Electrical circuits electric potential energy

Figure 6 A schematic band diagram (electrical potential energy versus distance) of a conventional p-n homojunction solar cell at equilibrium (left) and at short circuit under spatially uniform illumination (right). The energies of the conduction- and valence-band edges are Ecb and Evb. respectively. EF is the Fermi level at equilibrium and EFn and EFp are the quasi-Fermi levels of electrons and holes, respectively, under illumination.

SECTION 20.4 A voltaic cell generates an electromotive force (emf) that moves the electrons from the anode to the cathode through the external circuit. The origin of emf is a difference in the electrical potential energy of the two electrodes in the cell. The emf of a cell is called its cell potential, cell>3nd is measured in volts (1 V = IJ/C). The cell potential under standard conditions is called the standard emf, or the standard cell potential, and is denoted u. 

A voltaic cell generates an electromotive force (emf) that moves the electrons from the anode to the cathode through the external circuit. The origin of emf is a difference in the electrical potential energy of... 

Power, P, defiaed as the rate at which work is performed, is expressed ia terms of energy divided by time and is most commonly given in units of horsepower, as for the power suppHed by mechanical devices such as diesel engines, or in the SI units of watts, especially when measuring electrical power. One horsepower is equivalent to the amount of power needed to lift 33,000 pounds (14,982 kg) one foot (30.5 cm) in one minute. One watt is equivalent to the power required to perform one joule of work per second. In a simple direct-current circuit where potential is represented by E ... 

The ends of a correctly constructed electrochemical circuit measuring electrical potential difference must always have metals or conductors with identical chemical composition. It is usually reached by simple connection of two metals by copper wires. The inclusion between two metal conductors of a third metal conductor according to Volta s law does not change the difference of potentials at the output of a circuit. The difference of potentials in an electrochemical circuit at equilibrium is caused by the change of Gibbs free energy during the appropriate electrochemical reaction ... 

An electrical potential difference between the electrodes of an electrochemical cell (called the cell potential) causes a flow of electrons in the circuit that connects those electrodes and therefore produces electrical work. If the cell operates under reversible conditions and at constant composition, the work produced reaches a maximum value and, at constant temperature and pressure, can be identified with the Gibbs energy change of the net chemical process that occurs at the electrodes [180,316]. This is only achieved when the cell potential is balanced by the potential of an external source, so that the net current is zero. The value of this potential is known as the zero-current cell potential or the electromotive force (emf) of the cell, and it is represented by E. The relationship between E and the reaction Gibbs energy is given by... 

This equation gives the relation between the electrical potential difference between the copper wires attached to the electrodes when the cell is at equilibrium and the change of the Gibbs energy for the change of state that would take place in the cell if the cell were short-circuited. We point out here that the chemical potentials of electrons refer to 1 mole of electrons or 1 faraday of electricity. Therefore, A G refers to the change of state per faraday. If the change of state requires n faradays,... 

The electrical potential difference between two points in a circuit causes the transfer of charge from one point to the other. The volt (V) is the SI unit of electrical potential. When a charge of 1 C moves through a potential of 1V, it gains 1J of energy. Summarized ... 

To summarize voltaic cells, let s review the components that create the cell. First, you need two half-cells, each of which contains an electrode immersed in an electrolytic solution (typically containing the cation of the metal in the electrode). A spontaneous reaction must occur between the electrode and the solution. A wire connects the two electrodes and will allow the external flow of electrons from the anode to the cathode. In Figure 18.1, a voltmeter is shown as part of the circuit between the two electrodes. This is not a necessary part of the circuit—it is simply there to measure the voltage across the circuit. The salt bridge completes the electric circuit and allows the flow of cations and anions between the two half-reactions. Sometimes a porous disc is used in place of a salt bridge. The driving force for the current is the difference in potential energies between the two half-cells. 

Since it is impossible to measure the individual electric potential differences at the phase boundaries, we shall hereinafter speak only in terms of the difference in electric potential across the two terminals connected to the electrodes of the battery. When in a battery the current is not flowing or tends to zero, the measurable potential difference across the two terminals is called the open-circuit voltage (OCV), fJc, and it represents the battery s equilibrium potential (or voltage). Since it is related to the free energy of the cell reaction, the OCV is a measure of the tendency of the cell reaction to take place. Indeed, while the conversion of chemical into electric energy is regulated by thermodynamics, the behavior of a battery under current flow (the current is a measure of the electrochemical reaction rate) comes under electrochemical kinetics. 

Electrical energy provided through an external circuit to an electrolytic cell drives reactions that are not spontaneous, converting electrical energy into chemical potential energy. 

Current is the rate of charge flow in a circuit or solution. One ampere of current is a charge flow rate of one coulomb per second (1 A = 1 C/s). Voltage, the electrical potential difference, is the potential energy that results from the separation of charges. One volt of electrical potential results when one joule of potential energy is required to separate one coulomb of charge (1 V = 1 J/C). 

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