What Is An Electric Cell: Working And Example - EMBIBE

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What is an Electric Cell?

The idea that certain fluids can generate continuous electric power when used as a conductor was proposed by Volta in \(1800\). This discovery led to the development of the voltaic cell. After this, several scientists got involved in the further sophistication of electric cell technology.

An electric cell is a device that converts chemical energy into electrical energy. It consists of two electrodes immersed in an electrolyte. Several electric cells are connected to form a battery. The electrons flow from the negative terminal to the positive terminal when a battery or cell is connected through the circuit. A battery produces potential differences across its terminals by using chemical reactions. This potential difference provides the energy vital to move the electrons through the circuit.

Thus, a cell or a battery is a chemically powered, self-contained device in which a limited amount of electrical output is generated whenever required. A cell converts the chemical energy stored within it into electrical energy slowly, used over time. The energy generated by a battery is different from the energy that we receive in our homes from an electric power plant. The energy-based on fuel needs to be transferred from one place to another by wires, but the energy supplied by batteries is portable, as seen in our phones, laptops or electric cars.

Learn Combination of Resistances

Components Within an Electric Cell

An electric cell contains three main parts. It consists of two electrodes or electrical terminals present within an electrolyte (a chemical). The entire setup is usually packed inside a metal or plastic outer case for convenience and safety.Two more handy electrical terminals are marked with a plus (positive) and minus (negative) on the surface connected to the inside electrodes. These are the terminals across which external connections are made. The main difference between a battery and a cell is that a battery consists of two or more cells. Thus, in a battery, each cell’s power adds together, and it, therefore, can supply a larger amount of electric energy.When a battery is connected to a circuit, its electrolyte starts buzzing with activity. Slowly, the chemicals inside it are converted into other substances. Ions, formed from the materials within the electrodes, participate in chemical reactions with the electrolyte.The device connected to the battery is powered by the electrons that move from one terminal to the opposite through the outer circuit. This process continues until the whole electrolyte is transformed. At that time, the ions stop moving through the electrolyte, and the electrons stop flowing through the circuit, rendering the battery flat or completely drained.

Working of an Electric Cell

We know that a cell contains three parts, an anode (- electrode), a cathode (+ electrode), and an electrolyte. The positive and negative sides, i.e. cathode and anode, are attached to an electrical circuit. These two electrodes are dipped in an electrolyte which can either be a liquid or dry powder. When this cell is connected to an external circuit, chemical reactions happen within the electrolyte.

Due to these reactions, positive ions and electrons are generated near the negative electrode. The electrons flow towards the positive electrode from the external circuit while the positive ions move into the electrolyte. The electrons recombine with the positive ions present in the electrolyte at the positive electrode. Thus, the circuit is complete, and our device starts working.

The working of a cell involves a chemical reaction called oxidation-reduction reaction. The reaction takes place between the cathode and the anode via the electrolyte. Due to the oxidation reaction, an electrode gets negatively charged, which is called the anode. And due to the reduction reaction, the other electrode, the cathode, gets positively charged.

When two different reactive metals are dipped in the same electrolyte solution, there will be a movement of electrons between them. One metal will lose electrons while the other metal will gain electrons. A potential gradient is created between the two metal electrodes due to the difference in the concentration of electrons around them. This potential difference acts as a source of voltage for any electrical device. It is important to remember that ions only flow through the electrolyte, and the electrolyte blocks the movement of electrons from anode to cathode. Thus, we can only obtain electric current from the terminals of the battery.

The cell’s chemical energy gets converted into electric energy, and gradually, the cell loses its ability to generate power, and its voltage is reduced and eventually, it runs out. Thus, the battery becomes unable to produce positive ions due to the depletion of chemicals, and it ultimately becomes incapable of generating an electric current.

The Electromotive Force of a Cell

A cell is considered to be a source of emf or electromotive force. Although it is important to remember that electromotive force is not a real force, it describes the potential difference generated by a cell in terms of volts. Thus, the emf of a cell can be defined as the voltage supplied by a battery when there is no current flowing through the external circuit.

The emf of a cell is represented by the symbol \(\xi \) and is pronounced as ‘xi’. It gives the amount of work a cell does to move a specific amount of charge across the electric circuit. For an ideal cell, the internal resistance is zero, and its emf is the same as the potential difference across the battery. Cells, in reality, consist of a pair of electrodes dipped in an electrolyte, and they offer resistance to the flow of charges in a battery. Thus, a real battery’s potential difference or terminal voltage is not equal to the battery’s emf. Thus, although a new cell has a non-zero value of internal resistance, its value increases as the battery gets old.

