11 julio, 2024

Electromotive force: concept, formulas, symbols, examples

What is electromotive force?

The electromotive force (fem) is the agent in charge of keeping electrical charges moving within an electrical circuit. The amount of fem of a source represents the energy per unit charge that it can provide.

Batteries and generators are sources of electromotive force whose function in an electrical circuit is analogous to that performed by a water pump to maintain flow in a closed-circuit pipe.

A source of voltage or voltage, as the source of current is also called femconverts chemical energy, electromagnetic energy or mechanical energy into electrical current, that is, it provides kinetic energy to the conduction electrons of an electrical circuit that connects its two terminals or poles.

The unit in the International System (SI) of measures for the electromotive force is he voltabbreviated V and equivalent to 1 Joule/coulombso in reality it is not a force in the usual Physics sense and its unit of measurement does not correspond to that of force, which is measured in newtons in units YEAH.

Difference Between Voltage and Emf

A battery with a fem of 3 voltsdo a job 3 joules about each coulomb of charge (positive) that moves from the negative to the positive terminal inside the battery.

Because the charges inside a voltage source are in countercurrent, a fem is not equal to a potential difference, since in the latter case the (positive) charges would move from the region of higher potential to that of lower potential.

The name of electromotive force was coined by Alessandro Voltathe inventor of the battery, at the beginning of the 19th century, when the distinction between a force and the energy that it is capable of producing was not yet clear.

In explaining his discovery, Volta alluded to the force that keeps the electrical charges inside the battery apart, calling it electromotive forcename that endures until now.

measure of emf

In a simple resistive direct current circuit, the battery is the source of the electromotive force either fem.

Quantitatively, the fem measures the energy per unit charge supplied by the battery. Denoting the electromotive force as εis the quotient between the work ΔW made to carry a small load Δqfrom the positive terminal of the cell or battery, to the negative terminal through the external circuit:

In a battery, this work is generated by chemical reactions inside it, the consequence of which is the separation of charges between its electrodes, maintaining a constant voltage created by a non-conservative electric field.

fem symbol

The symbol used for the fem in a circuit, it usually consists of two unequal parallel lines indicating polarity. The long line is the positive pole and the short line is the negative.

Although these are almost always negative electrons, by convention charge carriers are assumed to be positive, which is why the current is drawn leaving the + pole of the source.

Ideal and real sources of electromotive force

The sources of fem They are classified according to various criteria, for example, there are ideal and non-ideal ones.

ideal source

A fountain ideal is the one that maintains a constant voltage between the terminals, before any current that is demanded. Therefore, the fem Ideally, it does not present internal resistance to the movement of charges within it.

royal source

A real source has some opposition to the movement of charges, manifested through a small internal resistance. When making the calculations in a circuit, it is necessary to take into account the value of said resistance, together with that of the external resistances and other elements present.

So when the terminals of the fem are disconnected, the value of the voltage between them is equal to the nominal value of the fem. If, for example, it is a 9 V battery, this will be the value read by a voltmeter. But when connecting it to a circuit that also feeds an external resistance, a certain current will flow that decreases the voltage between the terminals of the femin an amount that depends on the current and the internal resistance.

Be Vab the voltage between the terminals of the fem, r its internal resistance and Yo the current flowing through the circuit. Applying Ohm’s law we get:

Vab = ε − I∙r


The following are examples of fem well-known, in which use is made of various principles to generate electricity: chemical reactions, electromagnetic induction, electromagnetic waves and temperature differences.


They include all kinds of batteries and accumulators, which transform chemical energy into electricity, through reactions between compounds inside. There are nickel, nickel in combination with other elements, zinc-carbon, lithium and alkaline based on potassium hydroxide, among other substances.

Automobile batteries, typically made of lead-acid, can be charged and discharged using electrolysis. Common dry batteries, on the other hand, are not rechargeable.

Electric generators

They convert motion energy into electrical energy, through the combined work of a rotating element called rotorand another static one, known as stator.

Depending on their design, electric generators produce alternating current (alternator) or direct current (dynamos).

The origin of the fem in electric generators it is in the phenomenon of electromagnetic induction, which consists of the action of a magnetic field on mobile electric charges. In this process an induced voltage is generated, without the need for chemical reactions.

Solar cells

As its name indicates, the fem of a solar cell comes from sunlight, although they can also carry out the conversion starting from another type of incident light.

The constituent element of solar cells is silicon, obtained from the purification of sand. With this, thin sheets of a semiconductor material are manufactured, in which the electrons tend to accumulate on the illuminated surface, thus creating a separation of charges and the consequent electric field between them.

They are frequently used for street lighting, as well as for irrigation in rural areas and for powering telecommunication equipment.

thermoelectric devices

They convert temperature differences into electrical energy through a thermocouple, which consists of two conductors joined through a solder.

If two points on a conductor are at different temperatures, a electromotive force among them. This phenomenon is known as seebeck effectalthough it was discovered by Alessandro Volta, the creator of the electric battery.

combustion cells

They operate in a similar way to that of a common battery, but with the difference that the reagents inside are supplied from an external source. The chemical principle that generates energy is combustion, for which a variety of compounds are used, including methanol, hydrogen, carbon and more.

wind turbines

These devices convert wind energy into electrical energy, through three rotating blades, similar to a windmill or fan. The turning movement of the blades drives a turbine connected to an electric generator by means of a shaft.

The heart

The heart is a muscle that contains cells capable of originating and transmitting electrical impulses, just like a fem.

Deja una respuesta

Tu dirección de correo electrónico no será publicada. Los campos obligatorios están marcados con *