Three quantities, one story
Almost every electronics question comes back to three ideas: voltage, current and resistance. They describe one situation from three angles, and once they feel natural, datasheets and diagrams become much easier to follow.
The water analogy is imperfect but helpful. Picture a header tank feeding a pipe.
- Voltage is like the water pressure, set by how high the tank sits. It is the push available.
- Current is like the flow rate: how much water passes a point each second.
- Resistance is like the narrowness of the pipe, or something partly blocking it. It limits the flow for a given pressure.
More pressure through the same pipe gives more flow. A narrower pipe at the same pressure gives less. That intuition is nearly all of the subject.
Voltage: the push
Voltage is the difference in electrical potential between two points. It is always a comparison, which is why you measure it between two places, and why “the voltage at a point” usually means relative to ground.
A battery has a voltage because its chemistry creates a difference between its terminals. A small hobby board often works from 3.3 V or 5 V, and a USB port is commonly 5 V. Voltage by itself does not flow; it is the available push. A battery sitting on a shelf with nothing attached has voltage but no current.
Current: the flow
Current is the movement of electric charge, measured in amperes, or amps. Hobby electronics commonly deals in milliamps, where one milliamp is a thousandth of an amp.
Current needs a complete loop. A source, a path and a load are all needed. In a simple series loop, the same current flows through every part, just as the same amount of water passes every point of a single pipe.
Current is what does the work and, in excess, the damage. It heats wires, drives motors and lights LEDs. Most component ratings, such as the maximum current of a pin or a wire, are really limits on current.
Resistance: the opposition
Resistance, measured in ohms, describes how strongly something opposes current. A good conductor, such as copper wire, has low resistance. An insulator, such as air or plastic, has very high resistance. Resistors are parts made to supply a chosen amount,.
Resistance also turns electrical energy into heat. That is useful in a heater and a nuisance in a wire that is too thin.
Ohms law as reasoning, not memorising
For many everyday materials and components, the three quantities are linked simply: the current is the voltage divided by the resistance. Written as letters, I = V / R. Rearranged, V = I x R and R = V / I.
Rather than memorising three formulas, hold the intuition:
- If voltage goes up and resistance stays the same, current goes up.
- If resistance goes up and voltage stays the same, current goes down.
- If you know any two, you can find the third.
Not everything obeys this neatly. LEDs and diodes, for instance, do not behave like fixed resistors; their current rises very steeply once they begin to conduct. Ohms law still applies to the resistor placed in series with them, and that is how it is used in practice.
Worked reasoning
Example 1: a resistor across a battery. A 9 V battery is connected across a 450 ohm resistor. Current is 9 divided by 450, which is 0.02 A, or 20 mA. This is a purely illustrative case, and real batteries have internal resistance and voltage that falls under load, so the real figure would differ somewhat.
Example 2: doubling the resistance. With 900 ohms instead of 450 at the same 9 V, current is 9 divided by 900, which is 0.01 A or 10 mA. Doubling the resistance halves the current, as the intuition predicted.
Example 3: what a pin might see. Suppose a 5 V signal is connected through a 1 kilohm resistor to an input pin that draws almost nothing. Almost no current flows, so there is almost no voltage across the resistor, and the pin sees close to the full 5 V. This is the reasoning behind how pull-up resistors work without wasting much power.
Power: why parts get warm
Power is the rate at which energy is used or turned into heat, measured in watts. It equals voltage times current.
Using Example 1: 9 V times 0.02 A gives 0.18 W. That is the power the resistor must dissipate as heat. A resistor rated well above that will cope; one rated far below will overheat. Any time current flows through resistance, some power is lost as heat, which is also why thin wires carrying large currents warm up, and why wire gauge matters.
Series and parallel in terms of these quantities
- Series. The same current flows through each part, and the supply voltage is shared among them. Adding resistors in series adds up their resistances.
- Parallel. Each branch sees the full voltage, and currents add up. Adding a branch gives the source an additional path, so total current rises.
This is why adding more parallel loads to a supply can overload it, even though each load is fine on its own. The supply has a current limit as well as a voltage.
Common misunderstandings
- “Voltage is what hurts.” Both matter. Voltage drives current through a body or a circuit, and it is the current that does damage. The relationship depends on resistance, and conditions vary widely, so treat any significant voltage with respect and follow safety guidance.
- “A bigger supply will push too much current into my part.” A part draws what its own resistance allows at the voltage it is given. A supply that can deliver more current does not force it, though too high a voltage can still damage something.
- “Resistance is bad.” It is a design tool. Without it, nothing would limit LED current.
- “Low voltage is always safe.” Low-voltage sources, especially batteries, can supply large currents into a short circuit and cause heat or fire.
Measuring them
A multimeter can check all three, with care. Voltage is measured across a part, resistance with power off, and current by putting the meter in series. Estimating with the rules above first gives you an expected answer, so a surprising measurement tells you something is wrong.
Safety
These explanations are meant for low-voltage hobby circuits. Mains electricity and large battery packs can be lethal or start fires, and rules and licensing vary by country. Follow manufacturer guidance and leave mains work to qualified people.