a. Connect one end of each
conductor to the low-value
resistor. Set the DMM to
measure DC resistance. Measure
the total circuit resistance as
shown below. Record your
resistance reading in the
appropriate cell of the Data Table.
Sub-Concept: Voltage, Amperage, and Resistance
Agricultural Application: Basic electronics principles are applied in common agricultural
equipment.
Exercise: Understanding Ohm's Law
Applied Principle(s): Ohm's Law
Goals:
1. Describe the mathematical relationship between voltage, amperage, and resistance in an electrical circuit.
2. Use this relationship to determine voltage, amperage, or resistance in a circuit when two of the
three values are known.
Materials:
Digital multimeter(DMM)
3 insulated conductors with clips
<li>2 6-volt batteries <li>Pencil
<li>Calculator <li>Safety glasses
<li>1resistor of low value<li>1 resistor of high value
References: Attached information sheet and/or any basic electricity and/or high school physics textbooks
1. Divide the class into small groups of no more than 3 students each.
2. Provide each student with an instruction/data sheet, and allow the class time to read the entire
activity and attached information sheet before proceeding.
3. Provide each group with the necessary materials.
4. Allow the student to proceed through the activity, providing assistance as necessary.
5. When all groups have finished, discuss the activity questions and results as a class. Discuss the practical applications of the knowledge gained through the activity.
2. Secure the following equipment and supplies:
<li>Digital multimeter(DMM) <li>3 insulated conductors with clips
<li>2 6-volt batteries <li>Pencil
<li>Calculator <li>Safety glasses
<li>1resistor of low value, and 1 of high value (select resistor values based on maximum DC
amperage rating of the DMM)
3. Complete Test 1 (Single Battery/Low Value Resistor) as follows:
a. Connect one end of each
conductor to the low-value
resistor. Set the DMM to
measure DC resistance. Measure
the total circuit resistance as
shown below. Record your
resistance reading in the
appropriate cell of the Data Table.
b. Connect the free end of each
insulated conductor to opposite
terminals of the 6-volt battery to
complete the circuit. Set the
DMM to measure DC voltage.
Measure circuit voltage as shown
below. Record your reading on
the Data Chart.
c. Set the DMM to measure DC
amperage. Insert the meter into
the circuit as shown below.
Record your amperage reading on
the Data Table.
4. Complete Test 2 (Single Battery/High Value Resistor) as follows:
a. Connect the insulated conductors to each end of the high-value resistor. Set the DMM to measure DC resistance. Measure the total cirucuit resistance as shown below. Record your resistance reading on the Data Table.
b. Connect the free end of each insulated conductor to opposite terminals of the 6-volt battery to complete the circuit. Set the DMM to measure DC voltage. Measure circuit voltage as shown below. Record your reading on the Data Chart.
c. Set the DMM to measure DC
amperage. Insert the meter into
the circuit as shown below.
Record your amperage reading on
the Data Table.
5. Complete Test 3 (Two Batteries in Series/Low Value Resistor) by following these instructions.
a. Using an insulated
conductor, connect the
negative terminal of
one 6-volt battery to
the positive terminal of
the other 6-volt battery
as shown.
b. Connect the insulated conductors to each end of the low-value resistor. Set the DMM to
measure DC resistance. Measure the total circuit resistance as shown below. Record your
resistance reading in the appropriate cell of the Data Chart.
c. Connect the free end of each
insulated conductor to opposite
battery terminals as shown.
d. Set the DMM to measure DC
voltage. Measure the circuit
voltage as shown. Record your
readings on the Data Chart.
e. Set the DMM to measure DC amperage. Insert the meter into the circuit as shown below. Record your amperage reading in the appropriate cell of the Data Chart.
6. Complete Test 4 (Two Batteries in Series/High-Value Resistor) by following these
instructions. Note: Leave batteries connected in series as in Test #3.
a. Connect the insulated conductors to each end of the high-value resistor. Set the DMM to measure DC resistance. Measure the total cirucuit resistance as shown below. Record your resistance reading on the Data Table.
b. Set the DMM to measure DC voltage. Measure circuit voltage as shown below. Record your reading on the Data Chart.
c. Set the DMM to measure DC
amperage. Insert the meter into
the circuit as shown below.
Record your amperage reading on
the Data Table.
DATA TABLE
| Test No. | Description | Resistance
( W ) |
Voltage | Amperage* |
| 1 | Single Battery/
Low-Value Resistor |
|||
| 2 | Single Battery/
Higy-Value Resistor |
|||
| 3 | Two Batteries in Series/Low-Value Resistor | |||
| 4 | Two Batteries in Series/High-Value Resistor |
* Convert from mA to Amperes if necessary before entering data into this column.
7. Use your observed measurements to determine and specify the relationship between voltage,
amperage, and resistance in an electrical circuit. Develop a mathematical formula to express this
relationship.
8. Complete the following discussion questions. Be prepared to participate in a class discussion
of your answers.
a. In a circuit with a constant voltage, what factor determines the amount of current (amperage)
that will flow in the circuit?
b. If a circuit has constant resistance, what will happen to the current flow if the voltage doubles?
c. If a 24 VDC electrical circuit has a current flow of 2.5 amperes, what is the circuit's total
resistance?
d. Explain Ohm's Law in your own words.
Understanding Ohm's Law
INFORMATION SHEET
In order to develop an understanding of Ohm's Law, it is useful to review the definition of volts,
amperes, and ohms and the relationship between the three. First, the definitions:
<li> Volt - A unit of measure of electrical pressure. A given electrical pressure (voltage) causes
a given amount of electrical current (amperes) to flow through a load of a given resistance.
<li> Ampere - A unit of measure of the rate of flow of electricity through a conductor. One
ampere is equal to 6.28 x 1018 electrons flowing past a given point in one second.
<li> Ohm - A unit of measure of the resistance to the flow of electrons in a circuit.
The relationship between volts, amperes, and ohms can be stated as follows:
One volt is the amount of electrical pressure required to cause one ampere of current to flow
through a circuit having one ohm of total resistance.
This relationship can be expressed mathematically as Ohm's Law:
E = I x R where, E = voltage
I = current in amperes
R = resistance in ohms
When any of the two values are known, the unknown quantity can be determined by substituting
the known values into the formula and completing the mathematical operation implied.
For example, if you wished to determine the current flow (amperage) in a 120 volt circuit having
a total resistance of 15 ohms, the solution may be found as follows:
I = E I = 120v I = 8
R 15
Summary:
Ohm's Law is based on the relationship between voltage, current, and resistance in an electrical circuit. An understanding of Ohm's Law allows an individual to solve for an unknown third value if the remaining two values are known (or can be measured). An understanding of Ohm's Law is important in working with electricity.
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