Electricity and Magnetism: fields, circuits and induction
Electricity and Magnetism: fields, circuits and induction
NEB Class 12 Electricity and Magnetism covers electrostatics, capacitors, DC circuits, magnetic fields, electromagnetic induction and alternating current. Questions range from Coulomb-force and capacitor calculations to Kirchhoff loops, force on a conductor, induced emf and RMS values.
Core rules
- Coulomb force F = kq₁q₂/r²; electric field E = F/q; potential V = kq/r.
- Parallel-plate capacitor C = ε₀A/d; with dielectric C = κε₀A/d. Energy U = ½CV² = Q²/(2C).
- Series capacitors: 1/C = Σ1/Cᵢ; parallel capacitors: C = ΣCᵢ. Resistors obey the opposite combination rules.
- Kirchhoff: junction ΣI = 0; loop ΣΔV = 0. Magnetic force on a wire F = BIL sinθ.
- Faraday law: ε = −dΦ/dt; AC RMS values I_rms = I_peak/√2, V_rms = V_peak/√2.
Worked example
A 100 μF capacitor is charged to 12 V. Find its charge and stored energy. Q = CV = (100 × 10⁻⁶ F)(12 V) = 1.2 × 10⁻³ C. U = ½CV² = 0.5 × 100 × 10⁻⁶ × 144 = 7.2 × 10⁻³ J. The energy is stored in the electric field; doubling voltage quadruples energy for the same capacitance.
Traps
Forgetting to convert μF to farads; using resistor combination rules for capacitors; losing the sign of induced emf; using peak instead of RMS in power calculations; assuming a capacitor blocks DC only after it is fully charged; mixing Tesla, ampere and metre units.