Back emf

A description of an electrolytic cell has already been given under cell features (Section 1.3.2, Fig. 1.1c). Another example is the cell with static inert electrodes (Pt) shown in Fig. 3.1 where an applied voltage (Eappl) allows a current to pass that causes the evolution of Cl2 gas at the anode and the precipitation of Zn metal on the cathode. As a consequence, a galvanic cell, (Pt)Zn 2 ZnCl2 Cl2 iPt+, occurs whose emf counteracts the voltage applied this counter- or back-emf can be calculated with the Nernst equation to be... 

In contrast to a capacitor, an inductor will allow the passage of DC and low-frequency AC much more freely than high-frequency AC. This is because the amount of back EMF generated is proportional to the rate of change of the current... 

A graph of current flow versus time aims to show how an inductor affects current flow in a circuit. It is difficult to draw a graph for an AC circuit, so a DC example is often used. The key point is to demonstrate that the back EMF is always greatest when there is greatest change in current flow and so the amount of current successfully passing through the inductor at these points in time is minimal. 

Current Draw a build-up exponential curve (solid fine) to show how current flows when an inductor is connected to a DC source. On connection, the rate of change of current is great and so a high back EMF is produced. What would have been an instantaneous jump in current is blunted by this effect. As the back EMF dies down, a steady state current flow is reached. 

Back EMF Draw an exponential decay curve (dotted) to show how back EMF is highest when rate of change of current flow is highest. This explains how inductors are used to filter out rapidly alternating current in clinical use. 

With regard to points in a distributor, these are opened mechanically with the resulting bounce, lag and dwell, however electrically operated relays rely solely on the induced magnetism of the operating coil. In this instance are we expecting each relay to operate, lets say, assuming 6000 rpm, that the relay will operate a hundred times per second How is the back emf of the relay coil accounted for Or does it just work anyway I must admit I am confused but not too doubtful especially if someone claims that it works, you never know ... 

During the excitation discharge phase, however, use of the closed current loop circuit results in a back emf (electromotive force) across the source... 

The back emf, or galvanic voltage, is the difference between the anode and cathode potentials, each of which is the sum of a reversible potential given by... 

If a net chemical change is to be efiected, the anodic reaction cannot be simply the reverse of the cathodic reaction. Therefore, owing to the formation of electrolytic products at the electrodes, a galvanic cell is set up when current is caused to flow. The polarity of this galvanic cell is in opposition to the applied emf, giving rise to a back emf. The amount of current that flows is given by Ohm s law,... 

The back emf can be regarded as being made up of three components (7) a reversible back emf, (2) a concentration overvoltage, and (3) an activation overpotential. 

The reversible back emf is the reversible emf of the galvanic cell set up by the passage of the electrolytic current, based on concentrations of solutes involved in the electrode reactions in the bulk of the solution. For example, if an acidic solution of copper sulfate is electrolyzed between platinum electrodes, the electrode reactions are... 

Concentration overpotential is also observed when the surface concentration is increased over the bulk concentration. The most common example is the anodic dissolution of a metal. Suppose that, after part of the metal ion has been plated out from the solution in the above example, the applied emf is decreased to a value below the reversible back emf. The ceU now will operate as a galvanic cell, with the metal-plated electrode acting as the anode. The metal ion concentration at the anode surface becomes greater thah the bulk concentration of metal ion. As anodic polarization is increased, however, there is no limit to the surface concentration of metal ion except that imposed by the solubility of a salt. Since the surface concentration would have to be 10 times the bulk concentration to produce a concentration overpotential of 0.059/n V, the anodic concentration overpotential for metal dissolution is generally small unless the bulk concentration is low. 

In some instances. Equation (14-24) does not hold at the beginning of the electrolysis, because the initial concentration may be so high that the proportionality between the current and the concentration does not hold. Thus, if the source is unable to supply the total voltage -I- iE) at the level of current that can be sustained by the rate of mass transfer, the current is determined initially by the back emf and the electrolytic resistance [Equation (14-1)]. This statement amounts to saying that the cathode potential corresponds to a rising portion of the curve for cathodic current against voltage rather than to a point on the plateau. 

The minimum working electrode potential to begin reducing cadmium [back-emf (electromotive force) required to force the reaction] can be calculated from the Nemst equation (Chapter 13) ... 

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