Dielectric properties and relative permittivity — pick the correct statement(s) Consider relative permittivity (dielectric constant) and its implications. Which combined statement is correct?
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AThe ratio of the permittivity of a medium to free space is called relative permittivity
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BDielectric constant is a pure number and has no dimensions
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CThe dielectric constant of pure water is about 80
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DHigher dielectric constant generally corresponds to poorer electrical conductivity (insulating behaviour)
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EAll of these
Answer
Correct Answer: All of these
Explanation
Introduction / Context:Dielectric properties govern how materials polarise in an electric field. In electromagnetics and microwave remote sensing, relative permittivity (εr) influences reflection, transmission, and wave speed in media (soil, rock, water).
Given Data / Assumptions:
- εr = ε / ε0 by definition.
- Relative permittivity is dimensionless.
- Liquid water at room temperature has εr ≈ 80 (frequency dependent).
- Conductivity and permittivity trends are discussed qualitatively.
Concept / Approach:Relative permittivity compares material response to vacuum. While exact εr and conductivity σ are frequency- and temperature-dependent, materials termed “dielectrics” typically have high εr and low σ (insulators), contrasting with good conductors that have very high σ and effectively infinite complex permittivity at low frequencies.
Step-by-Step Reasoning:
1) Define εr = ε / ε0 → dimensionless.2) Note reference values: water ~80 at ~20 °C (decreases with frequency/temperature).3) In practice, higher εr is associated with polarisable, non-conducting media compared with metals which are characterised by conductivity rather than finite εr.Verification / Alternative check:Microwave retrievals of soil moisture rely on the strong contrast between εr of water and dry soil; this enables dielectric-based sensing.
Why Other Options Are Wrong:
- Each individual statement is correct; the comprehensive choice is best.
Common Pitfalls:Assuming εr is constant across frequency; ignoring temperature and salinity effects (e.g., seawater has high εr but also notable conductivity).
Final Answer:All of these