The charge is developed in nonmetals like plastic, wool, nylon, polyester, ebonite, etc. in insulating materials due to friction. These materials when rubbed against each other develop the charge. When we put off cyclic or polyester dress, we hear the clicking sound due to the transfer of charges. The dress materials get charged due to the friction of the body.
A cell is a device where a potential difference between two conductors is maintained by a conversion of chemical energy into electrical energy. A cell consists of two metal rods called electrodes dipped in a solution called electrolyte.
When the cell is in pen circuit, the potential difference between electrodes is called its electromotive force (e. m. f). The strength of the cumulative e.m.f., which a cell can produce in a given circuit, depends on the e.m.f. The e.m.f of cells depend on
This cell was invented by Alessandro Volta in 1800. It consists of two plates, one copper and other of zinc, both are dipped in dilute sulphuric acid taken in the glass container. When two plates are connected by, a copper wire provided with a bulb in between, the bulb glows showing the flow of electric current. The copper and zinc plates are called the electrodes of the cell. Zinc is the negative electrode as it supplies the electron and copper is the positive electrode. Dilute sulphuric acid is the electrolyte. The electron current flows from zinc to copper. The potential difference between the terminals of the cell is 1.08V. Therefore, it is not suitable for use in devices like torches, calculators, transistors, etc.
Zn → Zn+ + 2e-
H2SO4 → 2H+ + SO4--
2H + +2e- → H2
Zn + H2SO4 → ZnSO4 + H2
We cannot draw current for a long time as this cell has defects. There are two types of defects in the voltaic cell. These are:
It is caused by the deposition of the hydrogen gas on the surface of the copper plate in the form of small bubbles. They prevent the hydrogen ions from reaching the copper plate and thus the current given by the cell, decreases gradually. Polarization stops functioning of the cell. Polarization can be avoided by brushing off the hydrogen bubbles or by putting a chemical called potassium dichromate in acid. Potassium dichromate oxide changes hydrogen to water.
The combined zinc used in the cell contains certain impurities like carbon, iron, etc. Because of this local current is set up between zinc and these impurities in addition to the main current. These local currents cannot be utilized and thus constitute wastage of energy. Local action reduces the life of the cell. Amalgamating is the coating of zinc plate with mercury. Mercury covers these impurities present in the zinc plate. Local action can be avoided by amalgamating the surface of the zinc plate.
It was invented by Laclanche Georges in 1865. A dry cell is made without using the liquid. It consists of a zinc container, which also serves as the negative terminal. The electrolyte is a paste of ammonium chloride. A carbon rod with a brass cap serves as the positive electrode. It is surrounded by a mixture of powdered manganese dioxide and carbon particles.
Hydrogen is formed in the chemical reaction that takes place in the cell. The manganese dioxide reacts with the hydrogen formed and prevents it from collecting on the zinc electrode. In this way, the presence of manganese dioxide prevents the polarization. A porous seal at the top prevents the chemicals from dying out. The cell is dry from outside, but the chemicals are wet. The local action cannot be removed completely so it may decrease the life of the cell.
The electrons flow in a wire connecting the two terminals from the zinc to the carbon electrode. The dry cell, being compact, light and portable, is very convenient to use. Dry cells cannot be charged and reused and therefore they are called primary cells. Cells used in vehicles for lighting purposes or as anemergency light in our home called lead-acid cell. They can be charged and used again. Therefore, these cells are called secondary cells.
Zn → Zn++ + 2e-
2NH4Cl → 2NH3 + 2H+ + 2Cl-
2N++ + 2Cl- → ZnCl2
Zn + 2NH4Cl → 2NH3 + ZnCl2+ 2H+ + 2e-
The electrons so released are collected by zinc rod making it as negative potential with respect to the electrolyte.
The ammonia gas (NH3) so produced escapes and hydrogen ions react with manganese dioxide (MnO2) as follows the forming water.
H+ + MnO2 → MnO3 + H2O+ 2e++
Thus, the positive charge is transferred to the carbon rod, which gains positive potentials with respect to the electrolyte.
Advantages of dry cell over simple cell
In our daily life, we use more than two cells for different purposes. These cells are connected for use and this arrangement is called combination or connection of cells.
Cells in series:
When a number of cells are connected in series, the negative pole of one cell is connected to the positive pole of the cells and so on. When the internal resistance the cells are connected in series, to get the maximum current.
