Download Current and Circuits

Current and Circuits
Current
current: the flow of charged particles.
E
amount of charge
Current 
time
q
I
t
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.
1 Coulomb 6.25 x 1018 electrons/ protons
1 Ampere 

1s
1s
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
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
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
Circuits
circuit: A continuous path for charges to
flow through.
Circuit simulation
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
-
~
-
Single cell battery Multi-cell battery
Multiple voltage
of a single cell
Voltage
Source
EMF (electromotive force): An open terminal battery. (A battery not part of
a circuit.)
DC
AC
Generator
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 (Ω).
•Ω – Greek capital letter Omega
•Voltage (Potential Difference) is lost through resistors.
Wire: The medium through which charges flow.
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.
Power Source Configurations
• Power sources can placed in various
configurations to achieve increased
current or voltage.
+
1.0 V
+
2.0 V
+
+
+1.0V
+6.0 V
1.0 V
3.0 V
1.0 V
1.0 V
Power -sources
In this configuration
(series) Increase
voltage
Power sources in this configuration
(parallel) Increase current.
Battery Configurations
Cells within a 9.0 V battery
Battery Configurations
Series/Parallel Paths
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Series Path
-
-
-
Parallel Paths
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
Circuit Measuring Devices
Circuit measuring devices have minimal influence on the circuit.
A
V
•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
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
Ammeter Placement
I
I
I
A
Device
+
or
I
I
Device
I
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
Voltmeter Placement
IV
IV
V
+
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
Simple Circuit
Simple Circuit Schematic
A
I
I
V
+
V
-
+
+
_
R
R
V
V
-
Ohms Law:
V=IR
V = voltage (V)
I = current (A)
R = resistance (Ω)
R is constant for a circuit
that obeys Ohm’s Law.
Power and Energy in a Circuit:
P=VI
P=I2R
P=V2/R
(specific to
resistors)
E=Pt
t = time (s)
Power is measured in Watts.
Energy is measured in Joules.
Simple Circuit Animation
Series Circuits
Series Resistor Circuits
Resistors are in series when there is only one current path between the
individual resistors.
The sum of the
The current in each resistor is the same.
voltages in a closed
V1
+
loop must equal zero.
V-V1-V2-V3 = 0
or
R1
V=V1+V2+V3
+
I
+
V
R2
_
V2
V=IR1+IR2+IR3
V=I(R1+R2+R3)
V=I(Req)
Req=R1+R2+R3+…
Req=equivalent resistance
- The equivalent resistance (Req)
R3
-
is the single resistor that has
the same effect in the
circuit as the resistors it
replaces.
V3
+
Equivalent Circuit
I
+
+
V
-
Req
V
-
I=V/Req
Series Circuit Animation
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.
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.
V
V
V
V



R eq R 1 R 2 R 3
I2+I3
I
+
I=I1+I2+I3
I1
V
I3
I2
R1
R2
R3
1
1
1
1



R eq R1 R 2 R 3
-
I1+I2+I3
I2+I3
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.
Equivalent Circuit
I
+
+
V
-
Req
V
-
I=V/Req
Parallel Animation
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.
Home/Business Wiring
Circuit
breaker
outlets
Transmission
lines from outside
Alternating
Current
~
Neutral/ground
line
Inside view of
outlet wiring