St. Elmo's fire is not the only natural phenomenon that is explained using electric field lines.  Sharks are capable of detecting the weak electric fields that other animals produce through their muscle activity.  This ability enables sharks to detect prey that has buried itself under shallow sand on the seabed.

Just as a source mass surrounds itself with gravitational field lines, a source charge surrounds itself with electric field lines.  Because muscle activity is triggered by the moving charges in nerve cells, muscle contractions produce electric fields.  In this case, the source charges are very weak, so the resulting electric fields are also very weak.

 

Hammerhead sharks have sensors located in their snouts that allow them to detect very weak electrical fields.  The shark swims close to the seabed seeking prey that has hidden under the sand.  The electric field lines produced by the prey can extend up into the water above the sand.  As the shark swims past a location where the density of the electric field lines suddenly changes, the sensors in the shark's snout are triggered.  The shark uses this information to pinpoint the exact location of its hidden prey.

 

Scientists have verified this ability of sharks by burying pairs of oppositely charged sources in the sand. As the shark swims closer to the sources, the special sensors in its snout detect a change in the density of electric field lines. Where the field lines are most concentrated, the shark disturbs the sand looking for a meal. Astonishingly, this type of behaviour was observed with electric fields as weak as 5 ×10- 7 N/C.


Read Electric Field Lines
Read page 554 to learn more about electric field lines and the patterns they form. Afterwards do Self-Check 1.
Minds On: Drawing Electric Field Lines
Read this section on page 555 of your textbook. Afterwards do Self-Check 2.

  1. Are the field lines in the illustration with the shark drawn in a way that is consistent with the rules for drawing electric field lines listed on page 554 of your textbook? 

  2. Use the rules listed on page 554 of your textbook to identify the location in the illustration where the magnitude of the electric field would be greatest. 

  3. Use your answer to SC 12.b. to identify the location that would likely draw the shark.

  1. Yes, the field lines are drawn in a way that is consistent with the rules stated in the textbook. Electric field lines start from the positive source and extend radially. Electric field lines terminate at the negative source. The electric field lines are tightly packed between the two sources, indicating that this is where the magnitude of the electric field is greatest. 

  2. The magnitude of the electric field would be greatest where the field lines are densely packed together. This occurs in the region between the two sources but closer to one source or the other. 

  3. Given that the sensors in the shark's snout are designed to detect changes in the density of electric field lines, these sensors would tend to direct the shark to move into the space between the two charged sources but closer to one source or the other.


On a sheet of paper, sketch the patterns of the electric field lines shown in each of the situations illustrated in the "Minds On: Drawing Electric Field Lines" activity.  Complete each illustration by adding arrowheads to each of the electric field lines.