Download 35 Electric Circuits

35 Electric Circuits
Any path along
which electrons can
flow is a circuit.
35 Electric Circuits
Mechanical things seem to
be easier to figure out for
most people than electrical
things. Maybe this is
because most people have
had experience playing
with blocks and
mechanical toys. Handson laboratory experience
aids your understanding of
electric circuits. The
experience can be a lot of
fun, too!
35 Electric Circuits
35.1 A Battery and a Bulb
In a flashlight, when the switch is turned on to
complete an electric circuit, the mobile conduction
electrons already in the wires and the filament begin
to drift through the circuit.
35 Electric Circuits
35.1 A Battery and a Bulb
A flashlight consists of a reflector cap, a light bulb, batteries,
and a barrel-shaped housing with a switch.
35 Electric Circuits
35.1 A Battery and a Bulb
There are several ways to connect the battery and bulb from
a flashlight so that the bulb lights up.
1. The important thing is that there must be a complete
path, or circuit, that
• includes the bulb filament ( load with resistance)
• runs from the positive terminal at the top of the battery
• runs to the negative terminal at the bottom of the battery
35 Electric Circuits
35.1 A Battery and a Bulb
2. Electrons- real flow
• from the negative part of the battery through the wire
• to the side (or bottom) of the bulb
• through the filament inside the bulb
• out the bottom (or side)
• through the wire to the positive part of the battery
The current then passes through the battery to complete
the circuit.
3. Conventional Flow of Electrons from positive part of
the battery to negative .
35 Electric Circuits
35.1 A Battery and a Bulb
a. Unsuccessful ways to light a bulb.
35 Electric Circuits
35.1 A Battery and a Bulb
a. Unsuccessful ways to light a bulb.
b. Successful ways to light a bulb.
35 Electric Circuits
35.1 A Battery and a Bulb
The flow of charge in a circuit is very much like the flow of
water in a closed system of pipes.
In a flashlight, the battery is analogous to a pump, the wires
are analogous to the pipes, and the bulb is analogous to
any device that operates when the water is flowing.
When a valve in the line is opened and the pump is
operating, water already in the pipes starts to flow.
35 Electric Circuits
35.1 A Battery and a Bulb
Neither the water nor the electrons
concentrate in certain places.
They flow continuously around a
loop, or circuit.
When the switch is turned on, the
mobile conduction electrons in the
wires and the filament begin to drift
through the circuit.
35 Electric Circuits
35.1 A Battery and a Bulb
Electrons do not pile up inside
a bulb, but instead flow through
its filament.
35 Electric Circuits
35.1 A Battery and a Bulb
What happens to the mobile conduction
electrons when you turn on a flashlight?
35 Electric Circuits
35.2 Electric Circuits
For a continuous flow of electrons, there must
be a complete circuit with no gaps.
35 Electric Circuits
35.2 Electric Circuits
4. Any path along which electrons
can flow is a circuit.
A gap is usually provided by an
electric switch that can be opened
or closed to either cut off or allow
electron flow.
35 Electric Circuits
35.2 Electric Circuits
The water analogy is useful but has some limitations.
• A break in a water pipe results in a leak, but a
break in an electric circuit results in a complete
stop in the flow.
• Opening a switch stops the flow of electricity. An
electric circuit must be closed for electricity to
flow. Opening a water faucet, on the other hand,
starts the flow of water.
35 Electric Circuits
35.2 Electric Circuits
Most circuits have more than one device that receives
electrical energy.
These devices are commonly connected in a circuit in
one of two ways, series or parallel.
5. When connected in series, the devices in a
circuit form a single pathway for electron flow.
6. When connected in parallel, the devices in a
circuit form branches, each of which is a
separate path for electron flow.
35 Electric Circuits
35.2 Electric Circuits
How can a circuit achieve a continuous
flow of electrons?
35 Electric Circuits
35.3 Series Circuits
If one device fails in a series circuit, current in
the whole circuit ceases and none of the
devices will work.
35 Electric Circuits
35.3 Series Circuits
If three lamps are connected in series with a battery,
they form a series circuit. Charge flows through each
in turn.
When the switch is closed, a current exists almost
immediately in all three lamps.
The current does not “pile up” in any lamp but flows
through each lamp. Electrons in all parts of the circuit
begin to move at once.
35 Electric Circuits
35.3 Series Circuits
Eventually the electrons move all the way around the circuit.
6. A break anywhere in the path results in an open circuit,
and the flow of electrons ceases.
Burning out of one of the lamp filaments or simply opening
the switch could cause such a break.
35 Electric Circuits
35.3
7.Series Circuits
In this simple series circuit, a 9-volt battery provides 3 volts
across each lamp.
35 Electric Circuits
35.3
8. Series Circuits
For series connections:
• Electric current has a single pathway through the circuit.
