Voltage — The Pressure of Electrons

Section Overview

This section takes the most important quantity in electronics — voltage — and builds it from the plain-English "pressure" picture you met in What Is Electricity? into a working concept you can apply to real devices. You will see why voltage only exists between two points, what the numbers printed on batteries and adapters actually promise, and how voltage behaves inside a working circuit.

Why This Matters

Most of the measurements you will ever make as a repair technician are voltage measurements. When a device is dead, the first diagnostic question is almost always "is the expected voltage present, and is it the right value?" You cannot answer that question — or even ask it properly — until voltage stops being a vague word and becomes a physical idea you can reason about.

Required Prerequisites

What Is Electricity? — atoms, electrons, conductors, and the open/closed circuit concept.

Recommended Consumables

No consumables required for this section. The practice exercises use household items you read or measure, not items you use up.

Recommended Practice Hardware

A few household batteries (AA, AAA, 9 V) — fresh and partially used
A USB charger and cable you can read the label of
A laptop power adapter (for label reading only — do not open it)

Do NOT practice on any device you intend to use, sell, or return.

Real-World Applications

Every "no power" diagnosis starts with voltage: confirming a charger really outputs what its label claims, confirming a battery is actually charged, confirming a device's internal supply rails are present. When you later probe a dead game console and find the 5 V rail reading 0 V, this section is the foundation that tells you what that means.

Common Challenges

Beginners most often struggle with two ideas. First, that voltage does not flow — it is a difference in electrical pressure between two points, and it can exist with nothing flowing at all (a battery sitting in a drawer still has 1.5 V across its terminals). Second, that a voltage reading is meaningless until you know which two points it was taken between. Both ideas feel abstract at first and become second nature with practice — every experienced technician once tripped over them too.

Safety Notes

Risk Level: Low. This section is conceptual and the practice uses household batteries only.

Professional Tips Before Starting

Get in the habit of checking a label or spec sheet before you connect anything — professionals verify voltage and polarity as a reflex, not an afterthought.
When you see a voltage number, silently ask "relative to what?" before you trust it. This habit alone prevents most of the confused readings beginners report.
Keep a small "practice bin" of fresh and partially used household batteries set aside just for exercises like these, separate from batteries you actually rely on — so you're never tempted to skip a hands-on step to avoid using something you need.

From Potential Difference to Voltage Drop

Voltage Is a Difference, Not a Thing

In the previous section you saw that voltage is the electrical pressure that pushes electrons through a conductor. Here is the refinement that makes the idea precise: voltage is always a difference in electrical pressure between two points. The technical name for this is potential difference.

Think of water in two tanks connected by a pipe with a closed valve. If one tank is fuller, there is a pressure difference between them. Nothing is flowing — the valve is shut — but the push is there, waiting. A 9 V battery on a shelf is exactly this: 9 volts of potential difference between its terminals, pushing on nothing until a circuit gives the charge somewhere to go.

This is why the question "what is the voltage at this point?" is incomplete. A measurement needs two points: the point you care about, and a reference point. Voltage is like distance — saying "this town is 40 km" means nothing until you say from where.

Ground — The Agreed-Upon Reference

Because voltage always needs two points, circuits adopt a shared reference point called ground. Ground is the point in a circuit that everyone agrees to call 0 V, so that every other voltage can be stated as a single number relative to it. When a schematic says "this pin sits at 3.3 V," it means 3.3 V between that pin and ground.

In battery-powered devices, ground is usually the battery's negative terminal. The name comes from mains electrical systems, which are physically connected to the earth — but in a phone or console, "ground" is the common reference point, not a literal connection to soil.

In repair work, this is immediately practical: when you eventually probe a board, one meter lead almost always rests on ground while the other explores the circuit.

Polarity — Which Way the Pressure Pushes

A voltage difference has a direction: one point is at the higher potential (+) and one at the lower (−). This is polarity. On batteries it is marked on the case; on DC power connectors it is shown by a diagram (commonly a center-positive symbol on laptop adapters); on circuit boards, components like electrolytic capacitors and LEDs are marked because they only tolerate one orientation.

Polarity matters because many components are direction-sensitive. Reversing the supply polarity on a board is one of the fastest ways to destroy it — which is why "confirm polarity before connecting power" appears in every competent repair procedure.

Beginner Note: Polarity describes the push direction of the supply. Conventional current — the direction schematics and markings use — is defined as flowing from + to −, even though electrons physically drift the other way. Every schematic you will ever read uses the conventional view.

The Voltage Levels You Will Actually Meet

Electronics uses a small set of recurring voltage levels. Learning them turns labels and schematics from noise into information. The table below lists nominal values. How much a real measurement may differ depends on the source: regulated supplies and logic rails typically measure within about ±5% of nominal, while battery voltage legitimately varies with charge state — as the ranges in the table show.

| Nominal voltage | Where you meet it | |---|---| | 1.5 V DC | AA/AAA alkaline cells (≈1.6 V fresh, ≈1.0–1.1 V depleted) | | 3.7 V DC | Lithium-ion cells — phones, controllers (≈4.2 V full, ≈3.0 V empty) | | 5 V DC | Classic USB power, many logic circuits | | 3.3 V / 1.8 V DC | Internal logic rails on modern circuit boards | | 9 V DC | Rectangular PP3 batteries, guitar pedals | | 12 V DC | Automotive systems, desktop PC rails, routers | | 19–20 V DC | Laptop power adapters; USB-C Power Delivery at its highest common level (20 V) | | 120 / 230 V AC | Mains wall power — North America / most of Europe |

Notice the last row says AC — alternating current, where the polarity swaps many times per second. Everything else in this section deals with DC (direct current), where polarity is fixed. AC behavior gets its own treatment in a later chapter (Coming Soon); for now it is enough to recognize the distinction on labels.

