Electric Circuits

1
Electric Circuits
Chapter 22 23
2
Requirements of a Circuit
Find four different ways to light the bulb using
these materials
Wire
Battery
Bulb
Caution some configurations may heat the wire.
3
Requirements of a Circuit
4
Requirements of a Circuit
What two requirements had to be met for you to
light the bulb?

• Closed, conducting path from the positive
  terminal to the negative terminal.
• There must be an electric potential difference
  across the two ends of the circuit. This is
  usually done by using a battery.

5
Closed Conducting Path
No closed path extending from the positive
terminal to the negative terminal of the battery

Entire path is not conducting
6
Establishing an Electrical Potential Difference
Within the battery, there is an electric field
established between the two terminals, directed
from the positive terminal towards the negative
terminal. Moving a positive test charge through
the battery from the negative terminal to the
positive terminal would require work, thus
increasing the potential energy of every Coulomb
of charge that moves along this path.
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Establishing an Electrical Potential Difference
The positive charge moving from the negative
terminal to the positive terminal is moving
________ the electric field, so the positive
terminal is described as the _____ potential
terminal. The battery supplies the energy
(through a chemical reaction) to do work on the
charge in order to pump it or move it through
the battery from the negative to the positive
terminal.
against
high
8
Establishing an Electrical Potential Difference
The movement of positive charge through the wires
from the positive terminal to the negative
terminal would occur naturally. Its moving _____
the electric field and would not require work. It
_____ potential energy. The maximum potential
difference supplied by the battery is called the
electromotive force, E, or emf. E in this example
12 V.
with
loses
9
Establishing an Electrical Potential Difference
This assignment of high and low potential to the
terminals of a battery uses the traditional
convention that electric fields are based on the
direction of movement of positive test charges.
10
Electrical Circuit Analogies

• An electric circuit is like a water circuit at a
  water park or a roller coaster ride at an
  amusement park.
• In all three cases, there is something which is
  moving through a complete loop - that is, through
  a circuit.
• The circuit has a section where energy is added
  in (to move it against its natural direction)
• Water is pumped from ground level to the top
• A motor driven chain pulls a coaster to the top.
• An electric circuit has a battery, or other
  energy supply that moves the charge from the low
  potential ( terminal) to the high potential (
  terminal)

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Electrical Circuit Energy Conversion system
An electric circuit is nothing more than an
energy conversion system.
Chemical energy transforms into EPE, electrical
potential energy.
Electrical potential energy transformed into
thermal energy (or other form, depending on
design).
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Electrical Circuit Energy Conversion system
13
Electric Current

• If the two requirements are met for a circuit,
  charges will flow through the external circuit.
• __________ is the rate at which charges flow past
  a cross sectional point in a circuit.

Current
I
same

• Direct current moves around the circuit in the
  _____ direction all the time.

14
Resistance
Water traveling through pipes is hindered by
friction between the water and the pipes and
other obstacles that might be in the path.
An electron traveling through the wires and loads
of the external circuit also encounters
resistance. For example, it collides with fixed
atoms as it moves through the conductor. Resistanc
e is the hindrance to the flow of charge. While
the electric potential difference established
between the two terminals encourages the movement
of charge, it is resistance which discourages it.
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Resistance
How would the total length of wire affect its
resistance?
More wire ? more collisions ? more resistance.
How would the cross-sectional area of a wire
affect its resistance?
Just like water in pipes, the bigger cross
section ? more flow ? less resistance.
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Resistance
How would the material type of a wire affect its
resistance?
Better conductor ? less resistance. The
conducting ability can be measured by its
resistivity (r). Resistivity depends on the
electronic structure of a material and the
temperature. Insulators have high resistivities,
conductors have low resistivities. For metals,
resistivity increases with temperature, but it
decreases with temperature for semi-conductors.
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Resistivity of common materials
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Resistance
Resistance
___________ with wire length, L ___________ with
cross-sectional area, A ___________ with higher
resistivity (r) materials Thus,
Increases
Decreases
Increases
Note that resistivity is a property of the
material. Resistance depends on both the
resistivity and the geometry of the material.
19
Resistance
Just as larger pumps move more water through a
pipe, a larger voltages lead to larger currents.
How much more current would a 12V battery provide
than a 6V battery when they are connected to the
same circuit?
As the resistance is lowered, _____ current can
flow, thus current and resistance are __________
proportional.
more
inversely
20
Ohms Law
The ratio of voltage over current is a constant,
R. V is the voltage applied across a piece of
material. I is the current through the
material. R is the resistance of the
piece of material.
V
Units
volts
I
R
R
ohms W
amperes
21
Resistance
The filament in a flashlight is a resistor in the
form of a thin piece of wire. The wire becomes
hot enough to emit light because of the current
in it.
This flashlight uses two 1.5V batteries
(effectively a single 3.0 V battery) to provide a
current of 0.40 A in the filament. What is the
resistance of the filament?

