What Is Electromotive Force?

That is, the tendency of electronic movement can overcome the resistance of the conductor resistance to the current, and make the charge flow in the closed conductor circuit. This effect is derived from the corresponding physical or chemical effect, and is usually accompanied by energy conversion, because the current consumes energy when flowing in a conductor (except for superconductors), and this energy must be compensated by the energy generating the electromotive force. If the electromotive force occurs only in a part of the conductor loop, this part of the area is called the power supply area. There is also a resistance in the power supply area, called the internal resistance of the power supply. The energy consumed in some conductor loops outside the power supply area directly comes from the electromagnetic field in the conductor, but the energy of the electromagnetic field still comes from the power source.

Chinese name: Emf
Foreign name: Electromotive Force
Subject: physical

Application: Circuit and electrical problems
Unit: Volt (V)
Symbol: E ()

Introduction to electromotive force

Electromotive force is a physical quantity that reflects the power source's ability to convert other forms of energy into electrical energy. Electromotive force causes a voltage across the power supply. In the circuit, EMF is often expressed by E. The unit is volts (V).

Inside the power supply, non-electrostatic forces move the positive charge from the negative plate to the positive plate to perform work on the charge. The physical process of this work is the essence of generating the electromotive force of the power supply. The work done by non-static forces reflects how much other forms of energy have become electricity. So inside the power supply, the process of non-static force doing work is a process of energy conversion.

The magnitude of the electromotive force is equal to the work performed by a non-static force that moves a unit of positive charge from the negative electrode of the power supply to the positive electrode of the power supply through the interior of the power supply. If let W be the ratio of the work performed by the non-electrostatic force (power force) in the power supply to transfer the positive charge q from the negative electrode to the positive electrode of the power supply through the internal power supply, the magnitude of the electromotive force is:

. For example, if the electromotive force is 6 volts, it means that when the power source moves 1 bank of positive charges from the negative electrode to the positive electrode through the internal circuit, the non-static force does 6 joules of work. There are other forms of 6 joules that can be converted into electricity.

The direction of the electromotive force is specified from the negative pole of the power source to the positive pole of the power source through the inside of the power source, that is, the direction opposite to the voltage across the power source. ^[1]

EMF generation mechanism

The electromotive force of a power supply is closely related to the work of non-static forces. Non-electrostatic force refers to the force that can affect the flow of charge except electrostatic force, and does not refer to all forces except electrostatic force. Different sources of non-static forces have different forms of energy conversion.

The non-electrostatic force of chemical electromotive force (dry cell, button cell, battery, etc.) is a

The magnetic field is cut to generate electromotive force. The chemical action associated with the dissolution and deposition of ions, the magnitude of the electromotive force depends on the type of chemical action, and has nothing to do with the size of the power source. For example, the dry cell's No. 1, No. 2 and No. 5 EMF are 1.5 volts. The battery that generates chemical electromotive force is called a chemical battery or an electrochemical battery, for example, a copper-zinc primary battery, and the electrolyte solution is a copper sulfate solution.

Induced electromotive force and dynamic electromotive force (generator). The non-static force of a generator originates from the effect of a magnetic field on a moving charge, which is the Lorentz force.

According to Faraday's law of electromagnetic induction: As long as the magnetic flux passing through the circuit changes, an induced electromotive force will be generated in the circuit. In fact, there are two reasons for the change in magnetic flux: one is that the loop moves relative to the magnetic field; the other is that although the loop has no relative motion in the magnetic field, the distribution of the magnetic field in space changes with time. The induced electromotive force produced by the former cause is called dynamic electromotive force, and the induced electromotive force produced by the latter cause is called induced electromotive force. ^[2]

Magnitude of induced emf

(

Is the change in magnetic flux,

For time,

Is the number of coil turns) ^[3]

The non-electrostatic force of the photogenerated electromotive force (photovoltaic cell) comes from the internal photoelectric effect.

Under illumination, if the energy of the incident photon is greater than the forbidden band width, the bound valence electrons near the semiconductor PN junction absorb the photon energy and are excited to transition to the conduction band to form free electrons, and the valence bands accordingly form free holes. These electron-hole pairs, under the action of the internal electric field, the holes move to the P region, and the electrons move to the N region, so that the P region is positively charged and the N region is negatively charged, so a voltage is generated between the P and N regions. Called photo-generated electromotive force, this is the photovoltaic special effect. Sensitive elements made using photovoltaic special effects include photovoltaic cells, photodiodes, and phototransistors. ^[4]

Piezoelectric emf (crystal piezoelectric ignition, crystal microphone, etc.) originates from the phenomenon of polarization caused by mechanical work.

