Voltage

Voltage is the electrical 'push' — the energy each coulomb of charge gains or loses between two points. Learn potential difference, EMF, polarity, ground references and how real sources behave under load.

Every electrical circuit starts with voltage. A battery, a solar cell or a generator separates positive and negative charges, and that separation stores energy. Voltage (also called potential difference) measures how much energy each unit of charge will give up as it travels from one point to another through a circuit.

Voltage is always measured between two points. When someone says 'this node is at 5 V', they silently mean '5 V above the reference point', usually called ground or 0 V. Getting comfortable with that two-point idea is the single most important step in reading circuits.

An analogy

Think of a water tank on a roof. The height of the water above the tap sets the pressure that pushes water through the pipe. Voltage is that pressure: a taller tank (higher voltage) pushes harder, but no water flows until you open the tap (close the circuit).

What voltage actually is

Charges feel forces from other charges. Moving a positive charge 'uphill' against those forces takes energy, and letting it roll back 'downhill' releases that energy. Voltage is the energy per unit charge between two points: 1 volt means each coulomb gains or loses 1 joule.

A source (battery, generator) raises charges to a higher potential. Loads (lamps, motors, resistors) let them fall back down, turning the electrical energy into light, motion or heat.

Polarity, ground and reading a schematic

Every voltage has a direction. The + terminal is at higher potential than the − terminal. Ground (⏚) is simply the point we choose to call 0 V, and every other node voltage is measured from it.

• A node voltage (e.g. 'V_out = 3.3 V') always means 'relative to ground'.
• Swapping the meter probes flips the sign: V_AB = −V_BA.
• Chassis ground, signal ground and earth are different references and are not always at the same potential.

EMF vs terminal voltage

The EMF (E) is the voltage a source produces internally with no current flowing. Every real source also has internal resistance r, so when it delivers current I, some voltage is lost inside it and the terminal voltage drops to V = E − I·r.

This is why a car battery reading 12.6 V at rest can dip to 10 V while cranking the starter motor, and why a 'weak' battery measures fine on a multimeter but fails under load.

Voltage drops and dividers

In a series loop the source voltage is shared among the components in proportion to their resistance. That is the voltage-divider principle, used to scale a signal down for an ADC, bias a transistor or set a reference.

A divider only gives its ideal output when the load connected to it draws negligible current. A load of resistance R_L appears in parallel with R₂ and pulls the output down — the 'loading effect'.

Electric field, insulation and breakdown

Voltage across a gap creates an electric field E = V / d (volts per metre). Every insulator has a dielectric strength: air breaks down at roughly 3 kV/mm, which is why high-voltage equipment needs large clearances and creepage distances on PCBs.

Grid voltages are classified as LV (< 1 kV), MV (1–35 kV), HV (35–230 kV) and EHV/UHV (above that). Higher transmission voltage means lower current for the same power, and therefore far lower I²R losses.

Floating, differential and common-mode voltage

Measuring a voltage that isn't referenced to ground (a 'floating' voltage) needs a differential measurement. Instrumentation amplifiers and isolated probes reject the common-mode voltage both inputs share and amplify only the difference.

Clipping an oscilloscope's ground lead onto a live mains-referenced node shorts that node to earth through the scope. It is a classic, dangerous mistake in power-electronics labs.

Formulas

• Definition: V = W / Q
• Potential difference: V_AB = V_A − V_B
• Real source under load: V_terminal = E − I · r
• Voltage divider: V_out = V_in · R₂ / (R₁ + R₂)

Worked example: Energy delivered per charge

Given: Q = 20 C moves through a lamp; Energy converted W = 240 J

Find: The voltage across the lamp.

• V = W / Q
• V = 240 J / 20 C
• V = 12 J/C = 12 V

Answer: 12 V

Worked example: Battery under cranking load

Given: EMF E = 12.6 V; Internal resistance r = 0.05 Ω; Starter current I = 150 A

Find: Terminal voltage while cranking and power wasted inside the battery.

• Internal drop = I · r = 150 × 0.05 = 7.5 V
• V_terminal = E − I·r = 12.6 − 7.5 = 5.1 V
• Internal loss P = I² · r = 150² × 0.05 = 1125 W
• So a battery with r = 0.05 Ω is badly degraded — a healthy one is closer to 0.01 Ω.

Answer: V ≈ 5.1 V, 1.1 kW lost inside the battery

Common mistakes

• Wrong: Saying 'voltage flows through a resistor'. — Right: Current flows through a component; voltage exists across it, between its two terminals.
• Wrong: Assuming a battery's no-load reading proves it's healthy. — Right: Test under load: a high internal resistance only shows up as a large voltage drop when current flows.

Applications

• USB and electronics rails: USB supplies 5 V (and up to 48 V with USB-PD); chips run from 3.3 V, 1.8 V or below 1 V. Regulators convert between these levels.
• Electric vehicles: EV traction packs run at 400 V or 800 V. Doubling the voltage halves the current for the same power, allowing thinner cables and faster charging.
• Sensor scaling: Voltage dividers scale a 0–12 V or 0–24 V industrial signal down to the 0–3.3 V an MCU's ADC can read safely.

Key takeaways

• Voltage is energy per charge (J/C) and is always measured between two points.
• Ground is just the chosen 0 V reference.
• Real sources sag under load: V = E − I·r.
• Dividers scale voltage but are affected by the load connected to them.

Going further

• Kelvin (4-wire) measurement: Measuring a small voltage drop (a shunt, a battery's internal resistance) with two wires includes the test-lead drop. Four-wire Kelvin sensing uses separate force and sense leads so no current flows in the sense path, removing that error.
• Voltage references: Precision systems derive every measurement from a reference such as a bandgap (≈1.2 V, temperature-compensated) or a buried-Zener reference. Its tolerance and drift in ppm/°C limit the absolute accuracy of every ADC reading.
• Transient overvoltage: Switching inductive loads, lightning and ESD produce voltage spikes far above nominal. TVS diodes, MOVs and snubbers clamp them, and insulation coordination standards (IEC 60664) size clearances for these impulses.