dielectric constant

Meaning: A dielectric is an insulator placed between capacitor plates. Its molecules polarize in the field, and the induced surface charges oppose the applied field, so the net field inside is smaller. The dielectric constant κ (relative permittivity) measures this effect: κ = 1 for vacuum, and κ is greater than 1 for real materials. For the same plate charge the voltage is lower, so capacitance is higher.

Equation: C = κC₀ and E = E₀/κ, where C is capacitance with the dielectric (F), C₀ is capacitance without it (F), E and E₀ are the fields with and without it (V/m), and κ is dimensionless. Also κ = ε/ε₀, the ratio of permittivities.

On the MCAT: A dielectric slab is inserted into a parallel-plate capacitor. Battery connected: V is fixed, C rises, so Q = CV and U = ½CV² both rise. Battery disconnected: Q is fixed, C rises, so V = Q/C and E fall by a factor of κ, and U = Q²/2C falls.

Common confusion: Students remember that capacitance rises but forget to ask which quantity is held fixed, V or Q. That choice decides what happens to charge, voltage and energy. Also, dielectric constant is not dielectric strength, which is the maximum field an insulator withstands before breaking down.