Download Lesson B4–3

RESISTANCE IN SERIES AND
PARALLEL CIRCUITS
Student Learning Objectives. Instruction in this lesson should result in
students achieving the following objectives:
1 Explain series and parallel circuits, the advantages and disadvantages of each,
and how to connect series and parallel circuits.
2 Determine voltages for resistors in series and parallel circuits, and determine
total current flowing through series and parallel circuits.
3 Explain the mathematical relationship that exists in circuits containing more
than one resistor and use Ohm’s law in calculating current, resistance, or voltage
for different circuits.
Anticipated Problem: What are parallel and series circuits?
I. The series circuit is an electrical connection method in which current flows through all
devices in a circuit before returning to the source. A parallel circuit is set up when the
resistors or capacitors are connected so as to allow current to divide and flow through
each individual device, then to combine and flow back to the source.
A. The two types of circuits can be combined to form a series-parallel circuit. This
circuit has at least one resistor connected in series with at least two resistors connected in
parallel.
B. The circuit is considered complete in its loop or path when the current can flow
through the load before returning to the source or point of origin.
Anticipated Problem: How do voltage and amperage differ in their movement through
series and parallel circuits?
II. In a series circuit, voltage is divided between the devices connected in the circuit
proportionately to the resistors. Each device in a series circuit receives the same
amperage. There is only one path that the current flow may follow. By contrast, current
may follow more than one path in a parallel circuit. Current (amperage) can flow through
any single device or all devices simultaneously.
A. Devices connected in parallel receive the same voltage. However, the total current
flowing through a parallel circuit splits proportionately at each resistor and is equal to the
sum of the current flowing through each device.
B. Kirchoff’s First Law states that current entering a junction is equal to the current
leaving the junction. Considering the behavior of voltage in these circuits, Kirchoff’s
Second Law follows up by stating that the sum of the voltage drops around a complete
circuit loop or path is equal to the applied voltage to the circuit.
Anticipated Problem: What is Ohm’s Law and how is it used to determine voltage,
current, and/or resistance for electrical circuits?
III. Ohm’s Law states that the current in an electrical circuit is directly proportional to the
amount of voltage which causes the current to flow, and inversely proportional to the
amount of resistance in the circuit.
A. One volt (electrical pressure) is needed to push one ampere (electrical current) through
a conductor with one ohm (electrical resistance) of resistance. Resistance is the
opposition to the flow of electrons. Ohm’s law can be mathematically expressed as
follows: Current = Voltage / Resistance or I = E / R
Where: Current = I
Voltage = E
Resistance = R
B. Examining the mathematical relationship, it is easy to see that as the voltage is
increased, the amount of current flowing in the circuit is increased. If the amount of
voltage is decreased, the amount of current flowing in the circuit is decreased.
1. The formula also shows that as the resistance in the circuit is increased, the amount
of current in the circuit is decreased. If the amount of resistance in the circuit is
decreased, the amount of current in the circuit is increased.
2. As the length of a conductor increases there is more resistance to the flow of electricity
along the conductor which results in a drop in voltage.
3. The resistance of the circuit can easily be measured across any device with an
ohmmeter.
RESISTANCE IN SERIES AND
PARALLEL CIRCUITS
Part One: Matching
Instructions: Match the word with the correct definition.
a. Ampere
b. Ohmmeter
c. Load
d. Ohm
e. Volt
f. Ohm’s Law
g. Parallel
h. Series
i. Kirchoff’s first law
j. Kirchoff’s second law
_______1. Term for circuit in which resistors or capacitors are connected so as to allow current to
divide and flow through each individual device, then to combine and flow back to the source.
_______2. Current entering a junction is equal to the current leaving the junction.
_______3. Unit of measurement of electromotive force (pressure).
_______4. Basic electrical law stating the relationships of voltage, current, and resistance.
_______5. Device used to measure electrical resistance.
_______6. Basic unit of electric current.
_______7. Anything that may draw current from an electrical power source.
_______8. The sum of the voltage drops around a complete circuit loop is equal to the applied
voltage.
_______9. Electrical connection method in which current flows in sequence through all devices in
a circuit.
______10. Basic unit of electrical resistance.
Part Two: Fill-in-the-Blank
Instructions: Complete the following statements.
1. The unit of measurement for electrical current is the __________.
2. Anything that may drain current from an electrical power source is __________.
3. Electrical resistance is measured in __________.
4. __________ is the opposition to the movement of electrons.