• The total resistance to current in the circuit is the sum of
the individual resistances along the circuit path.
• The current is equal to the voltage supplied by the source
divided by the total resistance of the circuit. This is Ohm’s
law.
• The voltage drop, or potential difference, across each
device depends directly on its resistance.
• The sum of the voltage drops across the individual
devices is equal to the total voltage supplied by the
source.
35 Electric Circuits
35.3
9. Disadvantage of Series Circuits
The main disadvantage of a
series circuit is that when one
device fails, the current in the
whole circuit stops.
Some cheap light strings are
connected in series. When one
lamp burns out, you have to
replace it or no lights work.
35 Electric Circuits
35.3 Series Circuits
think!
What happens to the light intensity of each lamp in a series
circuit when more lamps are added to the circuit?
35 Electric Circuits
35.3 Series Circuits
think!
What happens to the light intensity of each lamp in a series
circuit when more lamps are added to the circuit?
Answer:
The addition of more lamps results in a greater circuit
resistance. This decreases the current in the circuit (and in
each lamp), which causes dimming of the lamps.
35 Electric Circuits
35.3 Series Circuits
think!
A series circuit has three bulbs. If the current through one of
the bulbs is 1 A, can you tell what the current is through each
of the other two bulbs? If the voltage across bulb 1 is 2 V, and
across bulb 2 is 4 V, what is the voltage across bulb 3?
35 Electric Circuits
35.3 Series Circuits
think!
A series circuit has three bulbs. If the current through one of
the bulbs is 1 A, can you tell what the current is through each
of the other two bulbs? If the voltage across bulb 1 is 2 V, and
across bulb 2 is 4 V, what is the voltage across bulb 3?
Answer:
The same current, 1 A, passes through every part of a series
circuit. Each coulomb of charge has 9 J of electrical potential
energy (9 V = 9 J/C). If it spends 2 J in one bulb and 4 in
another, it must spend 3 J in the last bulb. 3 J/C = 3 V
35 Electric Circuits
35.3 Series Circuits
What happens to current in other lamps if
one lamp in a series circuit burns out?
35 Electric Circuits
35.4 Parallel Circuits
10. In a parallel circuit, each device operates
independent of the other devices. A break in any
one path does not interrupt the flow of charge in
the other paths.
35 Electric Circuits
35.4 Parallel Circuits
In a parallel circuit having three lamps, each electric
device has its own path from one terminal of the battery
to the other.
There are separate pathways for current, one through
each lamp.
In contrast to a series circuit, the parallel circuit is
completed whether all, two, or only one lamp is lit.
10. A break in any one path does not interrupt the
flow of charge in the other paths.
35 Electric Circuits
35.4
11. Drawing of a Parallel Circuits
In this parallel circuit, a 9-volt battery provides 9 volts
across each activated lamp. (Note the open switch in the
lower branch.)
35 Electric Circuits
35.4 12. Parallel Circuits
Major characteristics of parallel connections:
• Each device connects the same two points A and B of
the circuit. The voltage is therefore the same across
each device.
• The total current divides among the parallel branches.
• The amount of current in each branch is inversely
proportional to the resistance of the branch.
• The total current is the sum of the currents in its
branches.
• As the number of parallel branches is increased, the
total current through the battery increases.
35 Electric Circuits
35.4
12. Parallel Circuits
From the battery’s perspective, the overall resistance of
the circuit is decreased.
This means the overall resistance of the circuit is less than the
resistance of any one of the branches.
35 Electric Circuits
35.4 Parallel Circuits
think!
What happens to the light intensity of each lamp in a parallel
circuit when more lamps are added in parallel to the circuit?
35 Electric Circuits
35.4 Parallel Circuits
think!
What happens to the light intensity of each lamp in a parallel
circuit when more lamps are added in parallel to the circuit?
Answer:
The light intensity for each lamp is unchanged as other lamps
are introduced (or removed). Although changes of resistance
and current occur for the circuit as a whole, no changes occur
in any individual branch in the circuit.
35 Electric Circuits
35.4 Parallel Circuits
What happens if one device in a parallel
circuit fails?
35 Electric Circuits
35.5 Schematic Diagrams
In a schematic diagram, resistance is shown by a
zigzag line, and ideal resistance-free wires are shown
with solid straight lines. A battery is represented with
a set of short and long parallel lines.
35 Electric Circuits
35.5
13. Schematic Diagrams
Electric circuits are frequently
described by simple diagrams,
called schematic diagrams.
• Resistance is shown by a
zigzag line, and ideal
resistance-free wires are shown
with solid straight lines.
• A battery is shown by a set of
short and long parallel lines, the
positive terminal with a long line
and the negative terminal with a
short line.
