Electrical Fundamentals — Current, Power & Laws
Learn electrical fundamentals: AC vs DC comparison, active and reactive power with phase angle, and Ohm, Faraday and Kirchhoff laws.
1. What Flows
Electric current is charge in motion. In copper wiring the carriers are free electrons drifting from negative toward positive under potential difference; in batteries and electrolytes, ions carry it instead. Voltage pushes, current flows, resistance opposes — and every later law is bookkeeping on these three.
2. AC vs DC — Ten Differences That Matter
| AC | DC | |
|---|---|---|
| Carrying power | Travels far with little loss — cities run on it | Fades quickly with distance |
| Drive | Rotating magnet along the wire | Steady magnetism along the wire |
| Frequency / direction | 50/60 Hz, reversing every half cycle | Zero frequency, one direction only |
| Current shape | Magnitude varies with time; electrons shuttle back and forth | Constant magnitude; electrons march forward |
| Sources | Alternators, mains | Cells, batteries, DC generators |
| Opposition | Impedance (resistance plus reactance) | Resistance only |
| Power factor | Between 0 and 1 | Always 1 |
That is why ships generate and distribute AC (440 V 3-phase, 60 Hz) while control, alarms and emergency loads keep DC battery backup.
3. Which Power Does Work
With sinusoidal voltage and current, their phase relationship splits power in two. A purely resistive load keeps both in step — polarity reverses together, energy flows one way, and every watt works as heat or motion: active power. A purely reactive load (ideal inductor or capacitor) holds them 90° apart — energy sloshes into magnetic or static fields and back untouched: reactive power, the magnetising currency no motor can run without. Real loads mix both, and the power factor names the working fraction.
Units: active power is measured in kW, reactive power in kVAr, and apparent power in kVA.
4. Three Laws, Three Conservations
Ohm — proportion
At constant temperature, current through a conductor tracks applied voltage: I = V/R. Doubles voltage, doubles current.
Faraday — induction
Changing flux linked with a circuit induces EMF; its magnitude equals the rate of flux change. Generators and transformers live here.
Kirchhoff — accounting
Junction rule: currents sum to zero (charge conserved, parallel work). Loop rule: IR drops sum to the EMFs (energy conserved, series work).
Practise this in the question bank
- Electrical General — 30 questions
- Cables, Insulation & Earthing — 22 questions
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