Electricity and Magnetism
Core concepts
Master Coulomb's law, electric field and potential (E = kQ/r², V = kQ/r), and the behavior of conductors in electrostatic equilibrium (field inside zero, charge on the surface). Apply Ohm's law, series and parallel resistor rules, and Kirchhoff's laws for multi-loop circuits; know power dissipation P = I²R = V²/R. For magnetism, use the right-hand rule for moving charges and currents, the force F = qvB sinθ and F = BIL sinθ, and understand that magnetic forces do no work. Faraday's law and Lenz's law govern induced EMF and the direction of induced current.
Common traps
- Adding resistors in parallel as if in series (use reciprocals).
- Forgetting that electric potential is a scalar and adds algebraically with sign.
- Assuming a magnetic field exerts force on a charge at rest or moving parallel to the field.
- Reversing Lenz's law: induced current opposes the change in flux, not the flux itself.
Worked example
Two 6 Ω resistors in parallel give 1/R = 1/6 + 1/6 = 1/3, so R = 3 Ω. Connected to a 9 V battery, the current is I = V/R = 3 A and total power is P = VI = 27 W.
Rule of thumb
For circuits, reduce series and parallel groups step by step before applying Kirchhoff; for induction, ask what the flux change is and oppose it.