Electric fields, circuits, and magnetism concepts and formulas for the calculus-based AP Physics C: E&M exam. Front: the term or formula. Back: definition or formula with a brief usage note.
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- Electric field
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The force per unit positive charge exerted at a point; E = F/q, measured in N/C or V/m.- E = kQ/r^2
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Electric field from a point charge Q at distance r; k = 8.99 x 10^9 N*m^2/C^2 (Coulomb's constant); field points radially outward from positive charge.- Coulomb's law
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F = k*Q1*Q2/r^2; force between two point charges depends on product of charges and inverse square of separation distance.- Electric potential
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The potential energy per unit positive charge at a point; V = U/q, measured in volts; represents work per unit charge to move charge from infinity to that point.- V = kQ/r
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Electric potential from point charge Q at distance r; measured in volts; potential difference between two points is the work per unit charge moving charge between them.- Superposition principle
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The total electric field at any point is the vector sum of fields from all individual charges; the total potential is the scalar sum of potentials.- Electric flux
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Phi_E = E*A*cos(theta); measure of field lines passing through surface area A at angle theta to field direction; measured in N*m^2/C.- Gauss's law
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The electric flux through any closed surface equals the enclosed charge divided by epsilon_0; Phi_E = Q_enclosed/epsilon_0 (where epsilon_0 = 8.85 x 10^-12 F/m).- Gaussian surface
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An imaginary closed surface chosen to exploit symmetry in applying Gauss's law; commonly a sphere, cylinder, or infinite plane; the field magnitude is constant on the surface.- Conductor in electrostatic equilibrium
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Electric field inside is zero, excess charge resides entirely on outer surface, surface is an equipotential, and field just outside surface is perpendicular to surface.- Equipotential surface
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A surface where all points have the same electric potential; no work is required to move charge along equipotential; equipotentials are perpendicular to field lines everywhere.- Capacitance
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C = Q/V; the ratio of charge on one plate to potential difference between plates, measured in farads (F); represents ability to store charge at given voltage.- Parallel plate capacitor
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C = epsilon_0*A/d, where A is area of each plate and d is separation; electric field between plates is uniform and equal to E = sigma/epsilon_0 (sigma is charge density).- Energy stored in capacitor
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U = (1/2)*Q*V = (1/2)*C*V^2 = Q^2/(2C); represents work done to charge capacitor; all three forms are equivalent.- Dielectric
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Non-conducting material placed between capacitor plates; increases capacitance by factor kappa (dielectric constant); reduces electric field between plates by factor kappa.
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