Electricity and Magnetism
Core concepts
Master Ohm's law (V = IR), series and parallel resistor rules, electrical power (P = VI = I²R = V²/R), and the difference between charge, current, voltage and resistance. For magnetism, understand that a current produces a magnetic field (right-hand grip rule), that a moving charge in a field experiences force F = BIL sinθ, and that electromagnetic induction (Faraday's and Lenz's laws) explains generators and transformers.
Common traps
- Series: same current, voltages add. Parallel: same voltage, currents add; combined resistance is less than the smallest resistor.
- Ideal ammeters have zero resistance (in series); ideal voltmeters have infinite resistance (in parallel).
- Lenz's law opposes the change producing it - a frequent sign-error question.
- Confusing electric field direction (force on a positive charge) with electron flow direction.
- Forgetting that transformers conserve power (VpIp = VsIs) in the ideal case.
Worked example
Two resistors, 6 Ω and 3 Ω, in parallel across 12 V. Equivalent resistance = (6×3)/(6+3) = 2 Ω. Total current = 12/2 = 6 A; the 6 Ω branch carries 2 A and the 3 Ω branch carries 4 A. Check: currents sum to 6 A and voltages match.
Rule of thumb
Reduce circuits step by step, labelling voltage and current at each node, and verify with power conservation.