Charge, and why a circuit must be a loop

Electricity is the flow of electric charge, and a computer is just a lot of that flow put to work.

• Charge only moves through a conductor, a material with low resistivity where electrons are loosely held, like copper.
• An insulator (rubber, plastic, glass) holds its electrons tightly and blocks the flow, which is exactly why it wraps every wire.
• Charge needs a closed loop to move continuously: break the loop and the current stops. This is the single idea every later topic rests on.

The three quantities: V, I, R

Voltage, current and resistance are one relationship seen from three sides.

• Voltage V = W/Q is the work per unit charge: the push that moves a coulomb from one point to another.
• Current is the rate of charge flow through a conductor.
• Resistance R opposes that flow and is measured in ohms (Ω).
• Ohm's law I = V/R ties them together: raise the voltage and current rises; raise the resistance and current falls. The visualizer on the deck lets you feel this, the lamp brightens with current, not with voltage alone.

Active vs passive components

The divide that organizes every component you will meet is whether a part can *produce* energy.

• Active components produce energy as voltage or current but need an external source, transistors, op-amps.
• Passive components cannot generate energy and only shape what the circuit gives them: resistors, capacitors, inductors.
• A trap: passive does not mean unimportant. A capacitor stores energy in an electric field and an inductor stores it in a magnetic field, they just cannot make new energy.

Semiconductors and the diode

Semiconductors sit between conductors and insulators, and doping them is what makes modern electronics possible.

• A pure (intrinsic) semiconductor has no impurities; adding impurity atoms makes it extrinsic.
• Adding a trivalent impurity gives P-type; a pentavalent impurity gives N-type.
• Joining P and N makes a diode, which conducts in one direction only. Forward biased (+ to the anode) it acts like a closed switch; reverse biased it acts like an open switch and blocks current.
• Stack two junctions and you get a transistor, the amplifying, switching part that, packed by the million onto one chip, becomes an integrated circuit.

Reading real parts

The components on a board carry their values printed on them, and reading them is a practical skill.

• Through-hole resistors encode their value in colour bands; the decoder on the deck turns band colours into a resistance and tolerance.
• Surface-mount resistors use numbers instead: the first digits are significant, the last is the count of trailing zeros, 1252 reads as 12,500 Ω.
• A power supply converts wall AC into the low DC a computer needs, using a rectifier so the current never reverses: your laptop charger is exactly this.
• A battery converts chemical energy to electrical energy and comes as primary (single-use) or secondary (rechargeable).