Download Circuits and Circuit Elements

Electricity
Circuits and Circuit Elements
Circuits
• Electric circuit – a set of electrical
components connected so that they
provide one or more complete paths for the
movement of charges
– Charges move from the battery through a path
back to the battery
Circuits
• Switches are used to open and close the
circuit
– If the circuit is open, there is no complete path
for the charge to travel
– If the circuit is closed, there is at least one
complete path for the charge to travel
• Charge cannot flow, so there is no current
Circuits
• Schematic diagram – a graphic representation of
an electric circuit or apparatus, with standardized
symbols for the circuit components
Circuits
Series and Parallel Circuits
• Series – describes a circuit or portion of a
circuit that provides a single conducting
path
– One path for current to travel
– If one element is removed, the circuit will not
work
– If one element doesn’t work, the circuit will not
work
Series and Parallel Circuits
• Parallel – describes components in a
circuit that are connected across common
points, providing two or more separate
conducting paths
– More than one pathway
– Charge moves through the path of least
resistance
– If one part is removed or doesn’t work the
circuit may continue to work
Electric Power and Electrical
Energy
• Electrical energy – the energy associated with electrical charges,
whether moving or at rest
• Electric power – the rate at which electrical energy is used in a circuit
–
–
–
–
–
–
–
–
–
The rate at which electrical work is done
Power = Current * Potential Difference
P=IV
Power = (Current)2 * Resistance
P=I2R
Power = (Potential Difference)2 / Resistance
P= V2 / R
Measured in Watts (W)
Electric companies measure energy consumed in kilowatt-hours (kWh)
• The greater the power produced by a given current or potential
difference, the smaller the resistance
Fuses and Circuit Breakers
• Overloaded – condition when electrical
wires carry more than a safe level of
current
• Short circuit – accidental creation of an
alternative pathway
– Reduces the resistance in the circuit
– Increases the current
Fuses and Circuit Breakers
• Short circuits
– Leads to overload
– Grounding is used to prevent electric shock
from short circuits
• Fuse – an electrical device containing a
metal strip that melts when current in the
circuit becomes to great
– Used to prevent overload
Fuses and Circuit Breakers
• Circuit breaker – a device that protects a
circuit from current overloads
• Trip when the current overloads
– Acts like a switch
Resistors in Series
• Series – describes a circuit or portion of a
circuit that provides a single conducting
path without junctions
• When resistors are arranged in series, the
current flowing through each resistor is the
same
• Equivalent resistance – the total resistance
of a circuit
Resistors in Series
Resistors in Series
• For resistors in series, the equivalent
resistance is equal to the sum of all of the
resistors
• Req = R1 + R2 + R3…
• Total Potential difference (voltage) = Current *
Equivalent Resistance
• VTot = IReq
• The equivalent resistance of a series
combination of resistors is always greater
than any individual resistance
Resistors in Series
• Potential difference across a given resistor
can be calculated by multpying the current
(which is constant) by the resistance of the
given resistor
• V1 = IR1 or V2 = IR2
• In a series circuit, all of the elements must
be able to conduct electrical charge
– If one bulb goes out or one wire gets
disconnected, the entire circuit fails
Resistors in Parallel
• Parallel – describes two or more components in a
circuit that are connected across common points or
junctions, providing separate conducting pats for
the current
– There is more than one pathway through the circuit or
part of a circuit
• Resistors in parallel have the same potential
difference across them
– V is constant
• The sum of the individual currents in parallel
resistors equals the total current
• ITot = I1 + I2 + I3 …
Resistors in Parallel
Resistors in Parallel
• The equivalent resistance of resistors in parallel
can be calculated using a reciprocal relationship
• 1/Req = 1/R1 + 1/R2 + 1/R3 …
• The equivalent resistance is always less than the
smallest resistance in the group of resistors
• Parallel circuits do not require all elements to
conduct
– The circuit will continue so long as there is at least one
complete pathway from one terminal to the other
Resistors in Parallel
Resistors Combined Both in
Parallel and in Series
• Complex circuits have resistors arranged both
in series and in parallel
• You must simplify the groups of resistors
– Work with a small group that is all in series or in
parallel
– After you have simplified the small groups, start
combining groups until you have “created” a de
facto simple circuit
• Work backward to find current or potential
difference
Resistors Combined Both in
Parallel and in Series
Resistors Combined Both in
Parallel and in Series