Electromotive force and voltage difference
Although the electromotive force and the voltage have the same unit, they are two physical quantities that are essentially different.
(1) The objects they describe are different: the electromotive force is the physical quantity that the power source has, and describes the physical quantity that the power source converts other forms of energy into electric energy. The voltage is the physical quantity that reflects the electric field power.
(2) The physical meaning is different: the electromotive force is numerically equal to the amount of electric energy converted into other forms of energy during the process of moving the positive charge of the unit charge from the negative pole of the power source to the positive pole; and the voltage is equal in value to the positive charge of the mobile unit The work of electric field force is the amount of other forms of energy that converts electrical energy into electricity. They all reflect the conversion of energy, but the process of conversion is different.
(3) The force of the two work is different: the voltage is the potential difference between two points in the electric field, and the work done by the electric field force moving the unit positive charge in the electric field is the potential difference, that is, the voltage, W=UQ is the work done by the electric field force, visible The voltage U is related to the work of the electric field force. The electromotive force reflects the characteristic of the non-electrostatic force of the power source. Its value is equal to the work done by the non-electrostatic force of the power source to transfer the unit positive charge from the negative pole of the power supply to the positive pole. The non-electrostatic force is a chemical action associated with the dissolution and precipitation process of ions; in the thermoelectric power supply, the non-electrostatic force is a diffusion effect associated with the temperature difference and the electron concentration; in the ordinary generator, the non-electrostatic force acts as an electromagnetic effect. The electromotive force, that is, the flatness in q is the work done by these non-electrostatic forces, so the electromotive force g is associated with the work of non-electrostatic force.
(4) The process of energy conversion is different: voltage is a measure of the change of potential energy, which is a process of converting electric field energy into charge mechanical energy. Since the potential is numerically equal to the potential energy of the unit positive charge in the electric field, there is a voltage in the electric field. The positive charge can be moved from the high potential to the low potential by the action of the electric field force, and the potential energy is reduced. The higher the voltage, the larger the potential energy can be reduced, and the higher the potential energy can be converted into the mechanical energy of the charge. The situation in which the free-falling gravitational potential energy in the gravitational field is converted into kinetic energy is similar. The electromotive force is the measure of non-electrostatic force against the electric field force, transforming other forms of energy. In the closed circuit, some non-electrostatic force acts on the moved charge. The electric potential energy of the electric charge is increased, and other forms of energy such as chemical energy, solar energy, thermal energy, mechanical energy, etc. are converted into electric energy. Different power sources, such as non-electrostatic force, are different in electric power, so the electromotive force is different. For example, the electromotive force of a chemical power source is determined by the nature of the solution and the plate. The electromotive force of the generator is determined by the armature, the magnetic field and their relative motion.
(5) The causal relationship in the circuit is different: if there is no power supply in the circuit, even if there is voltage, the current is formed very short, and finally the voltage will not be maintained. Without a power source (electromotive force), the current is like passive water, and the voltage is not stable. Therefore, the generation and maintenance of voltages in various parts of the circuit are predicated on the existence of electromotive force. Take two isolated charged conductors to see It is also necessary to have a non-electrostatic property to migrate the charge, that is, it must have an electromotive force before it can be said that there is a stable and constant potential difference (voltage) on the conductor.
(6) It is different from the same in a given circuit: for a given power supply, once it is manufactured, the electromotive force is fixed, regardless of whether the external circuit is connected or not, and also in the circuit regardless of the composition of the external circuit. The voltage should be changed due to the change of the external circuit resistance. If the number of parallel branches increases or decreases, the current and voltage of each part of the circuit will be redistributed when the resistance changes, and the voltage will change until the voltage of the terminal is broken when the external circuit is disconnected. Equal to the power electromotive force is only a special result of this distribution, and does not mean that the voltage is the electromotive force.