dielectric material
Summary
A dielectric is an insulating material placed between capacitor plates. It polarizes and weakens the electric field inside, so capacitance rises by the dielectric constant κ (C = κC₀). The testable point is whether the battery stays connected (V fixed) or is removed (Q fixed).
Detail
Meaning: A dielectric is an electrical insulator placed between the plates of a capacitor. In the field between the plates its molecules polarize, which puts bound charge on its surfaces that opposes the plates' field. The net field inside falls to E₀/κ, so the potential difference for a given charge falls and the capacitance rises. κ is the dielectric constant, which is 1 for vacuum and above 1 for materials.
Equation: C = κε₀A/d, where C is capacitance with the dielectric (F), κ is the dielectric constant (unitless), ε₀ is the permittivity of free space (C²/N·m²), A is plate area (m²), and d is plate separation (m).
On the MCAT: A slab is inserted into a charged parallel-plate capacitor, and you predict what changes. If the battery stays connected, V is fixed, so C, Q = CV and U = ½CV² all increase by the factor κ. If the capacitor is isolated, Q is fixed, so C rises, V = Q/C and E fall by the factor κ, and U = Q²/2C falls.
Common confusion: Students treat the dielectric like a conductor or think it strengthens the field. No charge flows through it, and the field inside is reduced. The plate charge changes only when a battery is connected.
Sources
- Physics (Giancoli)
- University Physics (Young and Freedman)
- Fundamentals of Physics (Halliday, Resnick, Walker)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.