• .1333 ohms
• 7.5 ohms
• 1.2 ohms
• 3.4 ohms

22
Resistivity and temperature
Metals Resistivity as temperature Semi-conduct
ors Resistivity as temperature Superconductors
Resistivity goes to zero below critical
temperature (a few degrees above absolute zero.)
These materials become perfect conductors. A
current can continue indefinitely without the
need for an emf. Superconductors are used in MRI
imaging, magnetic levitation of trains, some
electric motors and electrical transmission.
23
Electric Power
Recall that
EPE
qDV

Since power is energy (work) over time
The charge per time is the current, thus
P IDV
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Electric Power
We can combine this formula with Ohms law to
obtain different versions
P IV
V IR
and
P I(IR) I2R
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Electric Power Example
The flashlight has a current of 0.40 A and a
voltage of 3.0 V. Find (a) the power delivered
to the bulb and (b) the energy dissipated in the
bulb in 5.5 minutes of operation.

• .1333 watts
• 7.5 watts
• 1.2 watts
• 3.4 watts

396 J
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Circuit Diagrams
Circuit diagrams provide a quick picture of the
circuit. Standard symbols are used for clarity
and consistency.
V
A
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Circuit Diagrams
Identify the objects in this diagram.
F
B
E
C
A
D
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Series Wiring
In series wiring, the devices are connected in
such a way that the current through each device
is the same. The charge only has one path to
flow through.
What would happen to the second and third light
bulb, if the first one burnt out?
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Series Wiring
In series wiring, the voltage supplied by the
battery is divided between each of the load
devices.
VT V1 V2
From Ohms Law, VT IR1 IR2
Since the current is constant, VT I (R1 R2)
VT IRS
RS is the Equivalent Resistance of the series
circuit
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Series Wiring
VT I (R1 R2) IRS
Note that two resistors in series are equivalent
to a single resistor whose resistance is the sum
of the individual resistors.
This can be extended to any number of resistors
in series
RS R1 R2 R3
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Series Wiring Example
Draw an equivalent circuit
10.61 watts
5.31 watts
Assuming the battery contributes no resistance,
what is the current in the circuit? What is the
power dissipated in each resistor? What is the
total power delivered by battery?
1.33 amps
15.96 watts
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Series Wiring Example
How does adding more resistors change the overall
current in a series circuit?
Which series circuit would provide the brightest
bulbs? One with 1, 2 or 3 bulbs? Why?
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Series Wiring Summary
Current --the current through each device is the
same. Voltage -- the sum of the voltage drops in
each individual resistor is equal to the battery
voltage rating. Resistance the overall
resistance of the collection of resistors is
equal to the sum of the individual resistance
values
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Circuit Organizer
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Series

• PATH One Path
• ONE LIGHT GOES OUT they all go out
• BULB BRIGHTNESS more bulbs, less bright
• CURRENT same through out circuit
• RESISTANCE sum of resistors
• VOLTAGE voltage drops at each resistor add up to
  battery
• ADDING RESISTORS Increases resistance pf circuit

