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Physics · Electricity

Volts (Voltage)

Reference entry · last updated 20260908

Volts measure electric potential difference, also called voltage. One volt (V) equals one joule per coulomb of charge.[1]

1. First principles and definitions

Charge is measured in coulombs (C), and energy in joules (J). A potential difference describes the change in electric potential energy per unit charge between two points. Voltage therefore requires a reference, such as another terminal or a chosen zero potential.[2]

\[1\,\mathrm{V}=1\,\mathrm{J/C}\]

Amps measure charge flow per second. Watts measure energy transfer per second. A voltage can exist across an open circuit even when no steady current flows.[2]

2. Voltage, current, and resistance

For a resistor obeying Ohm’s law, voltage equals current times resistance. Resistance is measured in ohms (Ω). At fixed resistance, doubling voltage doubles current.[3]

\[V=IR\]

In steady DC, electrical power is \(P=VI\). Voltage alone does not specify power; current must also be known.[3]

3. Worked DC example

Illustrative calculation: a 6 Ω resistor connected across 12 V carries 2 A and dissipates 24 W. Each coulomb passing through it loses 12 J of electric potential energy.

\[I=12/6=2\,\mathrm{A},\qquad P=12\times2=24\,\mathrm{W}\]

4. AC voltage

An AC voltage changes with time. Its root-mean-square (RMS) value depends on the waveform. For a sine wave, the peak is √2 times the RMS value. An illustrative 230 V RMS sine wave peaks at about 325 V.[4]

5. See also

6. References

  1. NIST, The Two Classes of SI Units and the SI Prefixes, Tables 1 and 3
  2. OpenStax, University Physics Volume 2, §7.2: Electric Potential and Potential Difference (2016)
  3. OpenStax, University Physics Volume 2, §9.5: Electrical Energy and Power (2016)
  4. OpenStax, University Physics Volume 2, §15.4: Power in an AC Circuit (2016)