Electromotive force

The electromotive force ( emf ) (English Electro Motive Force ( EMF) ), also called electromotive force, is the historical name for the source voltage of an electrical power source. It refers to the ability of a system to generate a voltage. The term has been used mainly in relation to galvanic cells, or for the induction of voltage in electrical machines such as electric motors and generators. The term describes despite its name, no force in the physical sense, but an electrical voltage.

History

The fundamental relation of chemical work done in galvanic cells to power energy, EMF and power was provided by the work of Hermann von Helmholtz and Josiah Willard Gibbs. Max Le Blanc used for the standardization of the electromotive force of the galvanic element, the normal solutions. Le Blanc found further that platinized platinum electrodes are reversible electrode and can be used for precise measurement of normal potentials, he hit the lapped with hydrogen gas platinum electrode as a standard electrode before. Walter Nernst presented a theory for EMF determination at different electrolyte concentrations and temperatures.

Examples

Galvanic cell

The Daniell cell is a historical example of an electrochemical cell. It is made of a zinc rod immersed in the aqueous solution of a zinc salt and a copper rod immersed in the aqueous solution of a copper salt. Both half- cells are combined with a current key containing the solution of an electrolyte (saturated KCl or NHNO ) or by a diaphragm into a galvanic cell.

The two metals together by a metal conductor, a current flows through the system. Here, the wire heats. Zinc dissolves in the zinc electrode, copper ions from the solution to separate out the copper electrode. To determine the emf to cut open the wire and turn a voltmeter between the wire ends.

Daniell the element takes place, the oxidation of the zinc at the anode.

At the cathode copper is reduced.

For each half-cell, the half -cell potentials calculated according to the Nernst equation.

For the calculation of the EMF for the overall reaction to form the difference between the two half-cell potentials after

For the Daniell cell is obtained for metal ion concentrations of 1 mol / l

Since under standard conditions ( temperature 25 ° C, concentration: 1 mol / L, pressure 1 atm), the half-cell potential corresponding to the standard potential.

Each half- cell is to be considered separately here. You can also use a half cell of sheet zinc in zinc solution to produce electrochemically hydrogen. The zinc sheet is connected with a wire and brought into contact with a platinum electrode. Well dipping the platinum electrode in hydrochloric acid. It forms hydrogen gas.

For calibration, or for the correct setting of exactly 1.000 V emf Clark element (zinc / zinc paste / mercury sulfate / mercury ) or the Westonelement was used in earlier times.

Applications

From the emf, the free enthalpy of a redox reaction can be calculated.

If you have the emf determined under standard conditions, the free standard enthalpy of reaction can thus be calculated.

Furthermore, it can be determined with a reference hydrogen electrode of the pH by the emf developed to probes is measured when they are immersed in the liquid to be measured. See, for example, pH - electrode. The emf change at this time to 59.16 mV per pH change by 1, ie each power of ten of the hydrogen - ion concentration when the measured temperature is maintained at 25 ° C ( Nernst slope). Other electrode systems bypass the difficult handling of the hydrogen electrode for pH measurement.

Electric motors and generators

An electrical conductor moving across a magnetic field in it, an electrical voltage is induced; it is all the higher, the faster the movement. Accordingly, the magnetic field in the stator rotating rotor of an electric motor or magnetic rotor of a generator in its windings induces a voltage. This induced voltage is referred to as the back emf in engines. It does not matter what voltage is actually applied to the motor or generator - the difference between the two voltages drops at the ohmic resistance of the windings or is caused by leakage currents.

Increases the speed of a DC motor to the extent that the emf approaches the voltage applied, the power consumption and the rotational speed falls does not increase further. With regard to the counter-electromotive force of a DC motor can thus calculate the speed limit for a given voltage.

The back EMF of a DC motor and other motors may be used in order to control and speed control. This approach is used, but also in electronically commutated motors as well as in modern drives for induction motors, for example, the case of small permanent-magnet motors for driving cassette tape recorders.

Separately excited direct-current motor can be increased by weakening the field in its rotational speed - the back EMF now requires a higher speed, to achieve the value of the operating voltage.

Even asynchronous induce a back EMF - here induces the squirrel-cage rotor rotating magnetic field in the stator windings, an alternating voltage, which counteracts the power consumption when the rotor reaches the rated speed.

The EMF from stepper motors limited their dynamics and torque at high speeds.

The electromotive force is almost equal to the generators in open circuit voltage. The generated voltage or the electromotive force of generators may be varied by varying the rotational speed or of the excitation field.

Galvanometer drives and speakers

In galvanometer drives and electrodynamic loudspeakers the back EMF also plays a role: they act back through the inertia of their coils on the feeding voltage source. Your EMF is shorted usually due to the low internal resistance of the voltage source driving it, thus they are damped - a ringing or overshoots are reduced.