being connected?
• If one mile of wire is connected between the
  bulbs, will there be a significant time
  difference between the lighting of the two bulbs?
  

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Electric Current

• Current definition
• Its the rate of flow of charge
• I Q / t
• Units Amperes (amps)
• Potential differences (voltage differences) CAUSE
  current.
• If theres no difference in voltage, no charge
  will flow no current.
• Sources that can create potential differences
  are batteries and generators
• Symantics Voltages are APPLIED ACROSS a
  circuit Current FLOWS through a circuit

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Measuring Voltages
Setup a Circuit as shown below

• Measure the voltage difference of the batteries.
• Measure the voltage difference of wire 1, wire 2,
  and the light bulb.
• How does the sum of the voltages of the wires and
  bulb compare to that of the battery?

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Electrical Resistance

• Electrical Resistance describes how much a
  conductor impedes the flow of charge
• Measured in units of OHMS (?)
• It depends on
• Type of material (resistivity)
• Thickness of the conductor (larger it is, the
  lower its resistance)
• Length of the conductor (longer it is, the higher
  its resistance)
• Temperature of the conductor (higher the temp,
  higher its resistance except for carbon)
• HONORS R rl/A

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Ohms Law

• Current Voltage / Resistance
• I V / R
• Exs
• How much current flows in a lightbulb that has a
  resistance of 60 Ohms and is connected to a 120 V
  source?
• Exs

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Measuring Current
Setup a Circuit as shown below

• Measure the current through the batteries.
• Measure the current through wire 1, wire 2, and
  the light bulb.
• How do the currents through the wires, bulb, and
  batteries compare?

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Electric Shock

• 0.001 Amps barely feel it!
• 0.005 Amps Ouch!
• 0.010 Amps Slight muscle contractions
• 0.015 Amps Loss of muscle control
• 0.075 Amps Disruption of heart fatal if lasts
  for more than 1 sec.
• The less resistance you have the more current
  may flow through you

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Direct Current / Alternating Current

• Direct Current Charge flows in just one
  direction
• Graph of Current vs Time
• Batteries supply DC
• Useful in circuits with transistors and
  microchips
• Alternating Current Charge moves back and
  forth, but never gets very far.
• Graph of Current vs Time
• Frequency of oscillation in U.S. is 60 Hz
  Generators often supply AC
• Useful for long distance transmission and the
  ability to transform voltages up and down.
• Converting AC to DC requires
• Diode acts like a one-way door keeping charge
  going just one direction
• Capacitor due to the time it takes to charge
  it up it will smooth out the oscillations

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Electric Power

• Power is the rate at which energy is transferred
  to heat, light, or mechanical forms
• Power Current x Voltage
• P I x V
• Units are Watts (Joule/sec)
• Exs
• How many Joules of Energy does it take to operate
  a 1000 Watt microwave for 1 minute?
• If PGE sells you energy at 16 cents per kWhr,
  how much did the above cost?

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Practice Ques

• Setup the circuit as shown to the right.
• Using the multi-meter, measure the current
  through the batteries.
• Measure the voltage across the batteries.
• Measure the voltage across Bulb A.
• Measure the current through Bulb B.

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Circuits

• Series Circuits elements are in-line with
  each other
• Characteristics
• Current stays the same
• Voltage divides up among the devices
• Req R1 R2 R3 .
• Parallel Circuits elements are parallel to
  each other.
• Characteristics
• Voltage stays the same
• Current divides up amongst the branches
• Req (1/R1 1/R2 1/R3 )-1

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Honors Kirchoffs Laws

• Loop Rule Conservation of Energy
• The sum of voltage gains and losses must equal
  zero.
• Ex
• Junction Rule Conservation of Charge
• The current flowing into a junction must equal
  the current flowing out of a junction.
• Ex

Example Circuit