What’s the difference between “voltage at a point” and “voltage between two points?” Also, what is ‘ground?’

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Voltage is a measurement of the (potential) force driving electrons between two points, and is always a relative measurement. For instance, if I said "it’s 30 degrees in here," that would be an absolute measurement of temperature. However, if I said "it’s 5 degrees colder in here than in the kitchen," that’s a relative measurement between here and the kitchen. Voltage is always relative ("absolute voltage" is called electric potential).

When discussing the voltage at a point in a circuit, we actually mean the voltage between that point and ground. Ground is an arbitrary point in the circuit that we decide to call 0 volts, and measure the voltage in the rest of the circuit relative to. In DC circuits, ground is usually the negative terminal of the power supply/battery. In AC circuits, ground refers to either neutral or a third connection aptly named ‘ground.’ See ‘What’s the difference between neutral and ground‘ for more information.

For example, when a battery states that it’s 9 volts, it really means the voltage between its two terminals is 9 volts. When people tell you wall outlets are 120 volts (220 in Europe), they mean the voltage between the left- and right-plugs is 120 volts*. If a circuit diagram states a point in the circuit is 0 volts, it means that point and the negative terminal have the same electric potential (and are probably directly connected to one-another).

*Since voltage is a wave in AC circuits and therefore changes (quickly) over time, 120 volts actually refers to a kind of average voltage over time

Additional Reading:


  1. Andreas says:

    Stumbled upon this while browsing through your blog: In Europe it's actually 230V RMS (+- 10%). It was 220V RMS at some point, but that is was while ago. It was changed in 1987 in IEC 60038:1983

  2. anonymous says:

    Great explanation, but I would just add that voltage is potential *energy* per unit charge (1 V = 1 J / C), not potential force.

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