If you've ever wondered why voltage always needs two points, or what GND on a schematic actually means, or whether GND is "the ground" you stand on — this is the episode that clears all of that up.
In Episode 5 of the Electric Circuits Textbook series, we'll work through what voltage really is (and how it's different from current), why a single point's potential can't be stated without a reference, and how ground (GND) plays that role in every circuit. We'll also disentangle GND from the earth ground (a.k.a. "earthing") your wall outlet uses for safety — they're not the same thing.
If you missed Episode 4 (why current and electrons flow in opposite directions), that's the previous post. This one stands on its own.
Today's Goal
Five takeaways:
- What voltage is — the difference between two points (not a property of a single point)
- Voltage vs. current — applied across vs. flows through
- Why you need a reference to state a single point's potential as a number
-
What GND is — the point you've chosen as
0Vfor that circuit - What changes (and what doesn't) when you re-pick the reference
Plus a 3-question quick check, and a critical field note: GND on a schematic ≠ earth ground on your wall socket.
What Is "Voltage"?
Voltage is the difference in electric potential between two points. That's it.
It's often described with the analogy of "a force pushing electricity" — and that's useful as long as you don't take it literally (it's not a force in the Newtonian sense). Physically, voltage tells you how much energy each unit of charge can be given as it moves between those two points. Episode 1's separation of charge and energy shows up again here: voltage is the energy-per-charge that can be released as positive charge moves from the high-potential point to the low-potential point (i.e. in the conventional-current direction from Episode 4).
The crucial property of voltage:
You can't measure the voltage at a point. You can only measure the voltage between two points.
That's not a quirk of the math. It's the definition. Voltage requires two points.
The height analogy
Voltage works exactly like height differences:
| Height world | Electrical world |
|---|---|
| The height of a place | Potential (electrical "height" at a point) |
| The height difference between two places | Voltage (potential difference between two points) |
| A bigger height difference makes a ball roll faster | A bigger voltage delivers more energy per unit of charge |
The unit of voltage is the volt (V). A wall outlet might be around 100V (Japan), 120V (US), or 230V (most of Europe). A standard AA battery is 1.5V.
The Decisive Difference Between Current and Voltage
This is the chapter's biggest point. Two words that sound similar but mean completely different things.
| Current | Voltage | |
|---|---|---|
| What it is | The amount of charge crossing a cross-section per second | The potential difference between two points |
| Unit | Ampere (A) | Volt (V) |
| How it relates to a part | Flows through the part | Is applied across the part's two ends |
| Water analogy | Water actually moving in the pipe | Difference in water level between two points |
The cleanest one-liner I've ever heard:
Current flows. Voltage is applied.
That's it. Internalize that phrasing and you'll never confuse the two again. The water analogy makes it concrete: water flows through the pipe (= current), driven by a water-level difference between two reservoirs (= voltage).
Without a Reference, a Point's Potential Isn't Defined
OK — voltage is a difference between two points. Easy enough.
But then there's a follow-up question that trips a lot of people up:
"What's the voltage at this point?"
In strict terms, that question is ill-formed. You can only state a voltage between two points. But practically, we often want to assign a number to a single point — "this pin is at 3.3V" — and we do. How?
By picking a reference. We choose one point in the circuit and declare it to be 0V. Once we've done that, every other point in the circuit has a well-defined potential relative to that reference.
A point's potential, measured relative to a chosen reference, is what we just call "the voltage at that point." It's shorthand for "the difference between that point and the reference."
The mountain-height analogy
Heights work the same way. "How tall is the difference between person A and person B?" is well-defined without picking a reference. But "How tall is Mt. Fuji?" depends on what you call zero. In most maps, the reference is mean sea level — the average height of the ocean surface. Mt. Fuji is 3,776m above mean sea level. Pick a different reference (e.g., the center of the Earth) and the number changes.
Same idea: to state a single point's potential as a number, you have to pick a 0V reference.
Ground (GND) — the 0V Reference Point
Enter today's second main character: ground.
Ground (GND) is the point in a circuit that you've chosen to call 0V. That's the whole definition. It's a convention. It's whatever the designer picks.
On schematics, you mark this point with a dedicated ground symbol (a few horizontal lines that taper, or an inverted triangle). The symbol means "this is the 0V reference for that circuit or section."
In the height analogy: ground is the "mean sea level" you chose for this particular circuit. Once you've planted that flag, every other point has a definite electrical "height" — its voltage relative to GND.
The GND node is real wiring — but its value of
0Vis not an absolute voltage handed to you by nature. It's a choice the designer makes. Different circuits choose different points.
The most common choice in DC circuits is the negative terminal of the battery / power source. That's just convention — there's nothing physically special about it.
