The potential energy, stored in the form of an electric charge imbalance and capable of provoking electrons to flow through a conductor, can be expressed as a term called voltage (or electric potential difference, electric pressure, and electric tension), which technically is a measure of potential energy per unit charge of electrons or something a physicist would call specific potential energy. Defined in the context of static electricity, voltage is the measure of work required to move a unit charge from one location to another, against the force which tries to keep electric charges balanced.

In the context of electrical power sources, voltage is the amount of potential energy available (work to be done) per unit charge, to move electrons through a conductor. When a charge q is immersed in an electric field generated by other charges, it is endowed with a certain aptitude for doing work, simply because it is immersed in this field. This attitude is called potential energy. In stationary conditions the potential difference is equal to the work done to move a unitary charge across the field from one point to another, changed in sign.

In metrology, the International System establishes that the unit of measurement of the electrical potential difference is the volt [V]. The measuring instrument for making the measurement is the voltmeter, generally integrated in an electrical “tester“.

Conventions and properties of potential differences

The potential difference existing between two points A and B of an electrical system can be represented with the symbol \(\Delta V_ {AB}\), where the lower indices (A and B) indicate between which points of the system is meant to refer the potential difference.

\[V_{AB} = V_A − V_B\]

remember that:

  • if \(V_A > V_B\) ⇒ \(V_{AB} > 0\)
  • if \(V_B > V_A\) ⇒ \(V_{AB} < 0\)

Additivity principle of potential differences

Consider two terminal circuit elements (dipoles) connected in series. The potential difference between the two extremes A and C (the point B is the one of conjunction between the two elements in series) is obtained by making the sum of the potential differences at the ends of each element.

\[V_{AC} =V_A – V_C =V_A -V_B +V_B -V_C =V_{AB} +V_{BC}\]

Things do not change if there are three or more elements connected in series. In general the following principle applies, called the principle of additivity of potential differences:

the potential difference at the ends of \(n\) elements connected in series is equal to the sum of the potential differences at the ends of each electric element.

No-load voltage

No load voltage” is a common term used for unregulated power supplies, generators, and batteries. We define the no-load voltage \(V_0\), the electric potential difference present at the ends of a dipole, or a port, when the current flowing through it is zero, or when the two terminals of the port are disconnected from the electric circuit. It is the output voltage when nothing is connected to the output.

The higher the current, the lower the voltage. This is due to the supply’s internal series impedance. When nothing is connected to the output of the power supply (“no-load”), the current is 0 A, and the voltage will be at its highest. So the “no-load voltage” is the highest voltage the power supply will produce.

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