When a dielectric (crystal) is deformed by an external force in a certain direction, it will cause the positive and negative charge centers within it to shift relative to each other and cause polarization, which will cause opposite signs of charges on the corresponding two surfaces. A voltage is generated on the two surfaces, which is called piezoelectric electromotive force. When the external force is removed, the surface charge also disappears, and the non-charged state is restored again. When the direction of the external force is changed, the polarity of the charge is also changed. ^[5]

The non-electrostatic force of the thermoelectric force (thermoelectric power) is a kind of diffusion effect related to the temperature difference and the difference in electron concentration.

In 1821, German physicist TJ Seeback discovered that when two different metal wires form a closed circuit, if a temperature difference is maintained between the two joints, the circuit will generate current and electromotive force, which was later called the Seebeck effect. The generated electromotive force is called the thermoelectric force. ^[6]

Relationship between electromotive force and road-side voltage

The road-side voltage of the power supply refers to the voltage applied by the power supply to both ends of the external circuit. It is the work performed by electrostatic force to move a unit of positive charge from the positive electrode to the negative electrode through the external circuit. For a certain power source, both the electromotive force E and the internal resistance r are constant.

The ideal electromotive force source does not have any internal resistance, and no energy is wasted by discharging and charging. The electromotive force given by an ideal electromotive force source is equal to its voltage across the road.

In practical applications , the EMF source inevitably has some internal resistance. The resistance of the actual electromotive force source can be regarded as an ideal electromotive force source connected in series with a resistor having internal resistance. The electromotive force of the power supply is constant for a fixed power supply, but the voltage at the end of the power supply varies with the load of the external circuit. The size of the internal resistance depends on the size of the electromotive force source, the chemical properties, the use time, the temperature and the load current.

EMF power charging

When the power supply is being charged, the current inside the power supply flows from the positive electrode to the negative electrode of the power supply. The direction of the internal voltage drop is opposite to the direction of the electromotive force.

The voltage at the circuit end is greater than the electromotive force.

EMF power discharge

In the case of power discharge, when there is no back-EMF in the external circuit,

Power Discharge The law of change obeys Ohm's law with source circuits, and its mathematical expression is:

Is the internal voltage of the power supply, also called the internal voltage drop. Available

That is, the magnitude of the current I changes with the external resistance R. When the current I increases, the internal voltage drop Ir increases, and the road-end voltage U decreases; conversely, when the current I decreases, the road-end voltage U increases. ^[7]

EMF power off

When the external circuit of the power supply is disconnected, R can be regarded as infinite, I becomes zero, and the internal voltage drop Ir also becomes zero. At this time, the non-static force inside the power supply is balanced with the electrostatic field force. The terminal voltage is equal to the electromotive force of the power supply.

EMF measurement method

Emf using voltmeter

The electromotive force of the power supply can be measured with a voltmeter. When measuring, do not connect the power supply to the circuit. Use a voltmeter to measure the voltage across the power supply. The resulting voltage value can be regarded as equal to the electromotive force of the power supply. If the power supply is connected in the circuit, the voltage across the power supply measured with a voltmeter will be less than the electromotive force of the power supply. This is because the power supply has internal resistance. In the closed circuit, the current has an internal voltage drop through the internal resistance r and an external voltage drop through the external resistance R. The electromotive force E of the power supply is equal to the sum of the internal voltage Ir and the external voltage IR, that is,

. Strictly speaking, even if the power supply is not connected to the circuit, the voltage across the power supply is measured with a voltmeter, the voltmeter becomes an external circuit, and the measured voltage is less than the electromotive force. However, because the internal resistance of the voltmeter is large and the internal resistance of the power supply is small, the internal voltage can be ignored. Therefore, the voltage measured across the power supply by the voltmeter can be regarded as equal to the power EMF.

Emf using potentiometer

When a limited current is passed, a potential drop is generated in the battery internal resistance, so that the potential difference between the two electrodes is smaller than the battery electromotive force. Therefore, the potential difference between the two electrodes is equal to the battery electromotive force only when there is no current flowing through the battery. In accurate measurement , you cannot directly measure the electromotive force of a battery with a voltmeter, because you must use a voltmeter to pass a limited current through the loop to drive the pointer. The result must not be the electromotive force of the battery, but only the road-end voltage between the two electrodes .

Compensation method is generally used to measure the electromotive force of the battery. A commonly used instrument is a potentiometer. Potentiometer is a balanced voltage measuring instrument designed according to the principle of cancellation measurement. It cooperates with standard batteries and galvanometers to become the basic instrument for voltage measurement. ^[8] Its working principle is as follows:

Measuring circuit composed of working power E, resistor R _AB and current limiting resistor R _P

Schematic diagram of potentiometer for measuring power emf flow

. The power supply to be tested E _X and the galvanometer G form a shunt, and the sliding rheostat P is adjusted so that the current in the galvanometer G is zero, then E _X = V _AP = R _AP = I ₀ . Use the standard battery E _{S as the} swing switch K. Adjust the sliding rheostat again to make the current in the galvanometer G zero. Open the switch K _{e and} measure the resistance of the sliding rheostat R _S to get E _S = R _S × I ₀ .

