What Is Voltage

You cannot see voltage, but every circuit depends on it. Voltage is the driving force that pushes electrons through conductors and makes circuits do useful work. Before you can apply Ohm's Law, read a multimeter or troubleshoot a power supply problem, you need a solid understanding of what voltage actually is, how it is measured and where it comes from. This lesson covers all of that, with practical examples drawn from the workbench, the car and the home.

The Water Pressure Analogy

The easiest way to build intuition for voltage is to think about water. Imagine two water tanks connected by a pipe. If both tanks are at the same height, the water levels are equal, the pressure at each end of the pipe is identical and no water flows. Now raise one tank higher than the other. The elevated tank has more water pressure at its outlet than the lower tank. Water will flow through the pipe from the high-pressure end to the low-pressure end until the levels equalise.

Voltage works exactly the same way. A voltage source — a battery, a power supply, a generator — creates a difference in electrical potential between two points, just as the elevated tank creates a difference in water pressure. Electrons experience that potential difference as a force and flow from the point of higher potential to the point of lower potential through any conducting path between them. No potential difference means no force on the electrons, which means no current.

The analogy extends further: a taller tank (higher voltage) drives a stronger flow of water (larger current) through the same pipe (same resistance). A narrower pipe (higher resistance) restricts the flow even with the same tank height. This is Ohm's Law in hydraulic form, and it is why voltage, current and resistance are inseparable concepts.

Higher water level creates greater pressure at the outlet, just as higher voltage creates a greater driving force for current. No height difference means no flow — no voltage difference means no current.

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Defining Voltage

Voltage — more precisely called electric potential difference — is defined as the amount of energy required to move one unit of positive charge from one point to another. If moving 1 coulomb of charge from point A to point B requires 1 joule of energy, then the potential difference between those two points is 1 volt.

The unit of voltage is the volt (V), named after Italian physicist Alessandro Volta (1745–1827), who built the first practical battery — the voltaic pile — in 1800. The formal definition is:

The symbol for voltage is V. In older engineering texts and some European references you may also see E (for electromotive force) or U (common in continental Europe). All three refer to the same quantity. In this course, V is used throughout.

An important point: voltage is always measured between two points. You cannot say that a single wire "has" a voltage of 12V without specifying what it is measured relative to. In most circuits, one point is designated as the reference — usually the negative supply rail or the chassis — and called ground (0V). All other voltages in the circuit are then quoted relative to that reference. When someone says "the collector of this transistor is at 5V," they mean 5V above ground.

EMF and Terminal Voltage

A battery or power supply produces voltage by converting energy — chemical energy in a battery, electrical energy from the line voltage in a PSU. The voltage it generates through this internal conversion process is called its electromotive force (EMF). EMF is the open-circuit voltage: the voltage you would measure across the terminals with nothing connected and no current flowing.

When a load is connected and current starts to flow, something changes. Every real source has a small amount of internal resistance — the resistance of the electrolyte and electrodes in a battery, or the transformer windings and rectifier diodes in a power supply. As current flows through this internal resistance, some voltage is dropped across it — voltage that never reaches the output terminals. The voltage that does appear at the terminals under load is called the terminal voltage:

For a fresh, good-quality battery or a well-regulated power supply, the internal resistance is very low and the terminal voltage is very close to the EMF. A flat battery or a cheap unregulated supply will show a much larger voltage sag under load. This is why a car battery may measure 12.6V with the engine off but drops to 10V or lower when you try to crank a cold engine — the huge starter motor current flowing through the battery's internal resistance drags the terminal voltage down.

Common Voltage Sources and Levels

To develop a practical feel for voltage, it helps to know the values you will encounter in everyday electronics:

Automotive electronics is a good example of why the nominal voltage is only a starting point. A car battery reads about 12.6V when fully charged and at rest, but when the engine is running the alternator raises the battery voltage to approximately 13.8–14.4V for charging. Anything designed for a "12V" vehicle socket (dash cams, phone chargers, LED light bars) must therefore work correctly from roughly 11V up to about 14.5V, and survive brief spikes above that. Reading the full operating voltage range on a datasheet, rather than just the nominal value, is a habit worth forming early.

