Download Series Circuits

Water Analogy
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A simple electrical circuit - consisting of
a battery and a resistor - can be modeled
by a pump to simulate a battery and a
paddle to simulate electrical resistance.
As the current turns the paddle it does
work and thus loses some energy similar to
electrical current flowing through a
resistor.
Simple Circuit
Series Circuits
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Only one path for circuit current
Current the same in all parts of circuit
Sum of voltage drops across circuit
elements equals battery voltage
Total circuit resistance equals sum of
separate resistances
Series Circuit
Parallel Circuits
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More than one conducting path for circuit
current
Two or more components connected
across two common points in circuit
Currents in parallel branches vary
inversely with branch resistance; total
current = sum of branch currents
Parallel Circuits
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Voltage drop the same across parallel
branches
Parallel resistances add following
reciprocal rule: reciprocal of total
resistance equals sum of reciprocals of
individual resistances
Parallel Circuit
Adding Parallel Resistances
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For two parallel resistors
R1 R2
Req 
R1  R2
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For two or more parallel resistors
1
1
1
1
 
  ...
Req R1 R2 R3
Complex Circuit Networks
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Combination of series and parallel
To analyze, first find total resistance
(RT), then total current
To simplify resistance networks, replace
several resistances with one equivalent
resistance(Req)
Start with series resistances and
combine
Complex Circuit
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Then collapse parallel branches into one
equivalent resistance
Combine series resistances created by
previous step
Continue until only one equivalent
resistance remains
Complex Circuit
Kirchhoff’s Rules
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Algebraic sum of currents at any circuit
junction equals zero; or currents into a
junction equal currents leaving the
junction; conservation of charge
Algebraic sum of all voltage drops
around a circuit loop equals zero;
conservation of energy
Overload Protection
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Homes have 120 volt lines from power
company
Home circuits are connected in parallel
Each parallel branch lowers total
resistance, increasing current in supply
lines
Overload Protection
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Lines can become overloaded causing
heating and fire danger
Circuits protected by fuses or circuit
breakers which open circuit if current
rises beyond certain value
Protect against short circuits where
bare wires touch together
Battery Cells
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Wet cells: use liquid electrolyte - car battery
Dry cells: use paste “dry” electrolyte flashlight batteries
Primary cells: replaced when reactants are
used up
Storage cells: easily recharged
Fuel cells: New reactants added as needed
Combinations of Cells
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Battery is combination of cells connected in
series, parallel, or combination of both
Cells in series: cells connected + to -, as in a
flashlight
battery voltage = sum of cell voltages;
battery current = current of one cell, the
same throughout; battery resistance = sum
of cell resistances
Combinations of Cells
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Cells in parallel: - terminals all
connected together and + terminals all
connected together
Battery voltage = voltage of one cell;
total current drawn by circuit is divided
equally among the cells; battery
resistance is reciprocal of the sum of
reciprocals of cell resistances
Electrical Measurements
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Voltmeter must be placed in parallel
with circuit element whose voltage drop
is being measured
Ammeter must be placed in series so
that all circuit current flows through
ammeter.
Never connect the ammeter in
parallel like the voltmeter
Measuring Resistance
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Resistance can be measured using a
voltmeter and ammeter and calculating
resistance using Ohm’s Law
Some error is introduced by the meters
Multimeter uses this method
Wheatstone bridge gives more precise
measurement