Chapter 17, Problem 68RPP

Electrostatic exploration Geologists sometimes analyze the distribution of materials under Earth's surface, materials such as iron, water, oil, or dry soil. The process they use is called electrostatic exploration. Electrodes are placed in the ground about 800 m apart. An electric generator is connected to the electrodes and causes them to become oppositely charged. The opposite-sign charges on the electrodes cause electrically charged ions in the matter below the surface to move. The moving ions are detected by equipment on the surface. This helps the geologist decide what type of matter is below the surface. What causes this motion? To help answer this question, determine the net electric force exerted on ions at different places below the surface.

The resistance to the motion of electrons through different types of materials is, in decreasing order, dry soil, moist soil, underground water, and iron ore. How can this knowledge and the measurement of the charge reaching the detector per unit time help identify what type of material is under Earth's surface?

a. More electrons will reach the detector from iron than from dry soil.

b. The electrons reaching the detector will carry water with them if water is under the surface.

c. The electrons reaching the detector are affected very little by what's under the surface

d. The detector will get more electric charge if the resistance to flow is less, and resistance is related to the type of material.

e. a and d