36
Parallel Wiring
In parallel wiring, the devices are connected in
such a way that the voltage through each device
is the same. The charge has multiple paths to
flow through.
What would happen to the second and third light
bulb, if the first one burnt out?
37
Parallel Wiring
Houses are wired in parallel. Each branch of the
circuit would have the same 120 V. An unused, or
a burnt out bulb __________ affect the other
devices.
would not
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Parallel Wiring
Adding more resistors in parallel actually
decreases the overall resistance of the
circuit. The current has more possible paths to
flow through.
Each branch of the circuit draws current as if
the other wasnt there, so combined they draw
more current. Since R V/I, a larger current
means less resistance.
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Effect of adding more resistors
Adding more resistors in parallel would be like
adding more tollbooths in parallel. The cars
(current) have more possible paths to flow
through.
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Equivalent resistance of parallel circuit
In parallel wiring, the current from the battery
is divided between each of the load devices.
IT I1 I2
From Ohms Law, IT V/R1 V/R2
Since V is constant
IT V (1/R1 1/R2)
V (1/RP)
1/RP 1/R1 1/R2 1/R3
RP is the Equivalent Resistance of the parallel
circuit
41
Parallel Wiring Example
Draw an equivalent circuit
1.5 amps
.75 amps
What is the equivalent resistance of the
circuit? What is the total current in the
circuit? What is the current through each
resistor? What is the average power through each
resistor?
2.67 Ohms
2.25 amps
P14.5 watts
P29 watts
42
Parallel Wiring
Draw an equivalent circuit
R1 2.0 W
R1 18.0 W
Which resistor has the largest impact in
determining the overall resistance?
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Parallel Wiring Example
How does a 3-way light bulb provide three
different light levels?
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Parallel Wiring Summary
R1 2.0 W
Current -- the sum of the current in each
individual branch equals the current outside the
branches. Voltage the voltage drop is the same
across each individual branch. Resistance the
reciprocal of the overall resistance of the
collection of resistors is equal to the sum of
the reciprocals of the individual resistance
values
R1 18.0 W
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Circuit Organizer
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Parallel

• PATH multiple paths
• ONE LIGHT GOES OUT the rest stay on
• BULB BRIGHTNESS all equally bright
• CURRENT change through out circuit, vary with
  resistor
• RESISTANCE inverse of the sum of the inverse of
  resistors
• VOLTAGE Voltage drop at each resistor is the
  same as battery
• ADDING RESISTORS decreases resistance of circuit

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Combination Parallel Series Circuits
Some circuits have both parallel and series
sections. Analyze these circuits by reducing to
an equivalent series circuit.

• Which resistors are in series?
• Which resistors are in parallel?

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Combination Parallel Series Circuits
Find the total current supplied by the battery.
Find the voltage across A-B.
Start on the outer section
110 W
A
220 W
24 V
180 W
250 W
B
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Combination Parallel Series Circuits
Find equivalent resistances and redraw circuit.
50
Combination Parallel Series Circuits
Use Ohms law to find the current supplied by the
battery.
Use VAB IABRAB to find the voltage between
points A and B.
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Combination Parallel Series Circuits Practice
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Combination Parallel Series Circuits Practice
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Combination Parallel Series Circuits Practice
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Combination Parallel Series Circuits Practice
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Practice
greater than, equal to, less than?

• The current at location A is _____ the current at
  location B.
• The current at location B is _____ the current at
  location E.
• The current at location G is _____ the current at
  location F.
• The current at location E is _____ the current at
  location G.

gt
lt
gt
56
Practice
greater than, equal to, less than?
gt

• The current at location B is _____ the current at
  location F.
• The current at location A is _____ the current at
  location L.
• The current at location H is _____ the current at
  location I.

lt
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Practice greater than, equal to, less than?

• The electric potential difference (voltage drop)
  between points B and C is _____ the electric
  potential difference (voltage drop) between
  points J and K.
• The electric potential difference (voltage drop)
  between points B and K is _____ the electric
  potential difference (voltage drop) between
  points D and I.

gt
gt
58
Practice greater than, equal to, less than?

• The electric potential difference (voltage drop)
  between points E and F is _____ the electric
  potential difference (voltage drop) between
  points G and H.
• The electric potential difference (voltage drop)
  between points E and F is _____ the electric
  potential difference (voltage drop) between
  points D and I.

59
Practice greater than, equal to, less than?

• The electric potential difference (voltage drop)
  between points J and K is _____ the electric
  potential difference (voltage drop) between
  points D and I.
• The electric potential difference between points
  L and A is _____ the electric potential
  difference (voltage drop) between points B and K.

gt

60
Electrical Safety
What is electrical grounding?
61
Electrical Safety
What is electrical grounding?
62
Electrical Safety
What is electrical grounding?
Third prong provides a path for the electrons to
ground (usually a copper pipe). This path has
much less resistance than person.
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Electrical Safety

• Electric shocks can be fatal, depending on the
  magnitude of the current and the part the body
  the current flows through.
• Currents of 0.001A (1 mA) cause a mild tingling
  feeling.
• Currents of 0.01 0.02 A (10-20 mA) can cause
  muscle spasms where the person cant let go of
  item causing shock.
• Currents of 0.2A are potentially fatal as they
  can make the heart fibrillate.
• Larger currents can actually stop the heart, but
  the heart often restarts after current stopped.
  Less dangerous than fibrillation.