There are some components that are an integral part of a circuit, like EMF and internal resistance of a cell, and they are all interconnected. In this article we will look at electrochemical cells, EMF and the internal resistance of a cell. Later we will also see the terminal potential difference and cell types in detail. Contents:
- electrochemical cell
- Electromotive force (EMF)
- internal resistance of a cell
- Connection potential difference
- Difference between EMF and potential difference
- Important points about Cell
- type of cells
electrochemical cell
Andelectrochemical cellIt is a device capable of generating electrical energy from chemical reactions or using electrical energy to produce chemical reactions. Electrochemical cells that produce an electric current are referred to as voltaic cells or galvanic cells, and those that produce chemical reactions such as electrolysis are referred to as electrolytic cells.A common example of a galvanic cell is a standard 1.5 volt cell.Cell intended for consumer use. A battery consists of one or more cells connected in parallel, in series, or in series and parallel.[fuente]An electrochemical cell is a device that maintains the flow of charge in an electrical circuit by converting chemical energy into electrical energy.It usually consists of two electrodes made of different materials and an electrolyte. The electrode with the highest potential is calledAnodeand the one with the lowest potential iscathode.[caption id="attachment_4602" align="aligncenter" width="177"]
Electrochemical Cell[/caption]
Electromotive Force (EMF)
The emf of a cell is defined as the work done by the cell to move a unit of positive charge around the entire circuit, including the cell once.
- EMF E = W/q; The SI unit is joule/coulomb or volt.
- EMF is the maximum potential difference between the two electrodes of the cell when no current is drawn from the cell.
- EMF is the characteristic property of the cell and depends on the type of electrodes and electrolyte used in the cell.
- EMF is independent of the amount of electrolyte, the size of the electrodes and the distance between the electrodes.
internal resistance of a cell
The resistance offered by the electrolyte in the cell to the flow of current is called the internal resistance of the cell. The internal resistance of the cell depends on it.
- Electrode gap (r ∝ d) The larger the electrode gap, the greater the length of the electrolyte through which the ions have to move, and the greater the internal resistance.
- conductivity or composition of the electrolyte (r ∝ 1/σ)
- Electrolyte concentration (r ∝ c)
- Electrolyte temperature (r ∝ 1/T)
- Type and area of electrodes immersed in electrolyte (r ∝ 1/A)
Connection potential difference
The potential difference between the two electrodes of a cell in a closed circuit, i. H. when current is drawn from the cell is called the terminal potential difference.
(a) When the cell discharges
When the cell discharges, the current in the cell flows from the cathode to the anode. [caption id="attachment_4603" align="aligncenter" width="230"]
Cell discharges[/caption]current ${\rm{I}} = {{\rm{E}} \over {{\rm{r}} + {\rm{R}}}}$ or $ \quad {\rm{E}} = {\rm{IR}} + {\rm{Ir}} = {\rm{V}} + $Ir $\quad $ or ${\rm{V}} = {\ rm{E}} - $IrWhen current is drawn from the cell, the potential difference is less than the emf of the cell. The higher the current drawn from the cell, the lower the terminal voltage. When a large current is drawn from a cell, its terminal voltage drops.
(b) When the cell is charged
When the cells are charged, current flows from the anode to the cathode within the cell. Current ${\rm{I}} = {{{\rm{V}} - {\rm{E}}} \over {\rm{r} }}\quad $ or $\quad {\rm{V }} = {\rm{E}} + {\rm{Ir}}$During charging, the terminal potential difference is greater than the cell emf.[caption id=" attachment_4604" align="aligncenter " width="234" ]
The phone is charging[/caption]
(c) When the cell is open
In an open circuit, ${\rm{R}} = \infty $${\rm{I}} = {{\rm{E}} \over {{\rm{R}} + {\rm{r} } }}} = 0$Then ${\rm{V}} = {\rm{E}}$In an open circuit, the potential difference across the terminal is equal to the emf and is the maximum possible potential difference a cell can provide.
(d) When the cell is short-circuited
In the short circuit $R = 0$ then $I = {E \over {R + r}} = {E \over r}\quad $ and $\quad V = IR = 0$In the short circuit the cell current is maximum and the Terminal potential difference is zero.