35 Electric Circuits
35.5 Schematic Diagrams
These schematic diagrams represent
a. a circuit with three lamps in series, and
35 Electric Circuits
35.5
14. Schematic Diagrams of Series and Parallel
These schematic diagrams represent
a. a circuit with three lamps in series, and
b. a circuit with three lamps in parallel.
35 Electric Circuits
35.5 Schematic Diagrams
What symbols are used to represent resistance,
wires, and batteries in schematic diagrams?
35 Electric Circuits
35.6 Combining Resistors in a Compound Circuit
The equivalent resistance of resistors connected in
series is the sum of their values. The equivalent
resistance for a pair of equal resistors in parallel is
half the value of either resistor.
35 Electric Circuits
35.6
Combining Resistors in a Compound Circuit
Sometimes it is useful to know the equivalent resistance
of a circuit that has several resistors in its network.
15. The equivalent resistance is the value of the
single resistor that would comprise the same load to
the battery or power source.
The equivalent resistance of resistors connected in
series is the sum of their values. For example, the
equivalent resistance for a pair of 1-ohm resistors in
series is simply 2 ohms.
35 Electric Circuits
35.6
16. Example of Combining Resistors in a Compound Circuit
The equivalent resistance for a pair of equal resistors in
parallel is half the value of either resistor.
The equivalent resistance for a pair of 1-ohm resistors in
parallel is 0.5 ohm.
The equivalent resistance is less because the current
has “twice the path width” when it takes the parallel path.
35 Electric Circuits
35.6 Combining Resistors in a Compound Circuit
The equivalent resistance of two 8-ohm resistors in
series is 16 ohms.
35 Electric Circuits
35.6 18. Combining Resistors in a Compound Circuit
a. The equivalent resistance of two 8-ohm resistors in
series is 16 ohms.
b. The equivalent resistance of two 8-ohm resistors in
parallel is 4 ohms.
35 Electric Circuits
35.6 Combining Resistors in a Compound Circuit
For the combination of three 8-ohm resistors, the two
resistors in parallel are equivalent to a single 4-ohm resistor.
They are in series with an 8-ohm resistor, adding to produce
an equivalent resistance of 12 ohms.
If a 12-volt battery were connected to these resistors, the
current through the battery would be 1 ampere.
(In practice it would be less, for there is resistance inside the
battery as well, called the battery’s internal resistance.)
35 Electric Circuits
35.6 19. Combining Resistors in a Compound Circuit
Schematic diagrams for an arrangement of various electric
devices. The equivalent resistance of the circuit is 10 ohms.
35 Electric Circuits
More Examples
20. Star Connection
21. Ladder Connection
35 Electric Circuits
: Experiment
Analyzing Series and Parallel Circuits
Using Ohm’s Law
I. Purpose
• Learn about resistors and Ohm’s Law
• Learn how to calculate resistance, current and voltage in
each circuit arrangement
• Discover how to use electrical circuits with:
-resistors
-breadboard
-Multimeter (Am-meter, Volt-meter, Ohm-meter)
-connectors (alligator leads/clips)
• Conduct experiments using resistors in:
-series and parallel arrangements
• Measure resistance, current and voltage in each circuit
arrangement
• Learn how to analyze test results
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35 Electric Circuits
II. Procedures A. Series
Connection
1.
2.
3.
4.
5.
6.
7.
Connector 1 resistor using the breadboard.
Measure the resistance across it using an ohmmeter.
Set the ohmmeter to 20 X. Values indicated on the
screen of the ohmmeter will be multiplied by 1000.
Add one resistor at a time up to 4 resistors of the same
value.
Measure the resistance across two, three and four
resistors in series .
Record .
Calculate the % difference = / Expected value –
Measured Value / divided by Measured Value
35 Electric Circuits
III. A. Data and Results
Expected
value
R1 =
R1 + R2 =
R1 + R2+R3 =
R1 + R2 + R3 + R3 =
Measured
value
%
difference
35 Electric Circuits
II.B Procedures Parallel
Connection
1. Connect 1 resistor equals to 1,500 ohms in the
breadboard.
2. Connect another resistor of the same value in
parallel .
3. Measure the resistance across the parallel
connection using an ohmmeter at X20K
4. Connect two parallel resistors in series with
each other and measure the resistance at
terminal A and B . See the circuit below .
5. Calculate the % difference .
35 Electric Circuits
IIIB. Data and Results Parallel
Expected Measured %
Value
Value
difference
R1
R1//R2
R//R2 + R3//R4
35 Electric Circuits
IV. Analysis and Conclusion
1. How are series and parallel connections done
using a breadboard?
2. Explain how an ohmmeter is used in the
circuit.
3. How is the total resistance or equivalent
resistance calculated in a series and parallel
circuits?
4. How is the % difference calculated ?
5. Enumerate the things you have learned from
this experiment .