Cells in parallel combination:
When a number of cells are connected in parallel, all the positive poles are connected to one point and all the negative poles are connected to the other. When the external resistance is negligible in comparison to the internal resistance, the cells are connected in parallel so as to keep the current constant.
What is static and current electricity? Write with an example.
The electricity in which the electric charges do not flow or remain at rest is called static electricity. The charged produced on the comb, glass rod, etc. are the examples of it. The electricity in which the electric charges flow is called current electricity. Eg: current through the copper wire, etc.
How does a comb rubbed with hair attract pieces of paper? Explain.
When we rub a comb on our hair, then electrons of the atoms of the hair are transformed to the comb so that comb becomes negatively charged. When this comb is brought near to the pieces of paper, positive charges accumulate at the edge of the paper, as opposite charges attract. Now, comb starts to attract pieces of paper.
Why do we hear a crackling (tik) sound with sparks when we take off nylon clothes during the night?
Nylon or acrylic clothes get charged due to friction with the body for the whole day. While taking off these clothes, transfer of charges take place and crackling sound with sparks is produced.
What are anode and cathode?
Electrode with the negative terminal is known as anode whereas electrode with the positive terminal is called cathode.
What are the two defects of a simple cell?
The two defects of a simple cell are:
What is polarization and how do you remove this defect?
During chemical reaction in a cell, hydrogen gas is produced and it accumulates, it is known as polarization.
It can be removed by using depolarizer like manganese dioxide, potassium dichromate, etc.
What is local action? How do you remove it?
Zinc plate used in the simple cell is not pure so impurities present in it provides a conducting path for the flow of charge. This makes the zinc wasted. It is called the local action.
It can be removed by using pure zinc or amalgamated zinc rod.
What is the effect of the local action on the simple cell?
Local action reduces the life of the cell.
The simple cell is not used to produce current for a long time, why?
The simple cell is not used to produce current for a long time because of the defects like polarization and local action.
Write two differences between simple cell and dry cell?
|Simple cell||Dry cell|
|It has liquid chemicals filled in a vessel.||It has dry chemicals.|
|Zinc plate is negative terminal and copper plate is positive terminal.||Carbon rod is positive and zinc can is negative.|
Dry cell is more useful than simple cell in our daily life, why?
Simple cell has liquid chemicals and it is difficult to carry from one place to another. Dry cell has no solution. So, it is easier to carry it from one place to another and there is no risk of spilling acid from the dry cell. So, dry cell is more useful than simple cell in our daily life.
What is the role of MnO2 in a dry cell?
MnO2 is used as depolarizer in a dry cell. When the hydrogen gas is produced and accumulated, then MnO2 reacts with the gas and does not let the hydrogen gas to accumulate at a place. i.e. 2MnO2+ H2 → Mn2O3+ H2O
What is circuit? What are open and closed circuits?
The conducting path of the electricity where cell, wire, switch and load are connected is called an electric circuit. An open circuit is a circuit in which the current is not flowing or switch is off.
A closed circuit is a circuit in which the current flows or switch is on.
What do you mean by the combination of cells? Name its types.
The process of arranging two or more than two cells in a circuit is called the combination of cells.
Its types are:
What is the parallel combination of cells? Write its one advantage and disadvantage.
If the positive terminals of two or more cells are joined together at a point and all the negative terminals are joined at another common point, then this combination is called the parallel combination of cells.
It's one advantage is:
Its one disadvantage is:
What is series combination of cells? Write its one advantage and disadvantage.
If the positive terminal of the first cell is connected to the negative terminal of the second cell and the positive terminal of the second cell is connected to the negative terminal of the third cell and so on, then it is known as the series combination of cells.
It's one advantage is:
Its one disadvantage is:
Distinguish between series and parallel combination of cells
|Series combination of cells||Parallel combination of cells|
|If the positive terminal of the first cell is connected to the negative terminal of the second cell and the positive terminal of the second cell is connected to the negative terminal of the third cell and so on, then it is known as the series combination of cells.||If the positive terminals of two or more cells are joined together at a point and all the negative terminals are joined at another common point, then this combination is called the parallel combination of cells.|
|The bulb glows with more brightness.||The bulb glows with less brightness.|
|The bulb glows for a shorter time||The bulb glows for a longer time.|
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