. ^[9]

Difference between electromotive force and potential difference

Electromotive force and potential difference (voltage) are two concepts that are easily confused. Electromotive force is the ratio of the amount of work and charge done by a non-static force to move a unit of positive charge from the negative electrode to the positive electrode through the interior of the power source; and the potential difference indicates that the electrostatic force moves the unit of positive charge from one point in the electric field to another The ratio of work to charge. They are two completely different concepts.

Although there is a difference between electromotive force and potential difference (voltage), electromotive force and potential difference are both scalar. For a given power supply, no matter what the external resistance is, the electromotive force of the power supply is always the same, and the road-side voltage of the power supply changes with the change of the external resistance, which is a physical quantity that characterizes the nature of the external circuit. ^[10]

Difference between emf and voltage

Although electromotive force and voltage have the same unit, they are two physical quantities that are fundamentally different.

(1) The objects they describe are different: electromotive force is a physical quantity of a power source, and it describes the physical quantity of a power source's ability to convert other forms of energy into electrical energy. Voltage is a physical quantity that reflects the ability of electric field forces to do work ^[11] .

(2) The physical meaning is different: when the electromotive force is numerically equal to the amount of electrical energy converted from other forms of energy during the process of moving the positive charge of the unit electricity from the negative electrode of the power supply to the positive electrode, the voltage is numerically equal to the positive charge of the unit electricity The work done by the electric field force is how much other energy is converted into electrical energy. They all reflect the transformation of energy, but the process of transformation is different ^[11] .

(3) The power of the two is different: the voltage is the potential difference between two points in the electric field, and the work done by the unit of positive electric charge when the electric field force moves in the electric field is the potential difference, that is, the voltage, and W = UQ is the work done by the electric field force. The voltage U is related to the work done by the electric field force. The electromotive force reflects the characteristic of non-static force of the power supply. Its value is equal to the work performed by the unit non-static force transferring a unit of positive charge from the negative electrode of the power supply to the positive electrode. Non-electrostatic force is a chemical action related to the dissolution and precipitation of ions; non-static force is a diffusion effect associated with temperature difference and electron concentration in a temperature difference power supply; non-static force is an electromagnetic effect in ordinary generators The electromotive force Luo II, that is, the flat in q is the work done by non-static forces such as the above, so the electromotive force g is related to the work performed by non-static forces ^[12] .

(4) The process of energy conversion is different: voltage is a measure of the change in potential energy, and it is the process of converting electric field energy into mechanical energy of charge. Since the electric potential is numerically equal to the potential energy that a unit positive charge has in an electric field, there is a voltage in the electric field. Positive charges can be moved from high potentials to low potentials by doing work under the action of electric field forces, and the potential energy decreases. The higher the voltage, the greater the potential energy decreases, and the greater the value of the potential energy converted into the mechanical energy of the motion of the charge. In the gravity field, free-falling gravity potential energy is converted into kinetic energy in a similar way. However, electromotive force is a measure of the ability of non-static forces to resist electric field forces and transform other forms of energy. In a closed circuit, a certain non-static force acts on the moved charge. The potential energy of the charge is increased, and other forms of energy such as chemical energy, solar energy, thermal energy, mechanical energy, etc. are converted into electrical energy. Different power sources have different abilities to convert work from non-static forces into electrical energy, so the electromotive force is also different. For example, the electromotive force of a chemical power source depends on the properties of the solution and the plate, and the electromotive force of a generator depends on the armature, magnetic field, and their relative motions ^[12] .

(5) The cause and effect relationship in the circuit is different: if there is no power supply in the circuit, even if there is a voltage, the current formation is very short, and the final voltage will not be maintained. Without a power source (electromotive force), the current is like passive water, and the voltage will not be stable. Therefore, the generation and maintenance of voltage in all parts of the circuit are based on the existence of electromotive force. Take two isolated live conductors as an example. It is also necessary to have a non-static effect to transfer charge, that is, there must be an electromotive force before it can be said that there is a stable and continuous potential difference (voltage) on the conductor ^[12] .

(6) Change and invariability in a given circuit: For a given power supply, once it is made, the electromotive force is fixed and has nothing to do with whether the external circuit is turned on or the composition of the external circuit. However, the voltage of the external circuit changes due to the change in the external circuit resistance.For example, the number of parallel branches increases or decreases, and the current and voltage of each part of the circuit will be redistributed when the resistance changes, and the voltage will change. The above is equal to the power emf, which is just a special result of this distribution, and does not mean that the voltage is emf ^[12] .