35 Electric Circuits
Experiment : Voltmeter and
Ammeter in Series Circuit
I. Purpose :
To measure the voltage across the battery
using a voltmeter.
To measure Voltage drops across each
resistor using a voltmeter.
To verify if the voltage drops equals the
voltage of the battery .
To measure current in the series circuit
using an ammeter.
35 Electric Circuits
II. Materials
2 resistors – 1,500 ohms each
Battery = 6v
Multi meter
Alligator wires
breadboard
35 Electric Circuits
III. Procedures and Results :
1. Set the voltmeter to VDC 20 V range. Vbattery=___v
2. Measure the voltage of the battery by connecting the
red probe of the voltmeter to the + positive of the
battery and the black probe to the – negative of the
battery . Record.
3. Connect 2 resistors in series using the breadboard.
4. Connect the + terminal of the battery and one terminal
of the series connection using a red alligator clip
5. Connect the – terminal of the battery and the other
end of the series connection using a black alligator
clip.
35 Electric Circuits
6. Using the voltmeter , measure the voltage across one
resistor R1 by connecting the red voltmeter probe to the
terminal of the resistor that is positive and the black
probe of the voltmeter to the other terminal of R1.
Record as VR1 = _____volts
7. Repeat procedure 6 with R2
VR2 = ______ volts
8. Calculate the total voltage by
Vs = VR1 + VR2 = _____volts + _____volts
Vs = _____ volts
9. Is the total voltage equal to the voltage of the battery ?
Yes or No ? ________
35 Electric Circuits
IV. Procedures and Results for Ammeter:
1. Set your multi meter to DCA 20 m range .
2. Disconnect the wire that connects the + positive
of the battery and one end of the series circuit .
3. Insert the ammeter by connecting the positive
red probe of the ammeter to the +positive of
the battery .
Connect the black negative probe of the
ammeter to one terminal of the series that was
disconnected.
4. Record the current reading .I = _____ A
35 Electric Circuits
Analysis and Conclusion
1.
2.
3.
4.
5.
6.
How do you set up an ammeter?
How do you set up a voltmeter?
How do you measure a current in a series circuit using
an ammeter ?
How do you measure the voltage drops (voltage
across the resistors) and the voltage of the battery
using a voltmeter?
What is the relationship of the voltage drops and the
source voltage?
Based on your results, what is the current flowing from
the battery ? Current through the resistors ?
35 Electric Circuits
What's a Resistor?
A resistor is anything that electricity can not travel through easily.
When electricity is forced through a resistor, often the energy in the electricity is changed into
another form of energy, such as light or heat. The reason a light bulb glows is that electricity is
forced through tungsten, which is a resistor. The energy is released as light and heat.
An Ohm is a unit used to measure the resistance value of a resistor. It is the name
of scientist Georg Ohm who did research on materials with resistive characteristics.
A conductor is the opposite of a resistor. Electricity travels easily and efficiently
through a conductor.
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35 Electric Circuits
63
35 Electric Circuits
Breadboard
Solid lines are impregnated
with flat conducting
stripped copper wires.
Wire inserted in a front view
hole makes contact with
one of the copper wires in
the back.
A breadboard is a piece of a plastic board perforated with small
holes and conduction channels that allow electronic parts to be
mounted and connected into circuits.
64
35 Electric Circuits
Electric Circuit Diagram
Engineers draw circuit diagrams to help them design the actual
circuits. These diagrams depict the arrangement of components.
R
This is the resistor symbol.
This is the battery symbol.
This is the switch symbol.
This is the Ammeter symbol.
This is the Voltmeter symbol.
65
35 Electric Circuits
Definitions
• What is a SHORT?
SHORT is a term used by engineers to describe the effect of
inadvertent connection of two power supply outlets (plus and minus). A
short circuit means that there is a short path between battery terminals,
or almost no resistance between them. This causes unintended heating
and sometimes “arcs and sparks” that can damage equipment.
• What is an OPEN or CLOSED circuit?
OPEN is a term used by engineers to describe the circuit condition
where current is not flowing (e.g., a switch is in the open position or a
light bulb has a broken filament). A circuit must be CLOSED for current
to flow allowing the circuit to perform its intended function (e.g., a light
switch is in the closed position allowing the light to function).
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35 Electric Circuits
Ohm’s Law
• Ohm’s Law establishes the relationship between voltage V, resistance R
and electric current I in an electric circuit.
• When a voltage, V is applied in the same circuit an electric current, I
flows and it is limited only by the circuit resistance, R.
or
V =RxI
(Eq. No. 2-1)
or Voltage = Resistance x Current
R is the resistance, measured in Ohms. Its symbol is
V is the voltage measured in Volts.
I is the current measured in Amperes.
1 Ohm = 1 Volt/Ampere
Ohm’s Law states that in a circuit with a given resistance the electric
current varies proportionally to the applied voltage.
