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On the syllabus: A-Level Physics
A camera flash charges up over a few seconds, then releases all its energy in a millisecond — far faster than a battery could deliver it. Defibrillators store energy to restart a heart in one powerful pulse. How do these devices store and release charge so rapidly?
A camera flash charges up over a few seconds, then releases all its energy in a millisecond — far faster than a battery could deliver it. Defibrillators store energy to restart a heart in one powerful pulse. How do these devices store and release charge so rapidly?
A capacitor stores electrical energy in an electric field between two conducting plates. Unlike a battery, it stores and releases charge almost instantly — making it ideal for rapid energy delivery and signal filtering in electronics.
A capacitor stores charge Q proportional to the voltage V across it. The proportionality constant is the capacitance C. Energy is stored in the electric field between the plates. Capacitance depends on plate area, separation, and the dielectric material between the plates.
Capacitance is measured in Farads (F). One Farad stores 1 coulomb per volt — this is an enormous amount, so practical capacitors are measured in microfarads (μF = 10⁻⁶ F) or picofarads (pF = 10⁻¹² F). For a parallel-plate capacitor: C = ε₀εᵣA/d, where A is plate area, d is separation, and εᵣ is the relative permittivity of the dielectric. Dielectrics (insulating materials like ceramic, mylar) increase capacitance by reducing the effective electric field inside. Energy stored: E = ½CV² = Q²/(2C) = ½QV. A 100 μF capacitor charged to 300 V stores ½ × 100×10⁻⁶ × 90,000 = 4.5 J — enough to illuminate a bright flash for 1/1000 s. Capacitors in series reduce total capacitance; in parallel they add: C_total = C₁ + C₂.