Intro
Definition of cells
Cells are devices that can convert chemical energy into electrical energy.
(or)
An electrolytic cell is a device that maintains a steady current in an electric circuit by converting chemical energy into electrical energy through electrolysis.
Primary Cells
Primary cells are non-rechargeable electrochemical cells that convert chemical energy into electric energy.
Examples
Leclanche Cell
Leclanche Cells are primary cells commonly used in household batteries.
Mercury Cell
Mercury Cells are primary cells commonly used in watches.
Secondary Cell
A secondary cell is a rechargeable cell and it can store electrical energy in the form of chemical energy.
Examples
Lead-Acid Cell
Lead-acid cells are secondary cells commonly used in automobiles.
Nickel-Cadmium Cell
Nickel-cadmium cells are secondary cells commonly used in portable electronic devices.
Lithium-Ion Cell
Lithium-ion cells are secondary cells commonly used in modern gadgets like smartphones and laptops.
Working Principle of Cells
A cell consists of two rods of different metals which are called electrodes. These are immersed in a liquid called electrolyte. One plate becomes positively charged and the other negatively charged. The electrodes exchange charges with the electrolyte. The positive electrode has a potential difference V+ (V+>0) generated at a point between the positive electrode and the electrolyte and in the same way negative potential difference V- (V- <0) is generated between the negative electrode and the electrolyte.
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| Electrolytic cell |
When no electric current flows through the cell then potential remains constant (same potential throughout) in the whole electrolyte. The potential difference between positive and negative terminals is
`V_+ - (V_-) = V_+ + V_-`
At this time the circuit is an open circuit and the potential difference is called the 'electromotive force (emf)' of the cell and is denoted by ε.
EMF (Electromotive Force)
Electromotive force is the work done by the cell per unit charge that passes through it. It is the potential difference between the two terminals of a battery when the circuit is open. It is measured in volt.
`\epsilon = \frac {W}{q}`
In an open circuit, the electric current is zero (I = 0). Hence, Terminal voltage and emf are equal.
Note: EMF is always greater than or equal to the potential difference V (terminal Voltage).
Internal Resistance
When we connect the plates of a cell by an external resistance R, an electric current flows through the external resistance from the positive plate of the cell towards the negative plate toward the positive plate. The resistance offered by the electrolyte of the cell to the flow of current (ions) within the cell is called the internal resistance of the cell. It is denoted by r and measured in ohm.
The internal resistance of an ideal cell is zero. In general, cells have finite internal resistance.
When a cell is in a closed circuit current flows from the cell to the external resistance then the potential difference between the electrodes of the cell is called terminal potential difference. It is denoted by V.
In closed circuit
`V = \epsilon - I r` .........(Eq. 1)
`R I = \epsilon - I r`
`R I + I r = \epsilon`
` I (R + r) = \epsilon`
` I = \frac{\epsilon}{R + r}` .........(Eq. 2)
From eq. 1
`V = \epsilon - I r`
`I r = \epsilon - V`
` r =\frac{ \epsilon - V}{I}`
` r =\frac{ \epsilon - V}{\frac{V}{R}}`
` r =\frac{ \epsilon - V}{V} R`
This is the formula for the internal resistance of a cell.
The unit of internal resistance (r) is the ohm.
When a cell is going to charge then current enters the positive electrode then the terminal potential difference will be
`V = \epsilon + I r` here `V > \epsilon`
Internal Resistance of a Cell Depends on
`\star` Nature of the electrode material: The internal resistance of a cell depends on the electrode material because different material have different resistances and reactivity.
`\star` Electrolyte Concentration: Internal resistance of a cell is directly proportional to the electrolyte concentration.
`\star` Area of Electrode: The internal resistance of a cell is inversely proportional to the area of electrodes (anode and cathode) in the electrolyte.
`\star` Temperature: The internal resistance of a cell decreases as the temperature increases.
`\star` Distance between Electrodes: The internal resistance of a cell is directly proportional to the distance between the electrodes (anode and cathode).
PHYSICS NOTES
- Electric Current Definition and Electric Currents in Conductors
- Ohm’s Law: Definition, Formula, Limitations, Derivation, Diagram, and Deduction of Ohm's Law
- Drift of Electrons and the Origin of Resistivity
- Resistivity, Resistivity of Various Materials, Temperature Dependence of Resistivity
- Mobility Formula in terms of Relaxation Time
- Electrical Energy, Power
- Cells, Emf, Internal Resistance
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