What Changes (and Doesn't) When You Move the Reference
This is the cleanest demonstration of the whole "voltage = difference" idea. Take a 1.5V battery and try two different reference choices:
Choice 1: negative terminal = GND (0V) — the usual convention
- Negative terminal:
0V - Positive terminal:
+1.5V - Voltage across battery:
1.5V
Choice 2: positive terminal = GND (0V) — just to prove the point
- Positive terminal:
0V - Negative terminal:
−1.5V - Voltage across battery:
1.5V
| Reference | Positive terminal | Negative terminal | Voltage across battery (= difference) |
|---|---|---|---|
| Negative = GND | +1.5V | 0V | 1.5V |
| Positive = GND | 0V | −1.5V | 1.5V |
Look at the right column. The voltage across the battery is 1.5V either way. What changed is the labels on each terminal. What didn't change is the difference between them.
The takeaway — when you re-pick the reference, the single-point potentials shift, but every difference in the circuit stays exactly the same. Voltages between any two points are reference-independent. Potential at a point is reference-dependent.
This is why GND being "just a convention" doesn't matter for the physics: nothing the components actually do depends on which point you labeled 0V.
Field Note: GND ≠ Earth Ground
Here's a distinction that catches a lot of beginners — and even some experienced people who never had it pointed out:
The GND on your schematic is not the same as the earth ground (the dirt under your house) that wall sockets use for safety.
They have different symbols. They serve different purposes. And confusing them can be dangerous.
Two distinct concepts
| Concept | What it is | Purpose |
|---|---|---|
| Circuit GND | A point in your circuit you've chosen as 0V
|
A reference for measuring voltages |
| Earth ground | A protective earth conductor bonded to the building's earthing system | A safety return path / reference for fault currents |
A circuit's GND can be floating — not connected to the physical earth at all. A battery-powered toy is a perfect example: its GND is just "the negative side of the battery," with no path to earth.
Why the confusion matters
If a fault inside a powered device causes its metal case to become electrically energized, and you touch the case while standing on the ground, you could complete the circuit through your body — that's an electric shock. Earth grounding is part of a system that prevents this: it gives fault currents a low-impedance path back to the source through the protective earth conductor, instead of through you. Overcurrent protection (fuses, circuit breakers) trips on that fault current, and GFCI / RCD devices catch leaks by comparing the outgoing and returning current in the live and neutral conductors — if any current is missing (e.g. it's flowing through a person or insulation defect), they shut the circuit down.
But earth grounding alone isn't a complete safety system — it needs to be paired with proper overcurrent protection and, ideally, residual-current devices, to actually keep you safe. The detailed mechanics are beyond today's scope.
What you should take away: circuit GND is about measurement; earth ground is about safety. They're not the same thing, even though they share a name. Pros always check both separately.
Quick Check — 3 Questions
Three questions to lock the ideas in. Pause before peeking.
Q1. Fill in the blank: "What is applied across the two ends of a component is ___."
Current, or voltage?
Q2. The GND symbol on a schematic is always physically connected to the actual earth ground.
True or false?
Q3. You leave the wiring alone but re-pick which point you call
0V(you move the GND reference). What changes?
- Each point's potential (the number assigned to it)
- The voltage between any two specific points
Got your answers?
Quick Check: Answers
The Q3 insight is what makes circuit analysis actually work. Every node has a "potential," but those potentials are only meaningful relative to a chosen reference. Every difference — every voltage that matters for physics — is reference-independent.Click to reveal the answers
#
Answer
Why
Q1
Voltage
Voltage is applied across the two ends of a component. Current is what flows through it. "Applied" vs. "flows" — that's the key.
Q2
False
GND on a schematic is just the point chosen as the
0V reference for that circuit. Many circuits' GND has no physical connection to the earth — battery-powered devices are an obvious example. Earth grounding (for safety) is a separate concept.
Q3
1. Each point's potential changes
When you re-pick the reference, the numbers on individual points change (e.g., what was
+1.5V might now be 0V). But voltage = difference between two specific points, which stays the same no matter where you put the reference.
Section Summary
Today's single thread:
- Voltage = potential difference between two points (V)
- It's the energy-per-charge that's available as charge moves between those two points
- Voltage is applied across a component; current flows through it. Both, not either
- A point's potential is only well-defined relative to a reference
- The reference you pick for the circuit is called ground (GND) — by convention
0V - Moving GND changes each point's potential value but not the voltage between any two points
- GND on a schematic ≠ earth ground used for safety. Different concepts, different symbols, different purposes
This reference-frame idea — that some quantities are observer-dependent (potentials) while others are not (potential differences) — runs through all of physics, not just circuits. Once you see it here, you'll spot it again in mechanics (heights), thermodynamics (entropy reference states), and relativity (coordinate transforms).
Next episode: DC vs. AC — direct current vs. alternating current, the difference between a battery and a wall socket, and why the world's power grid uses one but our electronics mostly want the other. See you in Episode 6.
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