Learn these precisely now and everything downstream is arithmetic rather than mystery.
| Quantity | Symbol / unit | Is | Water analogy |
|---|---|---|---|
| Voltage | V, volts | Difference in electrical pressure between two points | Pressure difference |
| Current | I, amps | Rate of charge flow past a point | Litres per second |
| Resistance | R, ohms (Ω) | Opposition to flow | A narrow pipe |
| Power | P, watts | Rate of energy conversion, usually into heat | Work done per second |
Where the analogy misleads. Voltage is always a difference between two points, never a property of one point. "This wire is at 5V" is shorthand for "5V relative to the node we agreed to call ground". And current is not consumed — the same current that leaves a battery's positive terminal returns to its negative one. A circuit must be a complete loop or nothing flows at all.
One more that catches people: a component does not "draw the voltage it needs". A resistor across a supply gets the full supply voltage and the current follows from Ohm's law. Only components with their own internal behaviour — diodes, regulators — impose a voltage of their own.
V = I × R I = V / R R = V / I P = V × I P = I² × R P = V² / R // 5V across a 220Ω resistor I = 5 / 220 = 0.0227 A = 22.7 mA P = 5 × 0.0227 = 0.114 W -> a 0.25W resistor is comfortable
The power calculation is the one beginners skip, and it is the one that decides whether a component survives. Any time current exceeds roughly 30mA, work out the dissipation before you build.
Fill any two fields; the other two are calculated.
k kilo ×1000 1k5 = 1500Ω (the k replaces the decimal point)
M mega ×1000000 4M7 = 4,700,000Ω
m milli ÷1000 20mA = 0.02A
µ micro ÷1000000 100µF, 10µH
n nano ÷10⁹ 100nF = 0.1µF
p pico ÷10¹² 22pF
Two shortcuts worth memorising: two equal resistors in parallel give half the value; and a parallel combination is always smaller than the smallest resistor in it. If your parallel arithmetic gives a bigger number, you have inverted something.
// series divider — 9V across 1k and 2k I = 9 / 3000 = 3mA V across R1 = 3mA × 1k = 3V V across R2 = 3mA × 2k = 6V // they add back to 9V
If a measurement contradicts either, the circuit is not what you think it is — usually an unintended connection, or an open one.
Ground is simply the node you choose as 0V and measure everything against. It has no cosmic significance; on a battery circuit it is just the negative terminal.
Two 1kΩ resistors in parallel, connected across 5V. What is the total current drawn from the supply?
DC flows one way at a steady level: batteries, USB, bench supplies — everything you will build at first. AC reverses direction periodically; UK mains is 230V at 50Hz, and audio and radio signals are AC riding on a DC level.
Two other things that can hurt you or your house even at low voltage: lithium batteries — a shorted LiPo can vent flame, so never short, puncture or charge one unattended — and large capacitors, which hold charge after power is removed. Neither appears in a first LED project; both appear sooner than people expect.