Start Here: What Electronics Is
Every gadget you own is a loop of components steering electric charge. This volume explains what that means in plain words: voltage as the push, current as the flow, resistance as the brake. It solves a first circuit, shows how to read a schematic, and sets out the few safety rules that matter.
- What electronics is, and what a circuit needs to work
- Voltage, current and resistance in plain words, with their units
- How to read a simple schematic
- Why mains electricity is dangerous, and why a short circuit makes heat
- Nothing. This is the first volume, and it assumes no electronics and no maths beyond multiplying and dividing.
0.1 What electronics is
Electronics is making electric charge flow where you want it. Every phone, charger and computer is a circuit of small parts that steer that flow - to carry energy, or to carry information.
A torch is the simplest example. Press the button, and current flows from the battery, through the bulb and back; release it, and the flow stops. A phone does the same thing billions of times over, with switches too small to see.
People sometimes separate electrical from electronic. An electric kettle just turns current into heat. An electronic circuit uses some currents to control others - a small signal switching a big one on and off. That control is what lets a circuit count, compute and talk.
The parts you will meet
Each part of a circuit is a component. This course meets the common ones, one at a time:
| Component | What it does |
|---|---|
| Resistor | Holds the current back by a fixed amount |
| Capacitor | Stores a little charge, like a tiny rechargeable battery |
| Diode | Lets current through one way only |
| LED | A diode that gives out light |
| Transistor | A switch that is turned on and off by another current or voltage |
A circuit is a loop
Current only flows round a complete loop: out of the battery, through the components and back into the battery. That loop is an electric circuit. Break it anywhere - with a switch, or a loose wire - and the current stops everywhere in the loop at once.
A torch's bulb is off. Which of these would explain it?
Show the answer
Answer: C. Current needs a complete loop. A broken switch, a loose wire or a dead bulb breaks the loop, and then no current flows anywhere in it. Current is never used up on the way round.
0.2 Charge, current and voltage in plain words
Voltage is the push, current is the flow, and resistance holds the flow back. More push gives more flow; more resistance gives less.
Everything is made of atoms, and atoms contain tiny charged particles called electrons. In a metal wire, some electrons are free to move. When they drift along the wire together, that moving charge is a current.
The water picture
A picture that helps at first is water in pipes:
- Voltage is like pressure. A battery is a pump that pushes. Voltage is measured in volts (V).
- Current is like the flow of water. It is how much charge passes each second, measured in amperes, or amps (A).
- Resistance is like a narrow pipe. It holds the flow back, and it is measured in ohms (Ω).
The picture has limits. A wire is never "empty" waiting to fill: it is full of electrons already, and they all start moving at once. And current is not used up. The same current that leaves the battery comes back into it; what gets used is the energy it carries.
A first circuit
Figure 0.1 shows a 9 V battery, a switch and a lamp. The battery's + end is the long plate at the top.
With the switch closed, the whole 9 V pushes current through the lamp's 90 Ω. With it open, nothing flows:
switch closed: current 100 mA, lamp power 900 mW
switch open: current 0 A, lamp power 0 W
The mA means milliamps: thousandths of an amp, so 100 mA is 0.1 A. The current came from one simple sum, which Volume 02 explains properly:
9 V / 90 ohms = 100 mA
Units at a glance
| Quantity | Letter in formulas | Unit | Unit symbol |
|---|---|---|---|
| Voltage | V | volt | V |
| Current | I | ampere | A |
| Resistance | R | ohm | Ω |
| Power | P | watt | W |
| Charge | Q | coulomb | C |
Current is written I, from an old French word for intensity. Prefixes shrink or grow the units: m for milli (a thousandth), k for kilo (a thousand), M for mega (a million).
In the first circuit, the switch is opened. What is the current in the lamp?
Show the answer
Answer: B. An open switch breaks the loop, and with no complete loop no current flows anywhere in it - including in the lamp.
0.3 How this course works
Each volume adds one idea and uses only what came before. Every voltage and current in the course was computed from the circuit drawn beside it, so you can check any answer against the drawing.
The route
| Part | Volumes | What you get |
|---|---|---|
| The basics | 00 to 02 | Voltage, current, resistance, Ohm's law, power, series and parallel |
| Parts that store and steer | 03 and 04 | Capacitors and RC timing, diodes and LEDs |
| Switching with transistors | 05 and 06 | Transistors as switches, and how a logic gate is built from them |
| Real circuits | 07 and 08 | Power supplies, decoupling, datasheets, schematics and bench tools |
How each lesson is built
- The big idea in one or two sentences.