67
35 Electric Circuits
More about Resistance, R
Electrical resistance is a property of a conductor that opposes the flow of
electric current through it. An object of uniform cross section has a resistance
proportional to its resistivity and length and is inversely proportional to its
cross-sectional area.
Length, l
Resistance, R = Resistivity, ρ x ----------------- (Eq. No. 2-2)
Area, A
Units: Resistivity, ρ [ m]
Length,
Area,
l [m]
A [m2]
Example: If a 1 m × 1 m × 1 m solid cube of material has sheet contacts on two
opposite faces, and the resistance between these contacts is 1 Ω, then the
resistivity of the material is 1 Ω⋅m.
68
35 Electric Circuits
Table 1 Material Resistivity, ρ (Ω·m) at
20 °C
Carbon (graphene) 1×10−8
Silver
1.59×10−8
Copper
1.68×10−8
Annealed copper
1.72×10−8
Gold
2.44×10−8
Aluminium
2.82×10−8
Calcium
3.36×10−8
Tungsten
5.60×10−8
Zinc
5.90×10−8
Nickel
6.99×10−8
Lithium
9.28×10−8
Iron
1.0×10−7
Platinum
1.06×10−7
Tin
1.09×10−7
Carbon steel (1010) 1.43x10−7
Lead
2.2×10−7
Titanium
4.20×10−7
Sea water
2×10−1
Drinking water
2×101 to 2×103
Glass
10×1010 to 10×1014
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35 Electric Circuits
• SERIES CIRCUITS
– The current that flows in a series circuit flows through every
component in the circuit.
– All the components in a series connection carry the same
current.
Rtot = R1 + R2 + R3 +………. Rn
(Eq. No. 2-3)
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35 Electric Circuits
Resistors in Series
Derive equation for calculating the total circuit resistance and
voltage drop across each resistor
I
R1
V
R2
R3
V1
V2
V3
We must use Ohm’s Law to calculate the
total resistance Rtot.
Vtot = V1 + V2 + V3
= R1 x I + R2 x I + R3 x I
= (R1+ R2+ R3) x I ;
Vtot = Rtot x I and we can deduce that Rtot
Rtot = R1+ R2+ R3 (See Eq. No. 2-3 )
Example:
R1 = 1500 Ohm; R2 = 1500 Ohm; R3 = 1500 Ohm; V = 6 Volt;
Calculate the total circuit resistance R tot, I and voltages V1, V2, V3
Rtot = 1500 + 1500 +1500 = 4500 Ohm
I = V/Rtot = 6/450 = 1.33 mA
V1 = 1500 x 1.33 = 1995 mV; V2 = 1500 x 1.33 = 1995 mV; V3 = 1500 x 1.33 = 1995 mV
71
35 Electric Circuits
• PARALLEL CIRCUITS
– The current in each individual resistor is found using Ohm’s Law.
Factoring out the voltage gives:
– To find the total resistance of all components, add the reciprocals
of the resistances (the conductances) and take the reciprocal of
the sum.
(Eq. No. 2-4}
I1
R1
Iin
I2
Iout
R2
Rn
In
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35 Electric Circuits
Resistors in Parallel
Derive equation for calculating the total circuit resistance and the
current in each resistor
Itot
V
R1
R2
I1
R3
I2
I3
We must use Ohm’s Law to
calculate the total resistance Rtot.
1. Itot = V / Rt ;
I1 = V /R1 ; I2 = V /R2 ; I3 = V /R3
Itot = V x (1/R1 + 1/R2 + 1/R3) therefore
1/Rtot = 1/R1 + 1/R2 + 1/R3
We’ll first calculate the resistance of R1 and R2 in parallel i.e.. R1,2 where
Itot
V
R1
R2
I1
I2
2. Itot = V / R1,2 and
1/R1,2 = 1/R1 + 1/R2
= R1 /(R2 x R1) + R2 /(R2 x R1)
= (R1 + R2)/(R2 x R1) and the reverse of 1/R1,2
R1,2 = (R1 x R2) /(R1 + R2) (Eq. No. 2-5)
We can use the same equation to calculate the total circuit resistance
3. R1,2,3= (R1,2 x R3) /(R1,2 + R3)
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35 Electric Circuits
Calculation of Series and Parallel Resistances
R1 and R2 in Series
Example no. 1:
R1,2 = R1+ R2
R1,2 = 3,000 Ohm
= 3 Kohm
Example no. 2:
R1,2 =1,970 Ohm
=1.97 Kohm
R1 and R2 in Parallel
R1 = 1500 Ohm = 1.5 KOhm
R2 = 1500 Ohm = 1.5 Kohm
R1,2 = (R1 x R2) /(R1 + R2)
R1,2 =
_1.5 x 1.5
3
= 0.75 Kohm
= 750 Ohm
R1 = 1500 Ohm = 1.5 KOhm
R2 = 470 Ohm = 0.47 KOhm
R1,2 = 1.5 x 0.47
1.97
= 0.36 Kohm
= 360 Ohm
Resistors in SERIES increase total circuit resistance.
