Electrical Fundamentals — Current, Power & Laws
What flows, what AC changes, which power does work, and the three laws every calculation stands on.
Key Principles at a Glance 5 points
- Current is charge drifting through conductors — free electrons in metals, ions in electrolytes — driven by potential difference.
- AC reverses with time (50/60 Hz, impedance, power factor below 1); DC holds one direction (zero frequency, resistance only, unity power factor).
- Resistive loads align voltage and current so all power works; reactive loads shift them 90° so power merely shuttles into fields and back.
- Ohm links V, I and R at constant temperature; Faraday links changing flux to induced EMF with magnitude from its rate.
- Kirchhoff counts what cannot vanish: currents sum to zero at a junction (charge), voltages round a loop to the EMFs (energy).
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.
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).