Voltage, Current & Resistance: The Core Quantities

Lesson 1 · Voltage (V) — Electrical Potential Difference

Level: Beginner

• Explain what voltage represents in your own words.
• Use the volt (and millivolt) correctly and read common voltage levels.
• Describe polarity and why voltage is a difference between two points.

What voltage is

Voltage is the electrical push. More precisely, it is the difference in electric potential between two points — a measure of how much energy each unit of charge would gain or lose moving from one point to the other. A battery or power supply creates this difference: its + terminal sits at a higher potential than its − terminal. Until you connect a path between the two, nothing moves, but the push is there and waiting. That's why a fresh battery on a shelf has a voltage you can measure with a meter even though no current is flowing.

A helpful analogy — water and height

Picture a tank of water. Water near the top has more gravitational potential energy than water near the bottom, and it "wants" to flow downward if you open a pipe. The height difference is what drives the flow. Voltage is the electrical version of that height difference. Two points at the same height (the same potential) produce no push at all — the water stays still, and the charge stays still.

Units and symbols

The symbol for voltage is V and its unit is the volt, also written V (named after Alessandro Volta, who built the first practical battery). For small voltages we use millivolts: 1 mV = 0.001 V. A useful formal definition is that one volt is one joule of energy per coulomb of charge — but for now, "volts = push" is the idea to keep in your head.

Polarity and potential difference

Voltage is always a difference, measured between two points, never meaningfully "at" a single point on its own. We label the higher-potential point + and the lower one −. When we say a battery is "9 V", we mean its + terminal sits 9 volts higher than its − terminal. This gap is called the potential difference (p.d.), and in practice "voltage" and "potential difference" mean the same thing.

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Lesson 2 · Current (I) — The Flow of Electric Charge

Level: Beginner

• Describe current as the rate of flow of charge.
• Tell conventional current and electron flow apart.
• Convert between amps, milliamps and microamps, and read typical currents.

What current is

If voltage is the push, current is the result: the actual flow of electric charge. Current tells you how much charge passes a given point in the circuit every second, and it is measured in amperes (amps, A). A circuit only has current when there is both a push (voltage) and a complete, closed path for the charge to travel around. Break the loop anywhere and the current stops instantly.

Which way does it flow?

Two directions are in play, and it's worth learning both now. Historically, engineers defined current as the flow of positive charge — this is conventional current, drawn from the + terminal toward the − terminal. In metal wires the particles that actually move are electrons (which are negative), and they travel the opposite way, from − toward +. Both descriptions are correct and consistent; by convention we draw arrows for conventional current (+ to −). As long as you stay consistent, every calculation works out.

Units you'll meet

Amps are large for everyday electronics, so we use smaller units. The milliamp (mA) is one-thousandth of an amp (1 mA = 0.001 A), and the microamp (µA) is one-millionth (1 µA = 0.000001 A). An LED runs on a few tens of milliamps, a sleeping sensor might sip microamps, and a phone charger pushes a whole amp or two.

How current behaves in a simple circuit

In a single-loop (series) circuit the current is the same at every point. Charge isn't created or destroyed in the loop, so whatever flows out of the battery flows through the resistor and back — like the same flow rate through every section of one pipe. This is the key difference from voltage, which is a "level": voltage can exist with no current (a battery on a shelf), but in an ordinary circuit you need that voltage to make current flow.

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Lesson 3 · Resistance (R) — Opposition to Current

Level: Beginner

• Explain what resistance is and why it matters.
• Use ohms, kilohms and megohms, and read a 4-band resistor.
• Predict what happens to current when resistance changes.

What resistance is

Resistance is how strongly a material or component opposes the flow of current. It is the reason a given voltage doesn't push an infinite amount of current. Metals like copper and silver are conductors — low resistance, current flows easily. Materials like rubber, glass and plastic are insulators — very high resistance, current barely flows at all. The unit is the ohm, written Ω (named after Georg Ohm). One ohm is the resistance that lets exactly one amp flow when one volt is applied — you can already see Ohm's Law hiding in that definition.

Common values and units

Resistors are made across a huge range of values, so we use prefixes: ohms (Ω), kilohms (kΩ = 1,000 Ω) and megohms (MΩ = 1,000,000 Ω). A 100 Ω resistor limits current far more than a 10 Ω one at the same voltage, while a 1 MΩ resistor is used where you want almost no current to flow at all.

How a resistor controls current — and its color code

Put a resistor in a circuit and it "uses up" part of the voltage, limiting the current to a safe value — which is exactly what delicate parts like LEDs need. Real resistors show their value with colored bands (or a printed number). A common 4-band resistor reads as: two significant digits, a multiplier, and a tolerance. For example, brown–black–orange–gold is 1, 0, ×1,000, ±5% → 10,000 Ω = 10 kΩ. Our Resistor Color Code calculator does this decoding for you.

What happens when resistance changes

With voltage held steady, resistance and current are opposites. Increase the resistance and the current falls; decrease the resistance and the current rises. Push the resistance toward zero and the current grows without bound — that's a short circuit, and it can overheat wires, drain or destroy a battery, and even start a fire. This single idea — voltage pushes, resistance opposes, and current is what's left — is the whole heart of the next lesson.

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Lesson 4 · How They Interact — Ohm's Law

Level: Beginner → Intermediate

• Explain the V–I–R relationship in words before using formulas.
• Apply V = I·R and rearrange it to find any of the three.
• Solve step-by-step calculations and connect them to real circuits.

Putting it together — in words first

Now we have all three players: voltage pushes, resistance opposes, and current is the flow that results. For an ordinary resistor the relationship is perfectly proportional. Double the voltage and you double the current (same resistance). Double the resistance and you cut the current in half (same voltage). Raise the voltage and raise the resistance, and you have to weigh both effects against each other. This proportionality — current is directly proportional to voltage and inversely proportional to resistance — is Ohm's Law.

The formula and its friends

Written as a formula, Ohm's Law is V = I × R. Because it's one equation with three unknowns, you can rearrange it to solve for whichever you're missing: current is I = V ÷ R, and resistance is R = V ÷ I. The triangle (V on top, I and R below) is a handy memory aid — cover the quantity you want and read off the operation.

Step-by-step calculations

Try it live — the interactive circuit

Now you can feel the relationship instead of just reading it. Change the voltage or resistance and watch the current respond: the animated flow speeds up as current rises, and the resistor glows as it dissipates more power. Try a preset, then drag the sliders and see what happens. (This is the same circuit from Lesson 1's diagram, now under your control.)

Ohm's Law in the real world

Real circuits are just this relationship in action. A 12 V car battery through a 6 Ω heater draws 2 A and gives out 24 W of heat. A 5 V USB port through a 250 Ω resistor produces 20 mA — just right for many LEDs. Whenever you look at a circuit, you can usually predict its behaviour by asking three questions: what's the voltage, what's the resistance, and so what's the current?

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