Current and Circuits

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Current and Circuits
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Current
current the flow of charged particles.
E
Current is measured in Amperes (A) which is made
of the unit of a Coulomb/sec (C/s).
An Ampere is defined as a fundamental unit of
electricity.
How much current passes through a wire as 6.0 C
flows in 1.5 s?
4.0 A
4.0 x 6.25 x 1018 electrons/sec 2.5 x 1019
electrons/sec
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Drift velocity the resulting velocity due to
the application of an
electric field and the random motions and
collisions by charges.
Types of Current
Direct Current (DC) current that travels in
only one direction -batteries supply current and
voltage that are direct Alternating Current (AC)
current that travels in two directions -home,
business, school outlets, AC generators
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Current Conventions
conventional current - the flow of positive
charges (protons). -Positive
charges flow from high to low potential
positive charge motion
E
electric current - the flow of negative charges
(electrons). Negative charges flow from low to
high potential.
negative charge motion
E
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Circuits
circuit A continuous path for charges to
flow through.
Circuit simulation
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Circuit Components

• Power source A device that can produce and
  maintain a potential difference.
• The source of the electric field that can exert a
  force on the charges in the circuit.
• The power source is measured in volts.

Schematic A diagram that uses symbols to
represent circuit components.
Types of power sources




1.0 V
3.0 V

3.0 V
50 V
-
-
-
-
Multi-cell battery
Single cell battery
Multiple voltage of a single cell
Voltage Source
AC
DC
Generator
EMF (electromotive force) An open terminal
battery. (A battery not part of a circuit.)
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• Resistor A circuit device that is designed
  specifically to limit
• current flow.
• Resistance is a restriction to current flow.
• Any device can be modeled with a resistor.
• The larger the resistor/resistance the smaller
  the current that
• flows through them.
• Resistor/Resistance is measured in Ohms (O).
• O Greek capital letter Omega
• Voltage (Potential Difference) is lost through
  resistors.

Wire The medium through which charges flow.
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Circuit Components Continued
Switch A device used to start and stop the flow
of current in a circuit.
Fuse A device designed to interrupt current
flow in a circuit if the current exceeds a
certain level. Fuses must be replaced after they
blow.
Circuit Breaker A device designed to interrupt
current flow in the circuit if the current
exceeds a certain level. Circuit breakers can
be reset and reused after being activated.
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Power Source Configurations

• Power sources can placed in various
  configurations to achieve increased current or
  voltage.

1.0 V



1.0V
2.0 V
6.0 V
1.0 V
1.0 V
1.0 V
3.0 V
-
Power sources In this configuration (series)
Increase voltage
-
Power sources in this configuration (parallel)
Increase current.
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Battery Configurations
Cells within a 9.0 V battery
Battery Configurations
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Series/Parallel Paths
-
Series Path
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Parallel Paths
-
-
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Series/Parallel Paths
Series a region in a circuit in with only one
current path Parallel a region in a circuit
with multiple current paths
Series
Parallel
Series
Parallel
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Circuit Measuring Devices
Circuit measuring devices have minimal influence
on the circuit.
A

• Ammeter A device that measures current in an
  electrical circuit.
• -ammeters have an extremely low internal
  resistance
• -ammeters are placed in series with circuit
  components

V
Voltmeter A device that measures voltage
(potential difference) in an
electrical circuit. - voltmeters have an
extremely high internal resistance -voltmeters
are placed in parallel with circuit components
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Ammeter Placement
I
I
I
A
Device

or
I
I
I
Device
A

• An ammeter will read positive if conventional
  current enters the positive
• ammeter terminal.

• The ammeter is in series because there is only
  one path for the current

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Voltmeter Placement
IV
V
IV

I
I
Device
ID
ID

• A voltmeter will read positive if conventional
  current enters the positive voltmeter terminal.

• The voltmeter is in parallel with the device
  because there are alternate paths between the
  device and voltmeter for the current

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Simple Circuit
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Simple Circuit Schematic
A
I


I
V

_
R
V
V
V
R
-
-
Ohms Law VIR V voltage (V) I current
(A) R resistance (O) R is constant for a
circuit that obeys Ohms Law.
Power and Energy in a Circuit
PVI PI2R PV2/R
(specific to resistors)
EPt
t time (s)
Power is measured in Watts. Energy is measured in
Joules.
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Simple Circuit Animation
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Series Circuits
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Series Resistor Circuits
Resistors are in series when there is only one
current path between the individual
resistors. The current in each resistor is the
same.
The sum of the voltages in a closed loop must
equal zero. V-V1-V2-V3 0
or VV1V2V3 VIR1IR2IR3 VI(R1R2R3) VI(Req
)
V1
-

R1

I

V
V2
R2
ReqR1R2R3
_
Reqequivalent resistance The equivalent
resistance (Req) is the single resistor that has
the same effect in the circuit as the resistors
it replaces.
-
R3
V3
-

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Equivalent Circuit
I
IV/Req


V
-
Req
V
-
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Series Circuit Animation
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Series Resistor Concepts

• The more resistors in series, the smaller the
  circuit current.
• The larger the resistance, the greater the amount
  of voltage lost across the resistor.
• The resistor with the largest value uses the most
  voltage and power for series resistors.
• Each resistor in series has an overall influence
  on the total circuit current.

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Parallel Resistor Circuits
Resistors are in parallel when there are multiple
current paths. Voltages across each resistor in
parallel are the same.
Home/Business circuits are wired in parallel.
II1I2I3
I2I3
I
I3
I1
I2

R3
R2
V
R1
-
The equivalent resistance for resistors in
parallel is always smaller than the smallest
resistor. More resistors in parallel decreases
the equivalent resistance of the circuit.
I1I2I3
I2I3
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Equivalent Circuit
I
IV/Req


V
-
Req
V
-
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Parallel Animation
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Parallel Resistor Concepts

• The voltage of each parallel resistor is the same
  as the source voltage.
• The equivalent resistance for resistors in
  parallel is always smaller than the smallest
  resistor.
• More resistors in parallel decreases the
  equivalent resistance of the circuit.
• The current in each branch is affected only by
  the resistance of each branch.
• The resistor with the smallest value uses the
  most power for parallel resistors.
• The more resistors in parallel, the larger the
  total circuit current.

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Home/Business Wiring
Circuit breaker
outlets
Transmission lines from outside
Alternating Current
Inside view of outlet wiring
Neutral/ground line