Focus question

How do different types of liquid affect the electrical resistance of earth materials?

Essential materials

• at least three paper or plastic cups
• measuring cup
• sand (recommended), soil, or fine gravel
• 200 ml (about one cup) of water
• 30 g (about 5 teaspoons) of salt
• 50 ml (about 1/4 cup) of vegetable oil (recommended) or lubricating oil
• ohmmeter or multimeter (electronic meter with ohmmeter setting)
• six pieces of stiff bare copper wire, each about 10 cm (4 in) long
• optional but recommended:
• 2 short wires with alligator clips on each end
• a spoon for stirring water
• a stick or screwdriver for stirring sand
• paper towels

Main ideas and background information

• Electrical resistivity is the extent to which the flow of electricity is hindered when passing through a particular kind of material. Oil exploration teams look at the resistivity of underground material (usually layers of rock) to determine likely locations of petroleum deposits.
• Electrical resistance is the extent to which the flow of electricity is hindered when passing through a particular object. This resistance depends on the electrical resistivity of the material in the object and the length and cross sectional area of the object.
• The ohm is a unit used for measuring electrical resistance.
• The electrical resistance of an object can be measured easily with an ohmmeter. Determining the electrical resistivity of a particular kind of material is much more difficult.
• In this activity, we will simulate the measurement of resistivity by looking at the electrical resistance between probes placed into earth materials that have been saturated with different liquids.
• Dry earth material generally has a high electrical resistivity. In our simulation, this is shown by a high electrical resistance reading on the ohmmeter.
• Earth material saturated with water has a relatively low electrical resistivity. Material saturated with salt water has an even lower electrical resistivity. This is due to the charged particles called ions that are present in the salt water. In our simulation, this lower electrical resistance is shown by a lower reading on the ohmmeter.
• Earth material saturated with oil has a high electrical resistivity. Like the dry material, this resistivity is simulated by a high electrical resistance reading on the ohmmeter.

Procedural tips

• Fill each cup to about the same level with sand. (Sand is recommended for the most consistent results.)
• Pour water into the measuring cup up to the 200 ml or one cup line. Then add 30 g (about five teaspoons) of salt and stir until all salt is dissolved. If some salt remains, add a bit more water and stir again.
• Pour a little water into the sand in Cup A. Stir the sand with a stick or screwdriver. Keep adding water a little at a time and stirring the sand until all of the sand is wet. If a layer of water appears on top of the sand, soak it up with a paper towel.
• For Cup B, we strongly recommend using vegetable oil instead of lubricating oil. Pour only a little oil at a time. Do not be in a hurry. Allow sufficient time for the oil to soak into the sand. Stir the sand with a stick or screwdriver until all of the particles have been coated with oil. Do not oversaturate the sand with oil.
• Wires with alligator clips (optional) can be used to connect the probe wires firmly to the ohmmeter or multimeter terminals.
• Set the ohmmeter or the multimeter initially to a scale for 10K (10,000) ohms. (The symbol for ohm is Ω.) Try other scales if you do not detect a reading.
• Be sure to zero the ohmmeter for any scale you select. This is done by touching the two probes together and turning the adjustment knob until the meter reads zero ohms. This improves the accuracy of your reading.

Discussion

1. What did the ohmmeter measure?
   Answer: The ohmmeter measured the electrical resistance of the dry sand, as well as the resistance of the sand saturated with different liquids.

2. What is an ohm?
   Answer: An ohm is the unit used for measuring electrical resistance. Just as we use centimeters to measure distance, we use ohms to measure electrical resistance.

3. How were the electrical resistance readings different for each cup?
   Answer: Dry sand and sand saturated with oil have relatively high electrical resistance, while sand saturated with salt water has a lower electrical resistance.

4. Why might the electrical resistance of the sand be different when saturated with different liquids?
   Answer: Each liquid has its own characteristic electrical resistance.

5. What do you think will happen if the wires in the cup with salt water were separated by a much larger distance?
   Answer: If the wires were much farther apart, you could expect the electrical resistance to increase. This is because electrical current traveling between the two wires would have to go through more material.

6. How might this activity relate to everyday life?
   Answer: Oil exploration companies use similar electrical characteristics to help them locate underground deposits of petroleum.

Assessment

Are students able to describe how different types of liquid affect the electrical resistance of earth materials?

Answer: Dry material and material that has been soaked in oil have higher electrical resistances than material that has been soaked in salt water.

Extensions and further investigations

• Have students repeat the experiment using
  – other earth materials.
  – different amounts of earth material in each cup.
  – different levels of liquid saturation in the earth material.
  – different proportions of salt in the water.
  – different liquids.
  – different distances between the wires in each cup.
  – different sizes and shapes of wire.
  Note: For best results, change only one aspect at a time.

• Before the students carry out each additional experiment, challenge them to predict what they think will happen and why. Then have them compare the results with their predictions and develop possible explanations for any discrepancies.

Career connections

• Any oil-related careers, especially those related to exploration
• Chemistry
• Electronics
• Electrical engineering

Correlations with Standards

United States: This activity correlates with portions of NSES Content Standard A, Science as Inquiry, and Content Standard G, History and Nature of Science, Grades 5-8 and 9-12, and with the following additional standards:

Grades 5-8
Standard B – Physical Science: B1, B3
Standard D – Earth and Space Science: D1

Grades 9-12
Standard B - Physical Science: B6

Britain: This activity correlates with the English National Curriculum standard Sc1, Science Enquiry, and the following additional standards:

KS3, Sc1, Sc4: 1b, BoS: 1a, b, d, e; KS4, Sc1, Sc2: 3b, Sc3: 1h, Sc4: 1b, BoS: 1a, b, d, e

Glossary/vocabulary

electrical resistivity
electrical resistance
ohm

Resource links

These sites give several different perspectives on resistivity and resistance:

HyperPhysics

Wikipedia

It's Resistivity, not Resistance
University of Melbourne