Electric current is the flow of charge through a conductor, measured in amperes. Voltage is the electrical pressure pushing that charge — properly, the energy given to each coulomb passing through. Resistance is how hard a material makes it for charge to move. The water analogy is imperfect but useful: voltage is the pressure, current is the flow rate, and resistance is the narrowness of the pipe. What matters is that these three are locked together by one equation, so knowing any two always gives you the third.
| Quantity | Symbol | Unit | Measured with | Connected |
|---|---|---|---|---|
| Voltage | V | volt (V) | Voltmeter | In parallel, across the component |
| Current | I | ampere (A) | Ammeter | In series, broken into the circuit |
| Resistance | R | ohm (Ω) | Ohmmeter | Across the component, power OFF |
| Power | P | watt (W) | Calculated, or a wattmeter | — |
Ohm's law
Power
Power is the rate at which electrical energy is converted into something else — heat, light, motion. It is what determines whether a component gets warm or catches fire, so it is the quantity that decides component ratings. Three forms exist, and they are all the same equation with Ohm's law substituted in.
Every resistor has a power rating, commonly ¼ W for through-hole parts. Exceed it and the resistor overheats, drifts in value, and eventually fails — sometimes open-circuit, sometimes as a small fire. Calculating dissipation is therefore not an academic exercise; it is how you choose the part.
| Prefix | Symbol | Multiplier | Example |
|---|---|---|---|
| mega | M | × 1 000 000 | 2.2 MΩ = 2 200 000 Ω |
| kilo | k | × 1 000 | 4.7 kΩ = 4700 Ω |
| (base) | — | × 1 | 5 V |
| milli | m | × 0.001 | 20 mA = 0.020 A |
| micro | µ | × 0.000001 | 470 µF = 0.000470 F |
AC, DC and safety
Direct current flows one way at a constant voltage: batteries, USB, the rails inside a computer. Alternating current reverses direction periodically — in Canada, mains supply is 120 V at 60 Hz. Everything in this course is DC. A power supply's job is to convert mains AC into the DC rails electronics need.
- Power down and unplug before changing any wiring. Rebuild, then apply power.
- Check polarity twice before connecting. Reversed polarity destroys most ICs and electrolytic capacitors instantly.
- Never exceed a component's rated voltage, current or power.
- A large capacitor stays charged after power is removed — discharge it through a resistor before touching it.
- Wear eye protection when cutting leads; trimmed wire ends travel surprisingly fast.
- Soldering irons run at about 350 °C. Return the iron to its stand every single time, and solder in a ventilated space.