Field Intensity: an 800-KHz Radio Signal Is Detected at a Point

question 20

Multiple Choice

Field intensity: An 800-kHz radio signal is detected at a point 8.5 km distant from a transmitter tower. The electric field amplitude of the signal at that point is 0.90 V/m. Assume that the signal power is radiated uniformly in all directions and that radio waves incident upon the ground are completely absorbed. What is the average electromagnetic energy density at that point? (c = 3.0 × 10⁸ m/s, μ₀ = 4π × 10^-7 T ∙ m/A, ε₀ = 8.85 × 10^-12 C²/N ∙ m²)

• A) 3.6 pJ/m³
• B) 5.1 pJ/m³
• C) 7.2 pJ/m³
• D) 10 pJ/m³
• E) 14 pJ/m³

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Definitions:

Concrete Operations

a stage in Jean Piaget's theory of cognitive development during which children gain the ability to think logically about concrete events.

Preoperational

A stage in Piaget's theory of cognitive development, occurring roughly between the ages of 2 and 7, characterized by symbolic thinking, egocentrism, and a lack of operational thought.

Sensorimotor

A developmental stage in early childhood, as proposed by Piaget, where children learn about and experience the world primarily through their senses and motor actions.

Formal Operations

The final stage in Piaget's theory of cognitive development, starting around age 12, where individuals develop the ability to think abstractly, logically, and systematically.