Resistors in PARALLEL reduce total circuit resistance.
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35 Electric Circuits
Test No. 1 Resistors in Series
R1
a1 to a5
R2
R3
Four resistors are connected in
SERIES with each other
Connect the four 1500 Ohm resistors
R1, R2, R3 and R4 as shown.
b5 to b10
a10 to a15
R4
Measure the resistance using the 20K
resistance scale on the meter.
b15 to b20
Expected Actual
R1 =
_______;_______
R1 + R2 =
_______;_______
R1 + R2 + R3 =
_______;_______
R1 + R2 + R3 + R4 = _______;_______
Note: Enter your Test No. 1 four resistors data on the Data Sheet Pg.18
75
35 Electric Circuits
R1
a1 to a5
R3
Test No. 2 Resistors in Parallel
Two Resistors are connected in PARALLEL
with each other.
R2
R4
b1 to b5
Each of the two pairs is in SERIES with the
other pairs.
Connect the four 1500 Ohm resistors R1,
R2, R3 and R4 as shown. Measure the
resistance using the 20K resistance scale.
Expected
Actual
(R1 & R2) =
______
(R1 & R2) + (R3 & R4)=______
______
______
Note: Enter your Test 2 data on the Data Sheet (pg. 18)
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35 Electric Circuits
Calculate the Current
a1
R1
B
I
V
a5
Power supply voltage: 5.8 to 6.4 V (Why the variation?)
Resistor R1: 1425 to 1575 Ohm (Why the variation?)
Current, I =
V
R1
=
?
See Eq. No. 2-1)
I = V = _6_ = 0.004 A = 4.0 mA (nominal)
R1
1500
Note: Enter your calculated current value under Test 3 on the
Data Sheet (pgs. 16 and 18)
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35 Electric Circuits
Test No. 3 Measure the Current
a1
B
R1
R
a5
•Set the Am-meter dial at 20m on the DCA scale
•Connect the Am-meter BLACK probe alligator with R1 at (a1)
•Touch the Am-meter RED probe with the Power Supply (Battery +)
•Measure the current:
I=
•Compare with the calculated value of the current: I =
mA
mA
Note: Enter you Test 3 data on the Data Sheet (pg. 18)
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35 Electric Circuits
Test No. 4 Measure the Current
a1
B
R1
R2
R1,2
a5
•Set the Am-meter dial at 20m on the DCA scale
•Connect the Am-meter BLACK probe alligator with R1 at (a1)
•Touch the Am-meter RED probe with the Power Supply (Battery +)
•Measure the current through two resistors:
I=
mA
Note: Enter your Test No. 4 data on the Data Sheet (pg. 18)
79
35 Electric Circuits
Student Name: ______________________
Experiment No. 2 Resistors and Ohms Law
Data Sheet with Estimated and Actual Test Results
Expected
Actual
Test No. 1 Four resistors in SERIES
____ Ohm _______Ohm
Test No. 2 Two resistors in PARALLEL with each other
____ Ohm _______Ohm
Test No. 3 Measure the current in the circuit with one resistor ____ mA _______mA
Test No. 4 Measure the current with two resistors in parallel ____ mA _______mA
Explain why is there a difference in voltage between No. 1 and 2 tests? ___________
____________________________________________________________________
____________________________________________________________________
Explain why is there a difference between the expected and actual current values in
Test No. 3? __________________________________________________________
____________________________________________________________________
Explain why is there a difference between No. 3 and No. 4
tests?____________________________________________________________________
____________________________________________________________________
5/23/2017
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35 Electric Circuits
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Resistor
A material that resists the flow of electrical current.
Resistivity
A property of a conductor that opposes electric current flow.
Conductor
A material through which electricity flows easily.
Insulator
A material that has exceptionally high resistance, such that
it essentially stops the flow of electricity.
Ohm’s Law
The relationship between voltage, current, and resistance.
In equation form, it is simply I =V/R or V=IR.
Circuit Diagram A drawing depicting an arrangement of components.
Open Circuit
No current flows. The electrical circuit is incomplete.
Closed Circuit Current allowed to flow. The circuit is complete.
Short Circuit
An unintended closed circuit.
Breadboard
A device that allows mounting and connection of electrical
components in circuits.
Ohm-meter
A way to measure resistance (R).
Am-meter
A way to measure current (I).
Volt-meter
A way to measure voltage (V).
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35 Electric Circuits
Summary
You learned:
-How to calculate resistances in series and parallel circuits
-Ohm’s law (I=V/R or its equivalent form V=IR):
~If we know two among three variables (voltage, current and
resistance), Ohm’s Law allows us to calculate the unknown.