Common voltage sources and their typical values. Note that line voltage AC voltages are quoted as RMS values — the actual peak voltages are significantly higher.

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Polarity

Voltage has polarity — it has a direction as well as a magnitude. The terminal at higher potential is marked positive (+) and the terminal at lower potential is marked negative (−). In any circuit, conventional current flows from the positive terminal, through the external circuit (resistors, LEDs, motors and so on) and back to the negative terminal. Inside the source, the energy conversion process drives current from negative to positive, sustaining the potential difference.

In circuit diagrams, a battery is represented by alternating long and short horizontal lines. The long line is the positive terminal and the short line is the negative terminal. When you connect components to a supply, polarity matters:

• Electrolytic capacitors have a positive and a negative lead marked. Connecting them with reversed polarity can cause them to overheat, bulge and fail explosively.
• Diodes only conduct when the anode is at a higher voltage than the cathode. Reverse the polarity and the diode blocks current entirely (which is sometimes the intended behavior — or causes damage if not).
• Transistors and ICs can be destroyed instantly by reverse polarity. Many DC power inputs (on Arduino boards, LED drivers and car accessories) include a series diode or a polarity-protection MOSFET for exactly this reason.

Always check polarity before applying power to a new circuit or connecting unfamiliar equipment to a supply. A multimeter set to DC voltage takes only a few seconds and can prevent expensive mistakes.

AC Voltage vs DC Voltage

DC voltage (direct current voltage) has a constant polarity: the positive terminal is always positive and the negative is always negative. The voltage level may vary slightly (as it does in an unregulated supply) but the polarity never reverses. Batteries and regulated power supplies produce DC. The internal circuits of almost every electronic device run from DC.

AC voltage (alternating current voltage) continuously alternates between positive and negative values, following a sinusoidal waveform. One complete cycle consists of a positive half-cycle and a negative half-cycle. The number of complete cycles per second is the frequency, measured in hertz (Hz). line voltage electricity in North America alternates at 60 Hz; in Europe and Australia at 50 Hz.

The voltage value printed on an AC adapter or quoted for the utility supply is the RMS (root mean square) value. RMS is a mathematical average that represents the equivalent DC voltage that would deliver the same power to a resistive load. For a pure sine wave, the RMS voltage is the peak voltage divided by √2 (approximately 0.707 × peak). This means:

• 120V RMS line voltage has a peak of 120 ÷ 0.707 ≈ 170V
• 230V RMS line voltage has a peak of 230 ÷ 0.707 ≈ 325V

The peak voltage is what you must consider when rating capacitors and other components for use in line voltage-connected equipment. You will study AC in full detail later in this module, including sine waves, frequency and period.

Voltage in Practice

Voltage appears at very different levels in the equipment around you, and understanding those levels is important both for safe working and for effective troubleshooting.

Logic and microcontroller circuits run from low, tightly regulated voltages: 5V, 3.3V and, inside modern processors, 1.8V or less. A 3.3V microcontroller pin can be damaged by connecting it directly to a 5V signal, so checking the supply and signal voltages of every board you connect together is one of the first habits to learn.

Battery-powered and automotive equipment typically runs from 12V nominal, which in practice means anything from about 11V to 14.5V. Current draw can be large (a car audio amplifier or an inverter can draw 20 A or more), so the supply wiring must be rated for the current and the connections must be clean and secure. A high-resistance joint drops voltage under load and causes equipment to under-perform, reset or shut down.

Mains-powered equipment contains far higher voltages. A switching power supply rectifies the 120V or 230V AC mains to around 170–325V DC before converting it down, and its main filter capacitor can stay charged after the plug is pulled. Microwave ovens, old CRT televisions and valve (tube) amplifiers run at many hundreds or thousands of volts. These voltages are lethal. Never work inside mains-powered equipment until you have unplugged it and confirmed that the high-voltage capacitors have been safely discharged, following the procedures in the Safety module.