Electricity and Magnetism
The Electricity and Magnetism domain tests electrostatic force and field, electric potential, DC circuits, capacitance, magnetic forces, and electromagnetic induction. Problems often combine circuit analysis with energy or field reasoning.
Core rules
Coulomb’s law gives F = kq1q2/r²; electric field E = F/q and, for a point charge, E = kq/r². Potential difference is work per charge, V = W/q. In series, current is the same and voltages add; in parallel, voltage is the same and currents add. Equivalent resistance: series R = R1 + R2 + …; parallel 1/R = 1/R1 + 1/R2 + …. Kirchhoff’s junction and loop rules follow from charge and energy conservation. A magnetic field exerts force F = qvB sinθ on a moving charge; Faraday’s law links changing flux to induced emf, and Lenz’s law gives the direction that opposes the change.
Worked example
A 12 V battery connects to 6 Ω and 3 Ω resistors in parallel. The equivalent resistance is 1/R = 1/6 + 1/3 = 1/2, so R = 2 Ω. Total current is I = V/R = 12/2 = 6 A. The voltage across each parallel branch is 12 V, so the 6 Ω resistor carries 2 A and the 3 Ω resistor carries 4 A. Those branch currents sum to the 6 A total, confirming the junction rule.
Traps
Current is not “used up” in a series circuit. Do not add parallel resistances directly. Keep charge signs in Coulomb’s law. For magnetic force, use the right-hand rule and remember that a charge moving parallel to B feels no force. Induced current opposes change in flux, not the flux itself.