Elektromagnetizmus
What you must know
Expect Coulomb's law, electric field and potential, capacitance, Ohm's law, series and parallel resistors, electric power, and the magnetic effects of currents (force on a wire F = BIl·sin α, force on a moving charge, and electromagnetic induction with Faraday's law). Know that electric field lines point from positive to negative, and that potential decreases along field lines. For circuits, series resistors add, parallel resistors add reciprocally; the same current flows in series, the same voltage appears across parallel branches.
Traps
- Confusing electric field (force per charge) with potential (energy per charge).
- Forgetting that a wire parallel to a magnetic field feels no force (sin 0° = 0).
- Using total resistance as if parallel resistors simply add.
- Mixing up the direction conventions for induced current (Lenz's law opposes the change).
- Forgetting to convert mA, kΩ and µF to SI units.
Worked example
Two resistors, 6 Ω and 3 Ω, are connected in parallel across 12 V. Equivalent resistance: 1/R = 1/6 + 1/3 = 1/2, so R = 2 Ω. Total current I = U/R = 12/2 = 6 A. Check: branch currents are 2 A and 4 A, summing to 6 A - a good self-check.
Rule of thumb: for parallel networks, the equivalent resistance is always smaller than the smallest branch. Draw the circuit, label nodes, and reduce step by step.