Voltage, Current, and Resistance
The Water Analogy
Electricity is invisible, so let's think of it like water:
- Voltage (V) is like water pressure — it pushes the current through the circuit. Measured in volts (V).
- Current (I) is like water flow — how much electricity is flowing. Measured in amps (A).
- Resistance (R) is like a narrow pipe — it restricts the flow. Measured in ohms (Ω).
Ohm's Law
The most important formula in electronics:
\( V = I \times R \)
If you know any two values, you can find the third:
- V = I × R (voltage = current × resistance)
- I = V / R (current = voltage / resistance)
- R = V / I (resistance = voltage / current)
Example
A 13.8V power supply drives a radio with 2 ohms resistance. What current flows?
I = V / R = 13.8 / 2 = 6.9 amps
Power
Power is how quickly energy is used. Your radio's output power is measured in watts (W).
\( P = V \times I \)
Also: P = I²R and P = V²/R
Example
Your radio draws 3 amps from a 13.8V supply. What power does it consume?
P = V × I = 13.8 × 3 = 41.4 watts
AC vs DC
- DC (Direct Current) — flows in one direction. Batteries produce DC. Your radio runs on 13.8V DC.
- AC (Alternating Current) — flows back and forth. Mains power is 240V AC at 50 Hz. Radio signals are AC.
For the exam: Know Ohm's law (V = IR) and the power formula (P = VI). These are the two most important formulas at Foundation level.