-Electric units of measurement:
Ohm (resistance), Ampere (current), Volt (voltage)
NEW CONCEPT: Power is the rate of doing work. The power dissipated in a
resistor is the product of voltage and current (VI) or V2/R (KW)
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35 Electric Circuits
Homework, Experiment No. 2 Resistors and Ohm’s Law, Sheet 1
Name
Mark True or False.
1. In class we use a breadboard to mount and connect electronic parts. (T) (F)
2. An open circuit means electric current is not flowing. (T) (F)
3. A short circuit means the wire connectors are not long enough. (T) (F)
4. Ohm's law lets you calculate the current through a resistor from the measured voltage across the resistor and the known resistance. (T) (F)
5. Current flowing through a resistor varies with the voltage applied across the resistor. (T) (F)
6. The Volt and the Ohm are units in the Metric System, also known as Systeme Internationale or SI. (T) (F)
7. The current flowing through a resistor is always the same. (T) (F)
8. The current flowing through a resistor may vary if something else changes in the circuit. (T) (F)
9. Ohm's law defines the relationship between current, resistance and voltage. (T) (F)
10. Ohm is the name of a scientist. (T) (F) .
11. An Ohm is a measure of electrical resistance. (T) (F)
12. Ohm's Law is the name of an important law in electrical science. (T) (F)
13. Two 100 Ohm resistors in series, replacing a single 100 Ohm resistor, increase circuit resistance. (T) (F)
14. Two 100 Ohm resistors in parallel, replacing a single 100 Ohm resistor, increase circuit resistance. (T) (F)
15. One kilogram is 1000 grams, so one kilo-Ohm is 10,000 Ohms. (T) (F)
16. One Mega-byte is 1,000,000 bytes, so one Meg-Ohm is 1,000,000 Ohms, or 106 Ohms. (T) (F)
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35 Electric Circuits
Homework, Experiment No. 2 Resistors and Ohm’s Law, Sheet 2
17. In the electric circuit shown below calculate first the total circuit resistance and the voltages across
each resistor R1, R2 and R3 based on these parameters:
V= 10 VDC; R1 =300 Ohms; R2 = 600 Ohms; R3 = 600 Ohms; What is the total electric power
delivered in Watts?
I=?
R1
V1=?
Cm cm cm2
V=10V
R2
R3
V2 =?
V3=?
Type
18. Calculate the resistance of a copper wire that is 10,000 m long and has an area of 1 cm2; Hint:
use Eq. No. 2-2 and Table 1.
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35 Electric Circuits
22. More Circuit Diagram
35 Electric Circuits
35.6 Combining Resistors in a Compound Circuit
think!
In the circuit shown below, what is the current in amperes
through the pair of 10-ohm resistors? Through each of the 8ohm resistors?
35 Electric Circuits
35.6 Combining Resistors in a Compound Circuit
think!
In the circuit shown below, what is the current in amperes
through the pair of 10-ohm resistors? Through each of the 8ohm resistors?
Answer:
The total resistance of the middle branch is 20 Ω. Since the
voltage is 60 V, the current = (voltage)/(resistance) =
(60V)/(2 Ω) = 3 A. The current through the pair of 8-Ω resistors
is 3 A, and the current through each is therefore 1.5 A.
35 Electric Circuits
35.6 Combining Resistors in a Compound Circuit
What is the equivalent resistance of resistors in
series? Of equal resistors in parallel?
35 Electric Circuits
35.7 Parallel Circuits and Overloading
To prevent overloading in circuits, fuses or
circuit breakers are connected in series along
the supply line.
35 Electric Circuits
35.7 Parallel Circuits and Overloading
Electric current is fed into a home by two wires called lines.
About 110 to 120 volts are impressed on these lines at the
power utility.
These lines are very low in resistance and are connected to
wall outlets in each room.
The voltage is applied to appliances and other devices that
are connected in parallel by plugs to these lines.
35 Electric Circuits
35.7 Parallel Circuits and Overloading
As more devices are connected to the lines, more
pathways are provided for current.
The additional pathways lower the combined resistance
of the circuit. Therefore, a greater amount of current
occurs in the lines.
Lines that carry more than a safe amount of current are
said to be overloaded, and may heat sufficiently to melt
the insulation and start a fire.
35 Electric Circuits
35.7 Parallel Circuits and Overloading
Consider a line connected to a toaster that draws 8 amps, a heater that
draws 10 amps, and a lamp that draws 2 amps.
• If the toaster is operating, the total line current is 8 amperes.
• When the heater is also operating, the total line current increases to
18 amperes.
• If you turn on the lamp, the line current increases to 20 amperes.
