Practice question

**Energy stored in capacitor** U = ½ C V² = ½ Q V = Q²/(2C) (J), C capacitance (F), V voltage (V), Q charge (C). For 4 μF charged to 250 V, U=0.5×4×10⁻⁶×62500=0.125 J. Energy resides in electric field, energy density u = ½ ε₀ E² (J/m³), E field between plates. Using Gauss’s law for an infinite charged sheet with surface charge density sigma , a Gaussian surface (e.g., a cylinder perpendicular to the sheet) shows that the electric field E is perpendicular to the sheet and constant. The flux through the cylinder's end caps is E · 2A , and the enclosed charge is