Electric Currents

1
Electric Currents Electric Circuits
2
Electric Circuits

• Schematic diagram a graphic representation of
  an electric circuit, with standardized symbols
  representing circuit components.
• Symbols for Circuits p. 731 Table 20-1

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Electric Circuits
A - battery C - light bulb B - switch D -
resistor
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Electric Circuits

• Electric circuit a set of electrical components
  connected so that they provide one or more
  complete paths for the movement of charges.

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Electric Circuits

• Closed Circuit path from one battery terminal
  to other terminal is complete, potential
  difference exists, and electrons move from one
  terminal to the other.
• So closed-loop path for electrons to follow.
• Construct diagram with battery and switch

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Electric Circuits

• Open Circuit the switch is open or the circuit
  is not complete. No charge flow and therefore no
  current.
• Since no current , light bulb does not light up.
• Draw open switch, battery and light bulb.

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Electric Circuits

• Load any element or group of elements in a
  circuit that dissipates energy.
• Light bulb or bulbs would be examples of load.
  See p. 733 for how a light bulb works.
• Dissipate a process in which energy is used or
  lost without accomplishing useful work. Light
  bulb or friction to mechanical energy

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Electric Circuits

• Assignment
• Holt page 735 1-5

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Electric Circuits

• Resistor small devices designed to provide
  electrical resistance. When inserted in an
  electric circuit, they control the amount of
  current in part(s) of the circuit.
• When a resistor is crossed a voltage drop occurs.
• A resistor dissipates heat when a current flows
  through it.

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Electric Circuits How does the current flow?

• So how do currents move so fast through wire, so
  that when I flip the light switch the light bulb
  appears to come on instantly?

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Electric Circuits How does the current flow?

• When switch is turned-on an electric field is
  established throughout the conductor.
• This electric field, which sets charges into
  motion, travels through the wire at nearly 3 E 8
  m/s.
• Soooo. Individual charges move slowly, but
  electric field moves fast.

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Electric Circuits How does the current flow?

• WHICH WAY???
• Conventional Current - physicists state that the
  direction of current flow in an external circuit
  (not in a battery) is the direction of motion of
  positive charges.

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Electric Circuits How does the current flow?

• WHICH WAY???
• Electron Flow -In metals, electrons are the
  moving charge, so the definition of current is
  opposite the actual flow of electrons, since
  electrons are negatively charged.

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Electric Circuits How does the current flow?

• Draw a simple circuit with a single cell battery
  and one resistor in a closed circuit.
• Label the direction of current flow

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Current, Voltage, Resistance

• Current
• a measure of the flow of electronics
• measures in Amperes (A)
• Voltage
• a measure of the potential energy per charge
• measures in Volts (V)
• Resistance
• a measure of the opposition to electron flow
• measured in Ohms (W)

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Series and Parallel Circuits

• Series Circuits
• only one end of each component
  is connected
• e.g. Christmas tree lights
• Parallel Circuits
• both ends of a component are
  connected
• e.g. household lighting

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Series and Parallel Light Bulbs

• Series Circuits
• the currents at the same through each
• if one light is removed the other go out
• adding lights dims all lights
• Parallel Circuits
• the voltages are the same across each
• if one is removed the other are not effective
• adding lights does not effect the other bulbs

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Electric Circuits Series

• Series circuit or portion of a circuit that
  provides a single conducting path without
  junctions.
• THINK No forks in the road, one way highway
  with no exit ramps.

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Electric Circuits Series

• In applying Ohms Law to circuits in Series, we
  must understand what is happening to our three
  variables
• Current (I)
• Potential Difference (?V)
• Resistance (R )

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Electric Circuits Series

• Current when many resistors are connected in
  series, the current in each resistor is the same.
• So..
• I the same for all
• I I1 I2 I3

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Electric Circuits Series

• Resistance for the entire circuit is the sum of
  all resistors.
• Req R1R2R3
• Req equivalent resistance of circuit

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Electric Circuits Series

• Potential difference in Series Circuit is the sum
  of all potential differences
• ?V ?V1 ?V2 ?V3

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Electric Circuits Series

• Problem
• A 9.0 V battery is connected to four light bulbs
  (4.0 ?, 2.0 ?, 5.0 ?, 7.0 ?) in series. Find the
  equivalent resistance for the circuit and the
  current in the circuit.

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Electric Circuits Series

• Solution
• Req R1 R2 R3 R4
• Req 4.0 ? 2.0 ? 5.0 ? 7.0 ?
• Req 18.0 ?
• I ?V/ Req 9.0 V/ 18.0 ?
• I 0.50 A

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Electric Circuits Parallel

• Parallel Circuit - two or more components in a
  circuit that are connected across common points
  or junctions, providing separate conducting paths
  for the current.
• Provides alternate path for current.

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Electric Circuits Parallel

• Since the current branches into more than one
  direction the current is less in the parallel
  section of the circuit.
• I I1 I2 I3

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Electric Circuits Parallel

• The potential difference for the parallel circuit
  is the same throughout for each resistor.
• ?V ?V1 ?V2 ?V3

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Electric Circuits Parallel

• Resistors in parallel - can be obtained
  indirectly.
• Note you must take the reciprocal to solve for
  the equivalent resistance of a parallel circuit.