35 Electric Circuits
35.7 Parallel Circuits and Overloading
To prevent overloading in circuits, fuses or circuit breakers are
connected in series along the supply line.
The entire line current must pass through the fuse.
If the fuse is rated at 20 amperes, it will pass up to 20 amperes.
A current above 20 amperes will melt the fuse ribbon, which
“blows out” and breaks the circuit.
35 Electric Circuits
35.7 Parallel Circuits and Overloading
Before a blown fuse is replaced, the cause of
overloading should be determined and remedied.
Insulation that separates the wires in a circuit can wear
away and allow the wires to touch.
This effectively shortens the path of the circuit, and is
called a short circuit.
A short circuit draws a dangerously large current
because it bypasses the normal circuit resistance.
35 Electric Circuits
35.7 Parallel Circuits and Overloading
Circuits may also be protected by circuit breakers, which use
magnets or bimetallic strips to open the switch.
Utility companies use circuit breakers to protect their lines all
the way back to the generators.
Circuit breakers are used in modern buildings because they
do not have to be replaced each time the circuit is opened.
35 Electric Circuits
35.7 Parallel Circuits and Overloading
How can you prevent overloading in circuits?
35 Electric Circuits
Assessment Questions
1.
In a light bulb, the amount of current in the filament is
a. slightly less than the current in the connecting wires.
b. the same as the current in the connecting wires.
c. slightly greater than the current in the connecting wires.
d. twice as great as the current that is in the connecting wires.
35 Electric Circuits
Assessment Questions
1.
In a light bulb, the amount of current in the filament is
a. slightly less than the current in the connecting wires.
b. the same as the current in the connecting wires.
c. slightly greater than the current in the connecting wires.
d. twice as great as the current that is in the connecting wires.
Answer: B
35 Electric Circuits
Assessment Questions
2.
The flow of charge in an electric circuit is
a. much like the flow of water in a system of pipes.
b. very different from water flow in pipes.
c. like an electric valve.
d. like an electric pump.
35 Electric Circuits
Assessment Questions
2.
The flow of charge in an electric circuit is
a. much like the flow of water in a system of pipes.
b. very different from water flow in pipes.
c. like an electric valve.
d. like an electric pump.
Answer: A
35 Electric Circuits
Assessment Questions
3.
In a series circuit, if the current in one lamp is 2 amperes, the current
in the battery is
a. half, 1 A.
b. 2 A.
c. not necessarily 2 A, depending on internal battery resistance.
d. more than 2 A.
35 Electric Circuits
Assessment Questions
3.
In a series circuit, if the current in one lamp is 2 amperes, the current
in the battery is
a. half, 1 A.
b. 2 A.
c. not necessarily 2 A, depending on internal battery resistance.
d. more than 2 A.
Answer: B
35 Electric Circuits
Assessment Questions
4.
In a circuit with two lamps in parallel, if the current in one lamp is
2 amperes, the current in the battery is
a. half, 1 A.
b. 2 A.
c. more than 2 A.
d. cannot be calculated from the information given
35 Electric Circuits
Assessment Questions
4.
In a circuit with two lamps in parallel, if the current in one lamp is
2 amperes, the current in the battery is
a. half, 1 A.
b. 2 A.
c. more than 2 A.
d. cannot be calculated from the information given
Answer: C
35 Electric Circuits
Assessment Questions
5.
In a circuit diagram there may be
a. no switches.
b. at most, one switch.
c. two switches.
d. any number of switches.
35 Electric Circuits
Assessment Questions
5.
In a circuit diagram there may be
a. no switches.
b. at most, one switch.
c. two switches.
d. any number of switches.
Answer: D
35 Electric Circuits
Assessment Questions
6.
Consider a compound circuit consisting of a pair of 6-ohm resistors in
parallel, which are in series with two 6-ohm resistors in series. The
equivalent resistance of the circuit is
a. 9 ohms.
b. 12 ohms.
c. 15 ohms.
d. 24 ohms.
35 Electric Circuits
Assessment Questions
6.
Consider a compound circuit consisting of a pair of 6-ohm resistors in
parallel, which are in series with two 6-ohm resistors in series. The
equivalent resistance of the circuit is
a. 9 ohms.
b. 12 ohms.
c. 15 ohms.
d. 24 ohms.
Answer: C
35 Electric Circuits
Assessment Questions
7.
One way to prevent overloading in your home circuit is to
a. operate fewer devices at the same time.
b. change the wiring from parallel to series for troublesome
devices.
c. find a way to bypass the fuse.
d. find a way to bypass the circuit breaker.
35 Electric Circuits
Assessment Questions
7.
One way to prevent overloading in your home circuit is to
a. operate fewer devices at the same time.
b. change the wiring from parallel to series for troublesome
devices.
c. find a way to bypass the fuse.
d. find a way to bypass the circuit breaker.
Answer: A