Dielectric constant
The dimensionless ratio κ = ε/ε₀ of a material's permittivity to that of vacuum. Tells you how much a dielectric amplifies a capacitor's storage capacity.
Definition
The dielectric constant κ (also written ε_r, the relative permittivity) is a dimensionless number characterising a material's electrical response. It is defined as κ = ε/ε₀, the ratio of the material's permittivity to vacuum permittivity, and it equals 1 + χ_e where χ_e is the electric susceptibility. By definition, vacuum has κ = 1 exactly, and every material medium has κ > 1.
Operationally, κ is the factor by which putting a slab of material between the plates of a capacitor multiplies its capacitance: a vacuum capacitor of capacitance C₀ becomes a capacitor of capacitance κC₀ when filled with the dielectric. This is the easiest way to measure κ in a lab — assemble a parallel-plate capacitor, charge it through a known voltage, fill it with the test material, and read off the new voltage at fixed charge. The voltage drops by a factor of κ, which is the polarization shielding the field inside.
Numerical values cover an enormous range. Vacuum κ = 1; air κ ≈ 1.0006; teflon κ ≈ 2.1; paper κ ≈ 3.5; rubber κ ≈ 7; alumina ceramic κ ≈ 9; methanol κ ≈ 33; water κ ≈ 80; barium titanate κ ≈ 1,200; calcium copper titanate κ > 10⁵. The high-κ materials are dominated by polar molecules (water) or by ferroelectric domains that flip easily in modest fields (barium titanate). The choice of dielectric in a capacitor is always a balance between high κ (more capacitance) and low losses, low temperature dependence, high breakdown strength — there is no universal best material.