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Electric Circuits Parallel

• Resistors
• Because of reciprocal relationship, the
  equivalent resistance for a parallel arrangement
  of resistors must be less than the smallest
  resistance in the group of resistors.

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Electric Circuits Parallel

• Problem
• A 9.0 V battery is connected to four resistors in
  parallel (2?, 4?, 5?, and 7?). Find the
  equivalent resistance for the circuit and the
  total current of the circuit.

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Electric Circuits Parallel

• Solution
• 1/Req 1/2 1/4 1/5 1/7
• 1/Req 1/1.09 ?
• Req 0.917 ?

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Electric Circuits Parallel

• Solution Continued
• I 9.0 V/ 0.917
• I 9.8 A

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AC/DC

• DC
• Direct Current
• Electrons flow in one direction
• e.g. battery
• AC
• Alternating Current
• Electrons flow changes direction
• e.g. wall outlet (110Volts, 60 Hertz)

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Electric Circuits Complex

• Complex circuit - a circuit that has both series
  and parallel portions.
• Best approach is to divide the circuit up into
  groups of series and parallel resistors. This
  allows problems to be worked in easy parts.

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Electric Circuits Complex

• Steps for Complex resistance determination
• 1) redraw resistor along one side of circuit
• 2) calculate series resistance first
• 3) redraw simplified schematic
• 4) calculate parallel resistance and redraw
• 5) now calculate series for resistance

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Electric Circuits Complex

• The current and potential difference across a
  resistor can be determined.
• We know that
• But since R, I, and ?V can vary depending if
  resistors are parallel or in series.
• Therefore, must solve smaller parts of circuit to
  get values.

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Electric Circuits Complex

• Step to find current and potential difference
  across a resistor.
• 1) determine equivalent resistance of the
  circuit.
• 2) calculate total current in circuit.
• 3) determine path from equivalent resistance
  (step 1) to resistor in question.
• 4) Follow path from step 3 to calculate the
  current in and potential difference across
  equivalent resistance

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Electric Power

• Electric Power is the rate at which charge
  carries do work.
• Or the rate at which charge carriers convert
  electrical potential energy to nonelectric forms
  of energy.
• P W/?t ?PE/ ?t
• Where Power (P) Work (W) Potential Energy (PE)

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Electric Power

• Power dissipation in a resistor is derived as
  follows
• P W/?t ?PE/ ?t
• ?PE q ?V
• And I q/?t
• So P I ?V
• Where Current (I), charge (q)
• Power SI unit is a Watt (W) and is equivalent to
  1 J of electrical energy being converted to other
  forms of energy per second.

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Electric Power

• Power dissipated by a resistor in the following
  forms
• For constant current use
• P I?V I(IR) I2R
• If potential difference is constant
• P I?V (?V/R) ?V (?V)2/R

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Electric Power

• Electrical companies charge for energy not power.
• Kilowatt-hour is the unit that electric
  companies price.
• Kilowatt-hour (kW h) is the energy delivered
  in 1 hour at a constant rate of I kW.
• 1 kWh 3.6 E 6 Ws 3.6 E 6 J
• Typical price for a kW h is 0.05 to 0.20

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Electric Meters

• Electric meters enable components of a circuit to
  be determined.
• Ammeter a meter used to measure electric
  current. Current is measured for a certain
  section of circuit, MUST BE PLACED IN SERIES.
• Ideally ammeter will have zero resistance,
  however, reality is they have resistance of about
  1 ?. Work best when resistance of meter is much
  less than other resistors connected in series
  with it.

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Electric Meters

• Voltmeter meter used to measure potential
  difference in a circuit.
• A voltmeter is CONNECTED IN PARALLEL with other
  element. Thus enable us to see voltage drop or
  potential difference between the two terminals of
  the meter.
• Ideally, a voltmeter should have infinite
  resistance so that the equivalent resistance
  between points of connection is unchanged by
  voltmeter. Reality small amount of current goes
  through meter and effect on circuit is
  negligible.

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Electric Meters

• Ohmmeter meter that measures resistance
  directly.
• Connected to the ends of a resistor in order to
  obtain accurate resistance. This means it is
  connected in PARALLEL to resistor.

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Safety Devices

• In an electric circuit, fuses and circuit
  breakers are switches that act as safety devices.
  
• When appliances are connected in parallel, each
  additional appliance placed in operation reduces
  the equivalent resistance in the circuit and
  causes more current to flow through the wires.
  The additional current may produce enough thermal
  energy (at a rate of P I2R) to melt insulation
  on the wires, causing short circuit or even a
  fire.

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Safety Devices

• Fuse a short piece of metal that melts from the
  heating effect of the current. The thickness of
  the metal is adjusted to determine the current
  needed to melt the fuse.
• Once melted the circuit is no longer complete and
  electrical service is discontinued.
• Fuses can only be used once, and then new fuse
  must be installed to complete the circuit.

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Safety Devices

• Circuit Breaker an automatic switch that opens
  when the current reaches some set value. If
  current greater than the set value flows in the
  circuit, it will be overloaded. The circuit
  breaker will open stopping all current flow.
• Circuit Breakers can be manually reset by simply
  moving switch to on position, once appliance
  that caused problem has be removed from the
  circuit.