EMF is understood as the specific work of external forces to move a unit charge in the circuit of an electric circuit. This concept in electricity involves many physical interpretations related to various areas of technical knowledge. In electrical engineering, this is the specific work of external forces that appears in inductive windings when an alternating field is applied to them. In chemistry, it means the potential difference that occurs during electrolysis, as well as during reactions accompanied by the separation of electric charges. In physics, it corresponds to the electromotive force generated at the ends of an electrical thermocouple, for example. To explain the essence of EMF in simple words, you will need to consider each of the options for its interpretation.
Before moving on to the main part of the article, we note that EMF and voltage are very close in meaning, but still somewhat different. In short, the EMF is at the power source without load, and when the load is connected to it, this is already the voltage. Because the number of volts on the power supply under load is almost always slightly less than without it. This is due to the internal resistance of power supplies such as transformers and galvanic cells.
Content:
- Electromagnetic induction (self-induction)
- Electric motors and generators
- A little more theory
- EMF in everyday life and units
- Conclusion
Electromagnetic induction (self-induction)
Let's start with electromagnetic induction. This phenomenon is described by the law Faraday electromagnetic induction. The physical meaning of this phenomenon is the ability of the electromagnetic field to induce an EMF in a nearby conductor. In this case, either the field must change, for example, in the magnitude and direction of the vectors, or move relative to the conductor, or the conductor must move relative to this field. In this case, a potential difference arises at the ends of the conductor.
There is another phenomenon similar in meaning - mutual induction. It lies in the fact that a change in the direction and strength of the current of one coil induces an EMF at the terminals adjacent coil, is widely used in various fields of technology, including electrical and electronics. It underlies the operation of transformers, where the magnetic flux of one winding induces current and voltage in the second.
In electrics, a physical effect called EMF is used in the manufacture of special AC converters, providing the desired values of the effective quantities (current and voltage). Due to the phenomena of induction and self-induction engineers managed to develop many electrical devices: from conventional inductors (choke) and up to the transformer.
The concept of mutual induction applies only to alternating current, during the flow of which in a circuit or conductor, the magnetic flux changes.
For an electric current of constant directivity, other manifestations of this force are characteristic, such as, for example, a potential difference at the poles of a galvanic cell, which we will discuss below.
Electric motors and generators
The same electromagnetic effect is observed in the structure asynchronous or synchronous motor, the main element of which is inductive coils. His work is described in an accessible language in many textbooks related to the subject called "Electrical Engineering". To understand the essence of the ongoing processes, it is enough to remember that the EMF of induction is induced when the conductor moves inside another field.
According to the above-mentioned law of electromagnetic induction, a counter is induced in the motor armature winding during operation EMF, which is often called "back-EMF", because when the engine is running, it is directed towards the applied stress. This also explains the sharp increase in the current consumed by the motor when the load increases or the shaft is jammed, as well as inrush currents. For an electric motor, all the conditions for the appearance of a potential difference are obvious - a forced change in the magnetic field of its coils leads to the appearance of a torque on the rotor axis.
Unfortunately, we will not delve into this topic within this article - write in the comments if you are interested in it, and we will tell you about it.
In another electrical device - a generator, everything is exactly the same, but the processes occurring in it have the opposite direction. An electric current is passed through the rotor windings, a magnetic field arises around them (permanent magnets can be used). When the rotor rotates, the field, in turn, induces an EMF in the stator windings - from which the load current is removed.
A little more theory
When designing such circuits, the distribution of currents and the voltage drop across individual elements are taken into account. To calculate the distribution of the first parameter, the known from physics is used Kirchhoff's second law - the sum of the voltage drops (taking into account the sign) on all branches of a closed circuit, is equal to the algebraic sum of the EMF of the branches of this circuit), and to determine their values, use Ohm's law for a section of a chain or Ohm's law for a complete chain, the formula of which is given below:
I = E / (R + r),
where E - EMF, R - load resistance, r is the resistance of the power source.
The internal resistance of the power source is the resistance of the windings of generators and transformers, which depends on the cross-section of the wire, with which they are wound and its length, as well as the internal resistance of galvanic cells, which depends on the state of the anode, cathode and electrolyte.
When carrying out calculations, the internal resistance of the power supply must be taken into account, considered as a parallel connection to the circuit. A more accurate approach, taking into account the high values of operating currents, takes into account the resistance of each connecting conductor.
EMF in everyday life and units
Other examples are found in the practical life of any ordinary person. Such familiar things as small batteries and other miniature batteries fall into this category. In this case, the working EMF is formed due to chemical processes taking place inside DC voltage sources.
When it occurs at the terminals (poles) of the battery due to internal changes, the cell is completely ready for operation. Over time, the EMF decreases slightly, and the internal resistance increases markedly.
As a result, if you measure the voltage on an unconnected finger-type battery, you see normal for it 1.5V (or so), but when a load is connected to the battery, let's say you installed it in some kind of device - it does not works.
Why? Because if we assume that the internal resistance of the voltmeter is many times higher than the internal resistance of the battery, then you measured its EMF. When the battery began to give current to the load at its terminals, it became not 1.5V, but, say, 1.2V - the device does not have enough voltage or current for normal operation. It was just these 0.3V that fell on the internal resistance of the galvanic cell. If the battery is very old and its electrodes are destroyed, then there may be no electromotive force or voltage at all at the battery terminals - i.e. zero.
This example clearly demonstrates what is the difference between EMF and voltage. The author says the same at the end of the video that you see below.
You can learn more about how the EMF of a galvanic cell arises and how it is measured in the following video:
A very small electromotive force is also induced within the receiver antenna, which is then amplified by special cascades, and we receive our television, radio and even Wi-Fi signal.
Conclusion
Let's summarize and once again briefly recall what EMF is and in what SI units this value is expressed.
- EMF characterizes the work of external forces (chemical or physical) of non-electrical origin in an electrical circuit. This force does the work of transferring electric charges to it.
- EMF, like voltage, is measured in Volts.
- The differences between EMF and voltage are that the first is measured without a load, and the second with a load, while the internal resistance of the power source is taken into account and affects.
And finally, to consolidate the material covered, I advise you to watch another good video on this topic:
Related materials:
- What is the difference between alternating current and direct current
- What is electric charge
- How to lower AC and DC voltage