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Part Three: Multiple Choice
Instructions: Write the letter of the correct answer.
_______1. The formula used to calculate electrical current is:
a. voltage/resistance
b. voltage*resistance
c. amperage/voltage
d. resistance/amperage
_______2. The basic electrical law stating the relationships of voltage, current, and resistance is
called _____ Law.
a. Raoult’s
b. Kirchoff’s
c. Pascal’s
d. Ohm’s
_______3. The total resistance in a series circuit is equal to the __________.
a. diameter of the electrical wire used.
b. sum of the individual resistance.
c. the sum of the voltage.
d. total voltage divided by amperage.
_______4. If a milk cooler compressor uses 12 amps and has a resistance of 20 ohms, what is
the voltage requirement of the compressor?
a. 1.5 volts
b. 1.6 volts
c. 0.6 volts
d. 240 volts
_______5. The unit of measurement for electrical pressure is the:
a. ohm
b. ampere
c. volt
d. current
_______6. An ammeter measures which of the following?
a. ohms
b. resistance
c. current
d. volts
Part Four: Short Answer
Instructions: Answer the following questions.
1. If a grain auger operates on 115 volts and uses 600 watts of electricity, what is the ampere
rating of the motor?
2. If a grain dryer operates at 240 volts with 5 amps of current flowing through the circuit, how
much resistance is present in the circuit?
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Assessment
Illinois Physical Science Applications in Agriculture Lesson B4–3 • Page 28
TS–A
Technical Supplement
RESISTANCE IN SERIES AND
PARALLEL CIRCUITS
1. What is electrical resistance?
The opposition of a circuit to the flow of electricity. It is expressed in ohms.
2. What is a series circuit?
A circuit in which element has its negative terminal connected to the positive terminal
of the next element.
3. How are resistors connected in a series circuit?
The negative terminal is connected to the positive terminal of the next element. The
current must flow through the first resistor to flow to the second resistor and so on.
4. What is a parallel circuit?
A circuit in which all components have their positive terminals connected to a common
positive line and their negative terminals connected to a common negative line.
5. How are wires connected in a parallel circuit?
Positive terminals connected to a common positive line and their negative terminals
connected to a common negative line.
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6. How is resistance determined in series and parallel circuits?
Ohmmeter.
7. What is the formula for Ohm’s Law?
A technique useful in remembering Ohm’s Law is to divide a circle into three parts.
Label one part E, electromotive force, volts in the upper half; the bottom half is
divided into half, labeled I, amps, and R, ohms. If one uses their finger and touches
the letter they wish to find, the remaining visible letters will determine the math
calculation used to find the unknown. E=I * R, I=E/R, and R = E / I are the formulas
derived from the Ohm’s Law diagram.
8. What effect is produced when the voltage in a circuit is varied?
A change in circuit voltage, resistance remaining constant, will result in a change in
the current. As voltage increases the current, amps will increase, and when voltage
decreases the current, amps will decrease. The current will parallel the voltage
change.
9. What effect is produced when the resistance in a circuit is varied?
When resistance varies and voltage remains constant, the current, amps, will change.
Variable resistors are commonly used in volume controls for radio and television. A
conductor that has a resistance of one ohm will allow a current of one amp to flow
when a voltage of one volt is applied. Conductors of higher resistance, or ohms, will
limit current setup by one volt to values less than one ampere, and a conductor with
less resistance will allow more current to flow. The current path through a resistor is
more difficult than through an ordinary conductor, consequently the amount of
current flowing in the circuit is limited. By selecting the value of resistors, the value
of the current flowing in the circuit can be precisely controlled. Variable resistors are
so designed that their resistance may be changed. As resistance changes, the current
change is opposite of the resistance change. As ohms decrease, the current in the circuit
increases. When the ohms increase, the current in the circuit will decrease.
10. Why do mathematical results, when calculating current using Ohm’s Law,
vary slightly from measured current on a meter?
Resistors are designed to be within a tolerance range varying from plus or minus 5%
to plus or minus 20%. The resistors with a gold color on the fourth band indicates a
plus or minus 5% tolerance, silver bands indicate a plus or minus 10% tolerance, and
no fourth band on the resistor indicates a plus or minus 20% tolerance range. The
tolerance range for resistors will account for the majority of difference between
meter readings and calculated current levels. Another source of variation in measured
and calculated current is error in reading the meter.
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