How to Calculate the Internal Resistance of a Cell?

Let the internal resistance of the cell be \({\rm{r}}\). The cell can be represented as a combination of an ideal cell of emf \(\xi \) connected in series with a resistor having resistance \({\rm{r}}\). To calculate the value of internal resistance of the cell, connect a high resistance voltmeter across the cell and the resistor as shown in the figure below:

Calculating the Internal Resistance of a Cell

Since the resistance of the voltmeter is quite high, it draws a very small amount of current to give a deflection, and the circuit can almost be considered an open circuit. Thus, in this case, the voltmeter reading will give the value of emf of the cell. Let the reading of the voltmeter be \({\rm{V}}\).

Now connect an external resistance \({\rm{R}}\) in the circuit, as shown in the figure below:

Let a current \({\rm{I}}\) flow through the circuit. Now, the potential difference generated across the external resistor can be given as:

\({\rm{V = I R – – (1)}}\)

The value of reading \({\rm{V}}\) obtained by the voltmeter is less than the emf of the cell \(\xi \) due to the internal resistance \({\rm{r}}\) of the cell. Thus voltage drop across internal resistance will be:

\({{\rm{V}}^\prime }{\rm{ = Ir – – (2)}}\)

Thus, the emf supplied by the cell can be given as:

\(\xi = {\rm{V}} + {{\rm{V}}^\prime }\)

\(\xi – {\rm{V}} = {\mathop{\rm Ir}\nolimits} – – (3)\)

Divide equation \((3)\) by equation \((1)\)

\(\frac{{{\rm{\xi – V}}}}{{\rm{V}}}{\rm{ = }}\frac{{{\rm{Ir}}}}{{{\rm{IR}}}}\)

\({\rm{r = }}\frac{{{\rm{\xi – V}}}}{{\rm{V}}}{\rm{R}}\)

Thus, if we are given the values of \(\xi ,{\rm{V}}\) and \({\rm{R}}\) we can determine the internal resistance of the given cell. Using the same result, we can also calculate the total current flowing through the circuit.

If the emf of the cell be \(\xi \) and its internal resistance \({\rm{r}}\), then the power delivered to the circuit of resistance \({\rm{R}}\) can be given as:

\({\rm{P = I\xi }}\)

Using equation \((3)\), we get:

\({\rm{P = I(V + I r)}}\)

Using the value obtained in equation \((1)\), we get

\({\rm{P = I(I R + I r)}}\)

\({\rm{P = }}{{\rm{I}}^{\rm{2}}}{\rm{R + }}{{\rm{I}}^{\rm{2}}}{\rm{r}}\)

The term \({{\rm{I}}^{\rm{2}}}{\rm{R}}\) represents the power delivered to the external device and \({{\rm{I}}^{\rm{2}}}{\rm{r}}\) represents the power delivered to the internal resistance. Although for a reliable battery, the power supplied across the internal resistance is quite small, thus, \({{\rm{I}}^{\rm{2}}}{{\rm{r}}^{\rm{2}}} \ll {{\rm{I}}^{\rm{2}}}{\rm{R}}\).

Cells Connected in Series

A battery is a group of cells. When cells are connected in series, the negative terminal of the first cell is connected to the positive terminal of the second cell. The negative terminal of the second cell is connected to the positive terminal of the third cell. Thus, the positive terminal of the first cell and negative terminal of the last cell constitute the terminals of a battery,

Consider the figure shown below. Here \({\rm{n}}\) cells, each having emf \(\xi \) volts and the internal resistance \({\rm{r}}\) ohms, are connected in series with an external resistance \({\rm{R}}\).

Then, total emf of the battery \( = {\rm{n\xi }}\)

The total resistance in the circuit \({\rm{ = n}}{\rm{.(r) + R}}\)

Using Ohm’s law, the current flowing through the circuit:

\({\rm{I = }}\frac{{{\rm{ total\, emf }}}}{{{\rm{ total\, resistance }}}}{\rm{ = }}\frac{{{\rm{n\xi }}}}{{{\rm{nr + R}}}}\)

If \({\rm{r}} \ll {\rm{R}}\),

\({\rm{I = }}\frac{{{\rm{n\xi }}}}{{\rm{R}}}\)

The total current will be \({\rm{n}}\) times the current of a single cell.

