Electronics/Circuit fundamentals

What voltage means

After thiswhat you will be able to doCalculate energy transferred per charge from a voltage, compare potentials only through their difference, and explain why voltage can exist without current.

Questionwhat this lesson answersA battery can have a voltage across its terminals even when nothing is connected. What is voltage, and why is it the quantity that makes charge transfer possible?

Not coveredwhat this lesson leaves outWe define voltage as energy transferred per unit charge and use an ideal source. We do not yet explain how a source creates the separation of charge or what current a connected circuit will draw.

Start with a circuit that is not connected. Put a battery on a table and do not attach a wire between its terminals. A voltmeter can still report a difference between those terminals. There is no sustained current through an open path, but there is a voltage waiting across the gap. That fact is the first break with the everyday picture: voltage is not the flow itself.

Energy per charge

When a charge moves from one point to another, an electric field can transfer energy to it, or a component can take energy from it. The useful question is not only how much total energy changed, but how much changed for each amount of charge. Potential difference is that ratio:

One volt is one joule per coulomb:

So a source marked ${\SI{9}{V}}$ supplies, in the ideal model, ${\SI{9}{J}}$ of energy to each ${\SI{1}{C}}$ of charge that it moves through its interior from its lower-potential terminal to its higher-potential terminal. The label does not say how many coulombs move. It says what each coulomb could receive.

That is why the word difference matters. A single point does not have a useful voltage by itself; it has a potential relative to a reference point. A circuit diagram often calls one chosen reference ground and writes its potential as zero. Moving the reference changes all the individual labels by the same amount, but it does not change any difference between two points. The component only responds to the difference across its own terminals.

A source separates charge

Inside a battery, chemical reactions push charge uphill in electric potential. The battery spends chemical energy doing this, leaving one terminal at higher potential than the other. Outside the battery, a connected path lets the electric field guide charge back through components toward the lower-potential terminal. The source and the outside path together make the arrangement useful.

The battery is not a tank containing a fixed supply of voltage. Voltage is not a substance that gets used up as charge passes through a resistor. Energy is transferred: the source raises the energy per charge, and a resistor or lamp lowers it by taking that energy. The voltage across a component describes that drop or rise between its two terminals.

We will use an ideal source to keep the first calculations clean. An ideal ${\SI{9}{V}}$ source holds a ${\SI{9}{V}}$ difference no matter what is connected. A real battery has internal resistance, its terminal voltage changes with load, and its chemistry eventually runs down. Those are important facts about sources, but they belong behind a door. The first quantity is the energy-per-charge difference we can measure at the terminals.

Voltage does not imply current

Close the circuit with a resistor and charge can move through the path. Leave the switch open and the source can still maintain its terminal voltage while the steady current in the gap is zero. The source has established a difference, but there is no continuous route for charge to cross.

This gives voltage a precise role in the sequence:

1. A source establishes a potential difference.
2. A connected path provides somewhere for charge to move.
3. The path’s resistance determines how much current results.

Voltage is therefore the amount of energy offered per unit charge, not the amount of charge, not the speed of charge, and not the current. To describe how much charge crosses a point in a given time, we need a different ratio. That is what current measures.