University of Arkansas - AgriScience Project

AGRISCIENCE EXERCISE

PHYSICAL SCIENCES IN AGRICULTURE

Sub-Concept: Voltage, Amperage, and Resistance

Exercise: Measuring Voltage, Amperage and Resistance

Applied Principle(s): Measuring electrical quantities

Goals:

1. Safely and accurately measure voltage, amperage, and resistance using analog and digital meters.

Materials:

• Digital multi-meter
• Wire bundles*
• Fuses**
• Battery sets***
• Insulated conductors with clips

References: Attached information sheet and any basic electricity or physical science textbook

• It is suggested that the instructor set up each part of this activity for him/her self prior to conducting the activity in class. This will aid in answering students' questions during the activity.



• *Wire Bundles: Prepare one per group by twisting together about six identical wires into a bundle, so that the students will be unable to tell which ends belong to the same wire without testing them. Label the wires on one end as "A" - "F", and the other end as "1" - "6".



• **Fuses: Each group of students should receive a set of four fuses, some good and some blown; they will have to determine which are which, so don't label them.

• ***Battery Sets: Each group should receive a set of four batteries of different sizes.

Procedures for Conducting the Activity:

Explain that today's activity will involve methods of measuring electricity, specifically voltage, amperage, and resistance.

2. Divide the class into small groups of no more than three students each.

3. Provide each student with an information/data sheet, and allow the class time to read the entire activity and information sheet.

4. Provide each group with the necessary materials for conducting the exercise.

5. Carefully supervise the groups as they work through the activity, providing assistance as needed.

6. When all groups have completed their data sheets, review their answers as a class, and discuss the practical applications of this information.

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AGRISCIENCE EXERCISE

- Measuring Voltage, Amperage and Resistance -

Student Data/Instruction Sheet

1. Review the attached information sheet, and read the remainder of the exercise before proceeding further.

2. Obtain materials from your instructor:

• Digital multi-meter
• Wire bundles*
• Fuses**
• Battery sets***
• Insulated conductors with clips

3. Determine the voltage (DC) of the four batteries and record below:

Battery A _______ Battery B _______

Battery C _______ Battery D _______

4. Connect Battery A and Battery B together in series; record the total voltage:

____________________

5. Connect the same batteries together in parallel; record total voltage ___________.

6. Based on your meter readings from steps 4 and 5, describe the effects of connecting batteries in (a) series and (b) parallel:

(a)______________________________________________________________

________________________________________________________________

________________________________________________________________

(b)______________________________________________________________

________________________________________________________________

________________________________________________________________

7. Using the proper meter setting, determine which of the fuses are good and which are "blown", and record below:

Fuse A _______ Fuse B _______

Fuse C _______ Fuse D _______

8. Using the proper meter setting, identify both ends of each wire (e.g. B-5):

9. Measure and record amperage in the circuit as directed by your instructor:

_____________________

MEASURING VOLTAGE, AMPERAGE AND RESISTANCE

INFORMATION SHEET

Voltage is the measure of electrical "pressure" which forces electricity (electrons) to flow in a circuit. One volt is defined as the amount of electrical pressure required to force one ampere of current to flow in a circuit having one ohm of resistance. The higher the pressure (voltage), the greater the flow of electrons (all other factors being equal). Common voltages in the US are 120V, 208V, and 240V. Industrial applications frequently use 480V electricity; however, this is not common in most agricultural uses.

A voltmeter is used to determine circuit voltage. A direct current (DC) voltmeter is used to measure voltage in DC circuits, while an alternating current (AC) voltmeter is used to measure voltage in AC circuits. Meters which are sued to measure both AC and DC current are commonly available.

A voltmeter must always be connected between two wires, between a wire and a ground, or across the terminals of an electrical device supplying or using electricity. Typical voltmeter connections are illustrated below:

Measuring Amperage:

The ampere is the unit of measure of the rate of flow of electrons. When 6.28 x 10¹⁸ electrons flow past a given point in a circuit each second, one (1) ampere of electricity is flowing in the circuit. A DC ammeter is used to measure amperage in a DC circuit; an AC ammeter is used to measure amperage in an AC circuit.

An ammeter must be placed in series in the circuit to measure amperage. This means that the circuit must be opened and the ammeter must be wired into one current carrying conductor of the circuit. It does not matter if the ammeter is installed in the "hot" wire or the grounded wire of the circuit, since the same number of amperes flow in both wires. The proper connection of an ammeter is illustrated below:

A "clamp-on" ammeter can also be used to measure the amount of current flowing through a wire. This type of meter is clamped around one of the current carrying conductors. The meter measures the strength of the magnetic field produced by the current flowing through the wire and converts this measurement to an amperage reading. The clamp-on ammeter (illustrated below) is especially convenient, since its use does not require that the ammeter be wired into the circuit.

Measuring Resistance:

When electrical current flows through a conductor, it encounters resistance. This resistance to the flow of electricity is measured in ohms. The Greek capital letter omega () is the symbol for ohms. For example, the number symbol combination "5" means 5 ohms of resistance.

Resistance to the flow of electricity in a conductor is measured with an ohmmeter. An ohmmeter contains a battery that forces a small amount of current through a conductor; the ohmmeter then measures the amount of resistance of the conductor.

The conductor must be disconnected from the electrical power source prior to measuring resistance. Failure to disconnect the conductor from the power source will damage the ohmmeter.

An ohmmeter can be connected to a circuit in any place. It can be connected either in series or parallel. The ohmmeter must be "zeroed" out in order to make an accurate reading. To zero out the meter, touch the two leads together and turn the adjustment knob until the meter reads zero on the ohm scale.

Using a Volt-Ohm-Amp (VOA) Meter:

Single meters are now available which can measure voltage, resistance, and amperage. These meters are commonly called VOA meters or multi-meters. They are used in the same manner as described above, however the selector knob must be set to select the type of measurement desired (voltage, resistance, or amperage).

As with any piece of equipment, always read the manufacturer's instructions prior to using any type of electrical meter. Failure to do so may result in personal injury or damage to the meter.

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