If \({\rm{r}} \gg {\rm{R}}\),

\({\rm{I = }}\frac{{{\rm{n\xi }}}}{{{\rm{nr}}}}{\rm{ = }}\frac{{\rm{\xi }}}{{\rm{r}}}\)

It is equal to the current of the single cell. Thus, connecting cells in series to form a battery will be utterly useless in this case. Therefore, a series combination is useful only when each cell’s internal resistance is quite small compared to the external resistance.

Cells Connected in Parallel

When a group of cells are connected in parallel, all the positive terminals of the cells are connected at one point, while all the negative terminals of the cells are connected at another point. These two points form the positive and negative terminals of a battery.

As shown in the figure below, \({\rm{n}}\) cells are connected in parallel, and a resistance \({\rm{R}}\) is connected in parallel to the cells between the points \({\rm{A}}\) and \({\rm{B}}\). If \(\xi \) be the emf and \({\rm{r}}\) be the internal resistance of each cell.

Then the equivalent internal resistance of the battery will be:

\(\frac{{\rm{1}}}{{{{\rm{r}}_{{\rm{eq}}}}}}{\rm{ = }}\frac{{\rm{1}}}{{\rm{r}}}{\rm{ + }}\frac{{\rm{1}}}{{\rm{r}}}{\rm{ + – – n\, term s = }}\frac{{\rm{n}}}{{\rm{r}}}\)

\({{\rm{r}}_{{\rm{eq}}}}{\rm{ = }}\frac{{\rm{r}}}{{\rm{n}}}\)

The total resistance of the circuit \({\rm{ = }}{{\rm{r}}_{{\rm{eq}}}}{\rm{ + R = }}\frac{{\rm{r}}}{{\rm{n}}}{\rm{ + R}}\)

The total current flowing through the circuit, \({\rm{I}} = \frac{{{\rm{ total\, emf }}}}{{{\rm{ total\, resistance }}}} = \frac{\xi }{{\frac{{\rm{r}}}{{\rm{n}}} + {\rm{R}}}}\)

If \({\rm{r}} \gg {\rm{R}}\),

\({\rm{I = }}\frac{{{\rm{n\xi }}}}{{\rm{r}}}\)

The total current will be \({\rm{n}}\) times the current of a single cell.

If \({\rm{r}} \ll {\rm{R}}\),

\({\rm{I = }}\frac{{{\rm{n\xi }}}}{{{\rm{nR}}}}{\rm{ = }}\frac{{\rm{\xi }}}{{\rm{R}}}\)

Thus, here, the total current supplied by the battery is the same as the current due to a single cell. It is only useful to connect cells in parallel when the external resistance is smaller than the internal resistance of the cells.

Applications of an Electric Cell

The invention of an electric cell changed the world. The energy, which was only limited to fuel and steam-based systems, got revamped. The portable and easily accessible form of energy as an electric cell made it possible to develop devices that needed smaller power to run. Presently, we have photovoltaic cells, solar cells, lithium cells, and batteries—the basic design for most of these are the same. Cells are used in watches, satellites, electric car batteries, inverters, toys, TV remotes, phones etc. These cells are being heavily used to power hearing aids and pacemakers in the medical industry.

Summary

An electric cell is a device that converts chemical energy into electrical energy. An electric cell contains three main parts. It consists of two electrodes or electrical terminals present within an electrolyte (a chemical). The entire setup is usually packed inside a metal or plastic outer case for convenience and safety. The working of a cell involves a chemical reaction called oxidation-reduction reaction. The reaction takes place between the cathode and the anode via the electrolyte. When this cell is connected to an external circuit, chemical reactions happen within the electrolyte.

Due to these reactions, positive ions and electrons are generated near the negative electrode. The electrons flow towards the positive electrode from the external circuit while the positive ions move into the electrolyte. Thus, the circuit is complete as the electrons and positive ions recombine near the positive electrode, and the device starts working. Due to the depletion of chemicals, the cell cannot produce positive ions, and it stops functioning eventually.

FAQs

Q.1. What is a battery?Ans: A battery is a group of cells joined together to provide a large amount of electric power.

Q.2. Write a few uses of an electric cell.Ans: An electric cell is used:1. In watches and toys2. In hearing aids and pacemakers3. Tv remotes and laptops4. Phones and Invertors

Q.3. What is an electric cell?Ans: An electric cell is a device that converts chemical energy into electrical energy.

Q.4. What is the EMF of a cell?Ans: EMF or electromotive force is defined as the voltage supplied by the battery when no current is flowing through the circuit.

Q.5. What type of reaction takes place within a cell?Ans: Oxidation-Reduction or “Redox” reactions take place within a cell.

Learn the Concept of Electric circuits here

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