That is, the tendency of electrons to move can overcome the resistance of the conductor resistance to the current and make the charge flow in the closed conductor loop. This effect comes from the corresponding physical effect or chemical effect, and is usually accompanied by energy conversion, because when current flows in conductors (except superconductors), energy is consumed, and this energy must be compensated by the energy that generates electromotive force. If the electromotive force only occurs 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, which is called the internal resistance of the power supply. The energy consumed in the part of the conductor loop 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 supply at this time.
(1) Electromotive force is a physical quantity that represents the ability of a power supply to transform other forms of energy into electrical energy. A power source is a device that converts other forms of energy into electrical energy. Each power source has a positive pole and a negative pole at any time. There is a certain non-electric field force inside the power supply, which can overcome the electric field force to do work, forcing the positive charge to move from the negative electrode of the low potential point to the positive electrode of the high potential point through the inside of the power supply, and in the process convert other forms of energy into electrical energy . In order to characterize the ability of the internal non-electric field force to do work on positive charges, or to express the ability of other forms of energy to convert into electrical energy, the concept of electromotive force is introduced.
(2) The electromotive force of the power supply is equal to the work done by its internal non-electric field force to move the unit positive charge from the negative electrode to the positive electrode. The unit is the same as the voltage. The magnitude of the electromotive force depends on the power supply itself, and has nothing to do with the external circuit. Arbitrary electromotive force is represented by e. For the DC electromotive force whose magnitude and direction do not change with time, it is generally denoted by E, but can also be denoted by e.
(3) It is customary to specify the true direction of the electromotive force, which is the negative pole of the low potential point pointing to the positive pole of the high potential point, that is, the direction of potential rise. This is exactly the opposite of the true direction of the voltage.
(4) Electromotive force and voltage are two different concepts. The former is the work done by the non-electric field force to move the positive charge from near the low potential point to the positive electrode at the high potential point; and the latter is the work done by the electric field force to move the unit positive charge from the high potential point to the low potential point. However, they can all be used to represent the potential difference between the positive and negative poles of the power supply. In a closed circuit, from the perspective of the objective effect of the performance of the power supply to the external circuit, as shown in Figure 1-3, it can be represented by the electromotive force e between the positive and negative electrodes ab, or the voltage U in between. . It should be noted that the reference directions of the two are exactly opposite.
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