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 e. Volt i. Kirchoff’s first law

b. Ohmmeter f. Ohm’s Law j. Kirchoff’s second law

c. Load g. Parallel

d. Ohm h. Series

_______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.

Illinois Physical Science Applications in Agriculture Lesson B4–3 • Page 7

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?

Illinois Physical Science Applications in Agriculture Lesson B4–3 • Page 8

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.

Illinois Physical Science Applications in Agriculture Lesson B4–3 • Page 29

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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