12 Electric Fields in Marine Biology

Learning Outcomes

• Define electric charge and electric field;

• Describe the relationship between electric force, charge, and electric field;

• Use Coulomb’s law to calculate the force between two charges; and

• Explain how electric fields are used by marine organisms for navigation and hunting (e.g., electric fish).

The Role of Electric Fields in Marine Organisms

Electric fields play a crucial role in the lives of many marine organisms, particularly in navigation, hunting, and communication. This ability, known as electroreception, enables animals to detect electric signals in their surroundings. Sharks and rays, for example, rely on specialized sensory organs called ampullae of Lorenzini to perceive weak bioelectric signals produced by the muscle contractions or nerve impulses of prey. Similarly, electric fish use electrolocation, emitting low-voltage electric fields to detect objects and navigate murky waters.

Some marine organisms, such as electric eels and rays, possess the unique ability to generate electric fields through specialized organs composed of electrocytes. These electric discharges serve multiple purposes. Low-voltage emissions help in navigation, while high-voltage shocks are used for hunting and defense, effectively stunning prey or deterring predators. This combination of passive detection and active field generation provides significant advantages in underwater environments where visibility is often limited.

Electric fields also shape broader ecological dynamics. Predators use them to locate prey, while prey species may minimize movements to evade detection. In addition, electric signals facilitate communication among some species of fish, allowing them to attract mates or establish dominance.

However, human activities introduce electro-pollution through submarine cables and sonar, potentially disrupting these natural systems. Understanding the role of electric fields in marine ecosystems is vital for conservation efforts, as it underscores the delicate balance between biological and physical processes in aquatic environments. By studying these phenomena, we can better protect the diversity and functionality of marine life.

Electric Charge and Electric Field

Electric charge (symbol q, sometimes 𝑄) is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. It can be positive or negative, with like charges repelling each other and opposite charges attracting. The unit of electric charge is the coulomb (C), and charge is quantized, meaning it occurs in discrete amounts. The basic unit of charge is the elementary charge 𝑒, which is the charge of a single proton (+1.6×10⁻¹⁹C) or electron (−1.6×10⁻¹⁹C).

The electric field is a region around a charged particle where other charged particles experience a force. It is a vector field, meaning it has both magnitude and direction, and is defined as the force per unit charge exerted on a small positive test charge placed at a point in space. The electric field 𝐸 at a distance 𝑟 from a point charge 𝑄 is given by Coulomb's law:

where 𝜖₀ is the permittivity of free space, and 𝑟 is the unit vector pointing from the charge 𝑄 to the point where the field is measured. The unit of the electric field is newtons per coulomb (N/C) or volts per meter (V/m).

The relationship between electric force, charge, and electric field is given by the formula:

F = qE

where:

• F is the electric force experienced by a charge,
• q is the magnitude of the electric charge,
• E is the electric field at the location of the charge.

Key Points of the Relationship:
1. Direction of the Force:

• If q is positive, the force F will be in the same direction as the electric field E.
• If q is negative, the force F will be in the opposite direction of E.

2. Magnitude of the Force:

• The strength of the force F is directly proportional to the magnitude of the electric charge q and the magnitude of the electric field E.
• Larger charges or stronger electric fields result in a greater electric force on the charge.

3. Electric Field Definition from Force:

• The electric field E is defined as the force per unit positive charge:
• This definition implies that the electric field is an inherent property of the space around the charge creating it, independent of any other charges present.

4. Force as an Interaction Between Charges:

• Electric force results from the interaction of one charge with the electric field created by another charge.

Coulomb's Law

Problem Set

1. Two charges q₁ = 3.0 μC and q₂ = 2.0 μC are separated by a distance of  r = 0.5m. Calculate the magnitude of the electrostatic force between them. Use 𝑘 = 8.99 × 10⁹ N•m²/C² .
2. Two charges q₁ = -6.0 μC and q₂ = 4.0 μC  are separated by r = 0.3 m. Determine the electrostatic force between the charges.
3. A pair of charges, q₁ = 5.0 μC and q₂ = -7.0 μC, are located r = 1.2 m apart. What is the magnitude of the force acting between the charges?
4. Two charges q₁ = 10.0 μC and q₂ = 3.0 μC are separated by a distance of r = 2.0 m. Find the electrostatic force between them.
5. If q₁ = 1.5 μC and q₂ = 4.5 μC are separated by r = 0.8 m, calculate the force between the charges.
6. Two charges q₁ = -9.0 μC and q₂ = 2.0 μC are placed r = 0.7 m apart. Find the magnitude of the force between them.
7. If q₁ = 0.5 μC and q₂ = 0.25 μC are separated by  r = 0.4 m, compute the electrostatic force between the charges.
8. Two charges q₁ = -3.0 μC and q₂ = -2.0 μC are r = 0.6 m apart. Calculate the magnitude of the force.
9. If q₁ = 7.0 μC and q₂ = 1.0 μC are separated by r = 0.9 m, find the electrostatic force between them.
10. Two charges q₁ = 2.0 μC and q₂ = 5.0 μC are separated by r = 1.5 m. What is the magnitude of the electrostatic force between these charges?