(e) Power transferred to the load per cell
${\rm{P}} = {{\rm{I}}^2}{\rm{R}} = {{{{\rm{E}}^2}{\rm{R}}} \ sobre {{{({\rm{r}} + {\rm{R}})}^2}}}$$\matrix{{{\rm{so}}} & {{\rm{P}} = {{\rm{P}}_{\max }}\quad {\rm{ si }}\quad {{{\rm{dP}}} \over {{\rm{dR}}}} = 0 } \cr{\rm{P}} & { ={{\rm{P}}_{\max }}\quad {\rm{ si }}\quad {\rm{r}} = {\rm{ R}}} \cr} $La potencia transferida por la celda a la carga es máxima cuando $r = R$ y ${P_{\max }} = {{{E^2}} \over {4r}} = {{{ E^2}} \over {4R}}$[caption id="attachment_4605" align="aligncenter" width="243"]
Power transferred to the load per cell[/caption]
Difference between EMF and potential difference
[title id="attachment_4606" align="aligncenter" width="594"]
Difference between EMF and potential difference[/caption]
Important points about Cell
- The current inside a cell is due to the movement of positive and negative ions, while outside it depends on the type of circuit elements such as conductor, semiconductor, gas or electrolyte.
- The cell is a constant emf source and not a constant current source because as the resistance of the circuit changes, the current I = E/r + R also changes, but the emf remains constant.
- Since I = E/r, more current can be drawn from a cell with larger EMF and smaller internal resistance. Example: in a lead-acid battery E = 2.05 V and rMinimum= 0.1 Ω.
- As the cell is used, its internal resistance increases sharply, but the emf drops slightly. The current ability to deliver is reduced.
- A cell neither creates nor destroys charge, but maintains the flow of charge by providing the necessary energy.
- The emf of the cell is considered positive in a circuit when the current in a cell is from negative to positive, i.e. during discharge, negative otherwise. [caption id="attachment_4607" align="aligncenter" width="594"]
EMF of a cell[/caption] - Capabilityof a battery is equal to the product of the current in amperes and the time in hours that a cell can operate. It depends on the amount of electrolyte and the size of the cell. Example: A capacity of 8 Ah means that we can draw a current of 8 A for 1 hour or a current of 2 A for 4 hours.
EMF of a cell[/caption]cell type
primary cells
Cells that cannot be electrically charged are called primary cells. Here, the original state of the cell cannot be restored by passing electrical energy through the cell from an external source after the cell is discharged. Example: voltaic cell, Daniel cell, Leclanche cell, alkaline manganese cell, mercury button cell, etc.
secondary cells
Cells in which the chemical process is reversible are called secondary cells. The original chemical state of the cell can be restored by passing electrical energy through the cell from the outside. Example: lead-acid battery, alkaline cells, etc.
Comparative study of different cells
[title id="attachment_4701" align="align center" width="571"]
Comparative study of different cells[/caption]
VOLTAIC CELL BREAKDOWN
local Action:This is due to impurities from copper, iron carbon, etc. in commercial zinc. When the zinc rod is immersed in electrolyte, impurities and zinc in contact form small local cells in which small currents are generated, resulting in wastage of zinc even when the cell is not used. This defect is called local action. This is solved by amalgamating zinc rods with mercury. When the cell is in use, fresh zinc continues to rise to the surface, allowing the chemical reaction to continue with local action.Polarisation:Polarization is the formation of hydrogen gas bubbles at the anode of the cell. This leads to an increase in internal resistance since the hydrogen shell is a poor conductor. Hydrogen ions moving to the anode cannot reach the anode and transfer their charge. These positively charged ions set up a field from hydrogen to zinc, resulting in a reverse emf that weakens cell action. This deficiency can be overcome by using a depolarizer, ie an oxidizing agent such as MnO&sub1;&sub0; or CuSO 20 which oxidizes hydrogen in water.The structure of some cells is shown below:[title id="attachment_4702" align="aligncenter" width="541"]
Construction of Voltaic and Daniel Cells[/caption][caption id="attachment_4704" align="aligncenter" width="529"]
Building Leclanche and Dry Cells[/caption]Also read:
- Types and Effects of Electric Current
- Ohm's law and resistance
- resistance combination
- EMF and internal resistance of a cell
- Series connected, parallel and mixed cells
- District law of Kirchhoff
- Electric currents in conductors
- wheat stone bridge
- Letter box
- Wheatstone tube bridge
- Moving coil galvanometer
- ammeter and voltmeter
- Potentiometer working principle