To understand transistors, keep voltage, current, resistance, and power separate.
They are related, but they are not the same thing.
Four Roles
| Concept | Question It Answers | First Role |
|---|---|---|
| Voltage | How much electrical potential difference exists? | Driving condition |
| Current | How much charge is flowing? | Flow |
| Resistance | How strongly is flow opposed? | Limiting condition |
| Power | How fast energy is being transferred? | Energy rate |
Voltage can exist with almost no current. Current requires a path. Resistance limits current. Power describes energy transfer and heat.
The simplest useful formulas are:
V = IR: voltage equals current times resistanceP = VI: power equals voltage times current
These are not the whole of electronics, but they prevent a common confusion: “electricity” is not one thing.
Why Voltage Can Represent a Bit
Digital circuits usually interpret voltage ranges as logical states.
A wire might be treated as:
- low enough to count as
0 - high enough to count as
1 - in between, where the value should not be treated as stable
The circuit is not reading a philosophical zero or one. It is reading an electrical condition.
That condition can be maintained only if the circuit has a path to a reference, a source of energy, and devices that can drive the voltage into reliable ranges.
Why Current Still Matters
If voltage represents the state, current is still needed to change or maintain real circuits.
Current flows while:
- a node is being charged or discharged
- a load consumes energy
- leakage paths allow small unwanted flow
- a circuit temporarily connects supply to ground during switching
This is why a digital value is not free. Even when the logical model says “set this bit to 1,” the physical circuit must move charge.
Boundary
Do not reduce transistor behavior to only voltage or only current.
For the rest of this route, use this mental model:
Voltage is the condition a circuit can interpret. Current is the movement that changes physical state. Resistance and device behavior shape how that movement happens. Power is the cost of moving energy through the circuit.