University of Arkansas - AgriScience Project

AGRISCIENCE EXERCISE

PHYSICAL SCIENCES IN AGRICULTURE

Key Concept: Electricity and Electronic Systems

Sub-Concept: Electronics Applications

Agricultural Context: Water is a vital resource for agriculture. Moisture detection is important in both growing and storing agricultural crops.

Exercise: Construct a Moisture Detector

Applied Principle: Ohm's Law

Goals:

  1. Build a moisture detector.
  2. Calibrate a moisture detector.
  3. Explain the relationship between resistance to current flow and conductivity.
  4. Calculate resistance of samples tested for moisture content.


Materials: (per group)



References: Osborne, E., Moss, J., Buriak, P., and Wallace, P. (undated). Physical Science Applications in Agriculture. Urbana, IL: University of Illinois, Urbana-Champaign.

Procedures for Conducting the Activity:

PART 1

1. Divide the class into small groups of no more than 3 students each.

2. Provide each group with an instruction/data sheet and necessary materials.

3. Demonstrate for the class how to construct a circuit for detecting moisture, using the schematic below:





















4. Cross the probes; allow students to observe the movement of the meter's needle.

5. Instruct the students to carry out the activity described on their instruction sheets.

6. Carefully supervise the groups, answering any questions they may have concerning the activity.



PART 2

1. Lead the class in a discussion of the following questions. Have each group record the answers on their data sheets.

a. How can the meter be calibrated to read the amount of moisture present in the soil sample?

b. Why is the 10K resistor necessary for the circuit to operate?

2. Ask the class to consider how and why a moisture detector works. Lead the students to make hypotheses. Once the hypotheses are articulated by the class, write them on the board. Have each group also record them on their data sheets.

3. Instruct students to discuss the practical applications of what they have discovered, writing their answers on their data sheets.

AGRISCIENCE EXERCISE

- Construct a Moisture Detector -

Student Instruction/Data Sheet

1. Construct a circuit for detecting moisture as demonstrated by the instructor, using the schematic below:























2. Cross the probes; observe the movement of the meter's needle, and record the reading in the chart on the next page.

3. Dip the probes into the following materials, recording the meter readings of each. Be sure to dry and clean off the probes between materials.

a. distilled water

b. tap water

c. sand

d. dry soil

Chart 1: Probes
Crossed Distilled H2O Tap H2O Dry Sand Dry Soil
Meter

Reading






4. Add water to the container of dry soil, using an eye dropper. Count the number of drops added. Record the meter reading. Add more drops, counting them as you go, and continue to record the meter readings.

Chart 2: Probes In Dry Soil Plus...
# drops H2O
Meter Reading




5. Calibrate the meter to read the amount of water you have added to the soil.

6. Disconnect the power supply, disassemble the circuit, and return the equipment to your instructor.

7. Complete the discussion questions; be ready to discuss your findings with the rest of the class.

a. How can the meter be calibrated to read the amount of moisture present in the soil sample?

____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ _____________________________________________________________________



b. Why is the 10K resistor necessary for the circuit to operate?

____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ ____







c. How and why does a moisture detector work? Write the hypotheses articulated by your class:

____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ ____________________________________________________________________



d. What are some practical applications of this device and others like it?

____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ ____________________________________________________________________



TEACHER BACKGROUND SHEET

- Construct a Moisture Detector -

Water is a vital resource for agriculturalists. It is necessary to monitor moisture levels in the soil from the time seeds are planted until the crops are harvested. In the past, producers have relied on observing plants and feeling the soil to indicate moisture levels. However, today they are able to use moisture detectors for precise readings of soil moisture level, as well as to control irrigation equipment.

It is also important to detect moisture in the storage of agricultural products, such as cereal and feed grains. Grains are usually harvested when moisture levels are higher than acceptable for storing them. The grain is dried to acceptable levels after harvest. If it is stored in conditions that are too wet, it will spoil. Moisture detectors can quickly and efficiently measure the amount of moisture present in the grain to determine the amount of drying necessary. Storage facilities also employ moisture detectors in monitoring changes which might occur during storage; this way, the problem can be corrected before causing damage to the crop.



In this activity, a glass of distilled water is used to obtain a reference reading. The students should find that pure water itself is not a conductor of electricity; pure water offers infinite resistance to current. The impurities found in water serve as conductors. As can be seen in the periodic table of elements (Figure 1), conductors of electricity are those elements with 3 or fewer electrons in the valence (outer) ring. Semi-conductors contain 4 electrons, while insulators have 5 or more electrons in the valence ring.

The 10K resistor and the 6 volt battery produce .6mA of current when the probes are crossed to complete the circuit. If a sample inserted between the probes produces a resistance of 10 kiloohms, the meter would read .3mA. This meter will detect moisture levels between 0 and approximately 30 kiloohms.

Some of the relevant principles of electricity illustrated in this activity include the following:

1. Current is the movement of electrons through a conductor. Resistance refers to the holding back of these electrons.

2. Solids, liquids, and gases can all be conductors of electricity. Water can be one conductor which allows a rapid flow of free electrons.

3. Dry sand and soil are poor conductors, since they resist the flow of electrons through their particles. The higher the moisture level of a soil sample, the lower its resistance to electrical current.

4. The electrical conductivity of any substance is increased when water is added to it; this is the basis for how a moisture detector works.



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