Voltage, current and resistance — study guide
The concept's fragments, read in order.
The three quantities behind every circuit
Electricity, at the level that matters for building things, is charge on the move. In a metal wire an enormous number of free electrons drift along when something pushes them, and that orderly drift is what lights a lamp, spins a motor, or wakes a chip. To reason about any of it you need just three quantities, and the whole of basic electronics is the interplay between them.
The three are voltage, current, and resistance. Voltage is the push that drives charge, current is how much charge actually flows, and resistance is how hard the path fights back. Change any one and the others respond, which is why they are always discussed together rather than one at a time.
They only do anything in a complete loop. Charge leaves a source, travels through the wires and whatever the circuit is powering, and returns to the source; break the loop anywhere and the flow stops everywhere. A circuit is that closed path, and voltage, current, and resistance are simply the three things you measure about it.
Voltage, the electrical push
Voltage is the push. More precisely, it is the difference in electrical potential between two points: a measure of how much energy each unit of charge carries from one point to the other. It is measured in volts, written V, and one volt means one joule of energy is available for every coulomb of charge that moves between the two points.
Because voltage is a difference, it is always between two points, never at a single one. Saying a battery terminal is at 1.5 V only means something relative to the other terminal. This is why voltage is also called potential difference: it compares two places, and the comparison is what does the pushing. like water pressure in a closed pipe: the push is there whether or not the tap is open, and it is the difference between two points that drives any flow once a path is opened
The push is there whether or not anything is flowing. A fresh 1.5 V cell sitting on a shelf, connected to nothing, still has its full voltage across its terminals — the potential is waiting, unused. Voltage is the cause that can drive charge, not the flow itself, and holding that distinction is what makes the next two quantities make sense.
Current, charge on the move
Current is the flow — the rate at which charge actually moves past a point in the circuit. Where voltage is the push, current is the response to it: give the charge a path and a push, and current is how much of it streams through. It is measured in amperes, written A and usually shortened to amps.
The definition is a rate. One ampere is one coulomb of charge passing a point each second, so current always answers how much per second, never just how much. A larger current means more charge crossing every second — more electrons streaming past any slice you pick along the wire.
In small electronics the numbers are modest. A single pin on a microcontroller can typically deliver only around twenty milliamps — twenty thousandths of an amp — and pushing much past forty can damage it. That is why current is the quantity to watch when you connect something hungry like a motor: the push may be fine, but the flow it demands can be far more than a delicate part is built to pass.
Resistance, opposition to flow
Resistance is opposition to flow — how hard a material fights the charge trying to move through it. Every real material resists to some degree: good conductors like copper resist very little, insulators like rubber resist so much that almost nothing flows, and the components called resistors sit deliberately in between. Resistance is measured in ohms, and the more ohms a path has, the less current a given push can drive through it. like forcing water through a narrow, rough pipe: it lets less through for the same push, and the friction warms the pipe
What sets a material's resistance is physical: the substance it is made of, how long the path is, how thick it is, and how hot it is. A long thin wire resists more than a short fat one of the same metal, and most materials resist a little more as they warm up. None of this needs a formula to feel — narrower and longer means harder to push through.
The opposition does not come free. The energy spent fighting through a resistance leaves the circuit as heat, which is why a resistor warms up, why an overloaded wire can get dangerously hot, and why heat is the constant background concern in anything carrying real current. Resistance is where electrical energy quietly turns to warmth.
How the three pull against each other
The three quantities are locked together. Fix a path and raise the voltage across it, and more current flows; hold the voltage steady and add resistance, and less current flows. Voltage pushes the current up, resistance holds it down, and the current that actually flows is the outcome of that contest — every circuit is two influences pulling in opposite directions on a single flow.
You can feel each direction on its own. Increase the push on the same path and more charge streams through. Swap in a component that resists harder, at the same push, and the flow drops. Neither result needs a number to see the direction: more push, more flow; more opposition, less flow.
There is an exact, simple law that turns this contest into arithmetic — a precise relationship that lets you calculate any one of the three from the other two. That law is the very next thing to learn, and it lands best once this qualitative picture is solid: push and opposition set the flow, and the flow is what a circuit is ultimately doing.
Units, magnitudes, and where you meet them
Each quantity has its own unit. Voltage is measured in volts (V), current in amperes — usually shortened to amps (A) — and resistance in ohms. Getting fluent with the units is half of reading any part's rating: they are the vocabulary every component is specified in.
The everyday magnitudes are worth carrying in your head, because they set what counts as normal. A single alkaline AA cell provides about 1.5 V, and a USB port supplies 5 V — the voltages of small electronics are mostly single digits. Currents there are small too: a status light draws a few milliamps, and a microcontroller pin safely sources only around twenty. Resistances run from near zero for a plain length of wire up to the hundreds or thousands of ohms of the resistors that shape a signal.
One habit pays off from the start. Because voltage is always between two points, every voltage reading is really a comparison — ask not just what the voltage is but between which two points, and the numbers stop being mysterious. Current, by contrast, is read along the path the charge takes, and resistance is a property of the component itself.
Why this is the floor of the build
Voltage, current, and resistance are the floor everything electronic stands on. Every part you will ever wire up is specified in them: a battery by its voltage, a motor by the current it draws, a resistor by its ohms. You cannot choose a part, size a power source, or protect a chip without reasoning in these three.
They are also where projects quietly fail. Feed a component more voltage than it is rated for and it burns; ask a source for more current than it can supply and it sags or cuts out; ignore the current a pin can pass and you damage the very chip running your code. Most fried electronics trace back to one of these three being wrong, not to a bug in the software.
For a build like a small self-driving car, this is the ground floor of the whole electrical side: picking a battery that supplies enough push and flow for the motors, adding resistance where a delicate part needs protecting, and keeping every current inside what its wire and its chip can bear. Get comfortable here and every later electrical idea — the exact laws, the circuit shapes, the power budgets — has something solid to attach to.