Voltage Drop — Where the Pressure Goes

When current flows through a circuit, the supply's voltage gets divided up across the components in the path. Each component "uses up" part of the pressure — this is called voltage drop. The drops along any single path from + to − always add up to the supply voltage: a 9 V battery driving two identical resistors in a loop puts about 4.5 V across each.

This single idea is the backbone of diagnostics. If a component should be dropping 5 V and you measure 0 V across it, either no current is flowing or the component is being bypassed. If a connector that should drop almost nothing is dropping 2 V, it is corroded or damaged and burning pressure that the device needs. You will formalize this with Ohm's Law shortly — for now, hold onto the picture: the supply provides the pressure, and the circuit spends it.

Common Mistakes

Treating voltage as something that flows. Voltage pushes; current flows. Saying "the voltage flows through the wire" leads to wrong diagnostic reasoning later. Consequence: confusion when a point shows full voltage but the device still does not work.
Quoting a voltage without a reference point. "Pin 3 is at 3.3 V" silently means "relative to ground." Forgetting this makes measurements between arbitrary points seem mysterious. Consequence: misread measurements and false fault reports.
Assuming the label equals reality. A "5 V" charger may output 5.2 V, or 0 V if it has failed. Nominal values are expectations to verify, not facts. Consequence: replacing good parts while the real fault — a dead supply — goes unnoticed.
Equating low voltage with zero risk. A 12 V car battery cannot shock you, but short its terminals with a wrench and the resulting current can cause burns and fire. Voltage is only half of the danger story — current completes it, as the next section explains.

Troubleshooting Guidance

This section is conceptual, so troubleshooting here means troubleshooting your understanding. If voltage drop feels slippery, return to the water analogy: a pump (supply) raises pressure once, and every narrowing in the pipe (component) uses some of it; the total used always equals the total supplied. If ground feels arbitrary, remember it is a convention, like choosing sea level for altitude — the physics only ever cares about differences. If the ideas still resist after the verification checklist below, re-read the previous section's electron model first; voltage builds directly on it.

Verification & Testing Methods

You can verify your understanding right now, without tools:

[ ] Explain to yourself (out loud) why a battery in a drawer has voltage but no current.
[ ] Name the two points involved when someone says "the rail is at 5 V."
[ ] State the polarity convention: which terminal does conventional current leave the supply from?
[ ] Predict the voltage across each of two identical components sharing a 9 V supply in a single loop.

The Chapter Quiz at the end of this chapter checks exactly these ideas — a score of 80% is required to continue.

Practice Exercises

Label hunt (10 minutes). Collect three power sources in your home — a phone charger, a laptop adapter, and any battery. Find and write down each one's output voltage and, for the DC sources, the polarity marking. You are building the habit of reading specifications before connecting anything.
Battery sort (5 minutes). Gather your loose household batteries and group them by nominal voltage. For each group, note where the + terminal is physically located.
Optional multimeter preview (5 minutes). If you already own a multimeter: set it to DC volts with the leads in the V/Ω jacks (never the A jacks), touch red to a AA battery's + terminal and black to −, and note the reading (a fresh cell reads about 1.6 V). Then swap the leads and observe the minus sign — that is polarity, live. Full meter technique is taught in [Practical Measurement: Reading Voltage and Current] (Coming Soon).

These ideas are tested in the Chapter Quiz at the end of this chapter — wrong answers there come with a link back to the material that teaches the concept.

Key Takeaways

Voltage is a potential difference — electrical pressure between two points, never at one point alone.
Voltage can exist without current; current cannot flow without voltage.
Ground is the agreed 0 V reference that lets single numbers describe circuit voltages.
Polarity marks the direction of the push; reversing it destroys direction-sensitive components.
Common levels — 1.5 V, 3.7 V, 5 V, 3.3 V, 12 V, 19 V, mains — each signal where you are in a device.
Labels state nominal values; regulated supplies typically measure within about ±5%, while battery voltage varies with charge state.
Voltage drop divides the supply's pressure across the circuit; the drops along any single path from + to − sum to the supply.
Low voltage means low shock risk, not low stakes — wrong connections still destroy boards.

Skills Learned

You can now explain voltage as a pressure difference between two points, not a substance that flows through a wire.
You can now read a power source's label and know what voltage to expect from it — batteries, USB, adapters, or mains.
You can now find and interpret the polarity marking on a battery or DC connector before connecting anything.
You can now judge which voltages you're handling are low-risk and which call for extra caution.

Glossary Additions

potential difference — The formal name for voltage: the difference in electric potential between two points, which determines how strongly charge is pushed from one to the other. A voltage value is meaningless without knowing both points it was measured between.
ground — The point in a circuit that is agreed to be the 0 V reference, so every other voltage can be stated as a single number relative to it. In battery-powered devices this is usually the battery's negative terminal; in mains systems it is physically connected to the earth.
polarity — The orientation of a voltage difference: which point is at the higher potential (+) and which is at the lower (−). Many components only tolerate one orientation, so reversing supply polarity can destroy a circuit.
voltage drop — The portion of a supply's voltage that appears across an individual component when current flows. The drops along any single path from + to − add up to the supply voltage, which makes measuring voltage drops a core diagnostic technique.

Suggested Next Sections

Must read next:

Current — The Flow of Electrons (Coming Soon) — the next section in sequence, and it resolves the conventional-current-vs-electron-flow distinction this section deliberately deferred.

Recommended:

Ohm's Law — The Foundation of Circuit Analysis (Coming Soon) — teaches how to actually use the V = I × R relationship this section only previewed.
Practical Measurement: Reading Voltage and Current (Coming Soon) — the full multimeter technique this section's optional practice exercise only previewed.