Circuits
• Schematic diagrams depict the construction of a
circuit
• Uses symbols to represent specific circuit
elements
• Documents how elements are connected so that
anyone reading diagram can understand the
circuit’s construction and maintenance
• Symbols are fairly standard, but variations do
exist
• Connected elements of an electrical device form
a circuit
▫ Path through which charges can be conducted
• Components that dissipate energy: loads
• Simple circuit: source of potential difference
and electrical energy, wires, load(s)
▫ Wires assumed to dissipate negligible amount of
energy
• If charges can flow from one terminal of
potential difference source to other without
interruption: closed circuit
▫ Potential difference exists and current flows
• Without complete path, there is no potential
difference
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No flow of charge
No current
Open circuit
Load does not operate
• Switches used to selectively open and close
circuits
▫ Fuses and circuit breakers open circuits
automatically in dangerous situations
• Without a load, resistance in circuit is very low
▫ Current increases due to lowered resistance
▫ High current flow can overheat wires and cause
fires
▫ Short circuits route charge around loads and
create increased currents
• All circuits require source of potential difference
and electrical energy
▫ Batteries and generators are common sources of
potential difference
• Any device that increases potential energy of
charges in a circuit is a source of emf
▫ emf: energy per unit charge supplied by a source
of electric current
▫ Charge pump: forces charges to move in a certain
direction
• For conventional current, terminal voltage is less
than emf
▫ emf source has internal resistance to charge flow
▫ Acts as both an emf source and a resistor
• Potential difference across terminals actually
less than emf
▫ Potential difference listed for emf sources should
be interpreted as the terminal voltage and not the
emf value of the source
• When charges leave emf source, encounters with
loads and resistors dissipate energy
▫ Potential decrease across loads must equal
potential increase across battery
▫ Conservation of energy must hold in circuits
• Multiple loads can have different
arrangements in circuits
▫ Series, parallel
• Series circuits: when charges move
through single path
▫ Must go through each load in order
• Law of conservation of charge says that
same charge that moves through one load
must move through other loads
• In series circuits, current through each load is
the same
▫ Same charge must enter and exit loads in same
time interval to prevent accumulation or
disappearance of charge at any given point
• Total current in series circuit depends on how
many resistors are present and the magnitude
each resistor possesses
▫ Total current found by summing individual
resistances to determine total resistance of circuit
• The equivalent resistance of a series circuit is
always greater than any individual resistance
▫ Use Ohm’s Law to find current in circuit: ΔV =
IReq
• Charges dissipate energy through each load
▫ Potential across each load in a series circuit varies
▫ Use Ohm’s Law to find potential difference across
each load: ΔVload = IcircuitRload
• Series circuits require all elements to function
for current to flow
▫ Burnt filament, broken wire, open switch
• Alternate pathways for charges to travel from
and to potential difference source
• Loads or resistors in parallel have same
potential differences across them
▫ Charges carrying same amount of energy from
potential difference source to each load
▫ Potential difference equal to terminal voltage of
potential difference source
• Current through each load can vary
▫ Different numbers of charges move through
different loads
 Loads with less resistance experience more current
▫ Sum of current in parallel circuits equals total
current
 Itotal = ∑ Iresistors
▫ Use terminal voltage and individual resistance
value to find current flow through load
• Total equivalent resistance for parallel circuit
calculated using reciprocal relationship
▫ 1/Req = 1/R1 + 1/R2 + 1/R3…
▫ Equivalent resistance for parallel circuit always
less than the smallest individual resistance
▫ Adding resistors in parallel lowers overall
resistance – overloading parallel circuit can
increase current to dangerous level
• Parallel circuits do not require all elements
function – alternate path(s) for charges
• Series and parallel circuits often combined in
overall electrical scheme
• When determining equivalent resistance for
complex circuit, simplify circuit into groups of
series and parallel resistors and find equivalent
resistance of each group
▫ Recombine equivalent groups into simplified
circuit and analyze circuit as a whole
• Work backward to determine current and
potential difference across parts of circuit