Understanding Cells, EMF and Internal Resistance

CBSE board exam may include a simple definition based questions for students to answer what is a cell. A cell is a single unit device used for converting chemical energy into electrical energy. This is the basic building block of batteries. In general, a cell has two electrodes (a cathode and an anode) and an electrolyte. There occurs a chemical reaction between electrodes and electrolytes, which results in the flow of electrons. This generates electrical energy.

A cell works through the conversion of chemical energy to electrical energy through a number of redox reactions. For JEE Main exam and IIT JAM exam, we will be showing how a cell works in steps:

1. At anode in a cell, a chemical reaction occurs where reactant loses electron. 
2. The electrons which are released at anode flow through external circuit and create an electric current.
3. This flow of electrons is used for powering devices that are connected to the cell.
4. Another chemical reaction takes place at the cathode where a reactant gains electrons.
5. This reaction consumes all those electrons that have flowed through an external circuit.
6. For maintaining electrical neutrality, ions flow within cell. In electrolyte, positively charged ions move towards cathode and negatively charged ions move towards anode. 
7. The overall reaction in cell is the sum of oxidation and reduction half-reaction.

Electromotive Force or EMF is the measure of energy that a cell or a battery can produce per unit of electric charge. For NEET exam and CUET exam aspirants, knowing the mathematical expression of EMF is also very important. Mathematically, it is expressed as:

$ℰ=\frac{dW}{dq}$

Here:

• ℰ represents the electromotive force in volts
• dW refers to the infinitesimal amount of work done in joules (J)
• dq refers to the infinitesimal amount of flowing charge 

EMF is generated by those chemical reactions that occur within a cell or battery. These reactions cause separation of charges, which create, a potential difference between two electrodes. In simple words, EMF is the potential difference where no current is flowing.

It is important to understand the factors that influence the electromotive force. One of the main reasons for understanding these factors is optimizing performance and efficiency of electrochemical cells:

• Nature of electrodes and electrolytes: Combinations of different materials produces different EMFs due to the variation in their chemical properties.
• Concentration of Reactants: Concentrations of all those reactants that are involved in electrochemical reactions impact the EMF. As per Nernst equation, EMF of cell is dependant on the concentrations of reactants and products. As the concentration of reactant increases, EMF also increases.
• Temperature: It is also an important factor that influences the EMF of a cell. Again, as per the Nernst equation, there are effects of temperature of EMF. An increase temperature leads to higher EMF. 
• Surface area of Electrodes: The surface area of electrodes impact the internal resistance of a cell. This, in turn, impacts the effective EMF. Larger surface area reduces internal resistance, which leads to higher terminal voltage closer to its EMF.

Internal resistance is the opposition or obstruction to the flow of electric currents in a cell or battery itself. This resistance is because of the material used for designing the cell. Due to this internal resistance, the efficiency and performance of the cell are all impacted. Many other factors, like the resistance of electrolytes, electrodes and the resistance at the interface between electrodes and electrolytes, cause internal resistance. The following factors cause internal resistance:

• When an electrolyte in a battery has resistance to the flow of ions, it contributes to the overall internal resistance.
• Many materials that are used for electrodes have their own resistance. These can vary from material to material and their physical properties.
• Resistance at the interface between electrodes and electrolytes also contributes to internal resistance.