- Plain words first, then a picture, then the exact version with numbers.
- A worked circuit, drawn and solved.
- A common mistake, and a quick check at the end of each sub-module.
Beside these lessons sits a small circuit solver. Each circuit drawing is also a description the solver can read, so it works out the voltages and currents of exactly the circuit you see. A test script then checks that the numbers on the page are the numbers it found.
Reading the drawings
Circuits are drawn as schematics: each component has a symbol, and lines are wires. A battery is a long and a short plate, with the long one marked +. A resistor is a small rectangle. A lamp is a circle with a cross. A switch is a lever between two contacts. Where wires join, a dot marks the join.
A schematic shows how things connect, not where they sit. Two wires that cross without a dot are not joined - they just pass each other on the page.
On a battery symbol, which plate is the + terminal?
Show the answer
Answer: A. The long plate is +. It matters, because many components - LEDs, for one - only work one way round.
0.4 Staying safe
Low voltages from batteries and small supplies are safe to touch. Mains electricity is not - it can kill - and this course never uses it. Even small batteries can get dangerously hot if they are short-circuited.
Mains electricity
Mains electricity from a wall socket is 230 V in India and the UK and 120 V in North America. It can drive a deadly current through a body. Nothing in this course uses it: every circuit here runs from a battery or a small supply of 12 V or less.
Short circuits
A short circuit is a path with almost no resistance straight across a supply. Every battery has a little internal resistance inside it, and in a short circuit that is all that holds the current back:
circuit short: current 4.5 A; heat inside the battery 40.5 W
Forty watts of heat inside a small battery makes it hot very quickly. It can leak or burst, and a lithium battery can catch fire. So never connect a wire straight across a battery.
Power ratings
Every component can only get rid of so much heat. A small resistor usually has a power rating of a quarter of a watt:
power = V x V / R = 9 x 9 / 100 = 810 mW, more than a quarter-watt resistor can take
Volume 02 shows where that formula comes from. For now, remember that resistors have a limit, and a hot resistor is a warning sign.
Assuming that a low voltage is always harmless to the circuit. It is safe for you, but a short circuit from a 9 V battery still pushes amps through the wires and heats the battery. Low voltage keeps you safe; it does not keep the parts safe.
Why does a shorted 9 V battery get hot?
Show the answer
Answer: D. With almost nothing else in the loop, the current is limited only by the battery's internal resistance, and that resistance turns the energy into heat inside the battery.
What you learned
- Electronics makes charge flow where you want it, using components that control currents.
- Current only flows round a complete loop, and it is never used up on the way.
- Voltage is the push (volts), current the flow (amps), and resistance holds it back (ohms).
- 9 V across 90 ohms drives 100 mA; with the switch open, nothing flows.
- A schematic shows connections with symbols; the long plate of a battery is +.
- Mains electricity can kill; this course uses 12 V or less.
- A short circuit drives a large current through a battery's internal resistance and makes heat; resistors have power ratings.
Key words from this volume
Every word below has a plain-English entry in the glossary.
- Electronics
- Component
- Circuit
- Voltage
- Current
- Resistance
- Electric charge
- Battery
- Mains electricity
- Short circuit (electrical)
- Internal resistance
- Power rating
Practice
A 12 V lamp
A 12 V battery lights a lamp of 48 ohms. What current flows, and how much power does the lamp use?
Show the solution
12 V across 48 ohms: current 250 mA, power 3 W
A resistor that is safe
A 1 kΩ resistor sits across a 9 V battery. Is a quarter-watt resistor big enough?
Show the solution
9 V across 1000 ohms: current 9 mA, power 81 mW
81 mW is well under a quarter of a watt (250 mW), so yes.
A worse short
A different 9 V battery has only 1 ohm of internal resistance. What happens if it is shorted?
Show the solution
a 9 V battery with 1 ohm inside, shorted: current 9 A, heat 81 W
The lower the internal resistance, the bigger the short-circuit current. Batteries that can deliver a lot of current are exactly the ones that are most dangerous to short.
Interview corner
Voltage and current
"Explain the difference between voltage and current."
Show the solution
"Voltage is the push that drives charge round a circuit, measured in volts. Current is the rate at which charge actually flows, measured in amps. You can have a voltage with no current - a battery on a shelf has 9 V across it but no current, because there is no loop. Current needs both a voltage and a complete path."
Volume 01 looks at voltage, current and resistance one at a time, and how to measure each one.