Electric Cell and its Internal Resistance

Introduction

We have all seen and used electric cells, more commonly known as cells or electrochemical cells, in our day-to-day lives. The cells we use in TV remotes, alarm clocks, or any other electrical appliance are examples of electric cells. They were invented by an Italian scientist named Luigi Galvani in the 1800s. Thus, they are also called galvanic cells. This article teaches about the electric cell, its components, working mechanism, and the internal resistance it provides when connected to a circuit.  

What is an Electric Cell? 

An electric cell is the powerhouse of the circuit. It acts as an electric power supply by storing the chemical energy present in the chemical solutions. These cells convert chemical energy into electric energy. Thus, generating electric current and acting as a driving force for the chemical reactions. Two or more cells together constitute a battery. 

 An electrochemical cell consists of two electrodes, mainly, the anode and the cathode. The anode is a positively charged terminal, whereas the Cathode is a negatively charged terminal. These terminals are dipped in an electrolyte solution, and redox reactions like oxidation and reduction take place at the end of the terminals.   

Working of Electric Cell 

The electric cell is primarily driven by chemical reactions. The cathode and anode are dipped in an electrolyte solution. An electrolyte solution is a solution that carries free ions, which can be either positively charged ions or negatively charged ions. 

In this case, the positively charged ions are called cations. They are called this because they are attracted to the cathode, a negatively charged terminal. Similarly, the negatively charged ions are called anions because they migrate to the anode, which is the positively charged terminal. 

In this entire process, the cations get reduced, and the anions are oxidized. Hence, the reduction takes place at the cathode, and the oxidation takes place at the anode. The electrolytic cell is thus a good example of redox reactions. These freely charged ions are responsible for conducting electricity in the outer circuit. This entire process is called electrolysis. Also, during the process, the electrodes do not touch each other, but rather the electrolyte solution acts as a medium through which they are electrically connected.   

The Electromotive Force 

The electromotive force of a cell, more commonly called the EMF of the cell, can be defined as the maximum potential difference between the electrodes of the electrolytic cell. It is the measure of the net voltage between the reduction and oxidation reactions. 

The value of the electromotive force depends on the degree of reduction and oxidation. In addition, it depends on the compound’s tendency to lose or gain electrons.   

The Formula

When there is no current flowing through a circuit, this condition is known as an open circuit. In this case, the electromotive force is given by the difference between the two electrodes.  Anode has a positive potential, denoted by (V+), whereas cathode has a negative potential (-V–). This potential difference between the electrodes is known as the electromotive force (EMF) of the cell, and it is equal to,

ξ = (V+) − (−V–) = V+ + V–

About the Internal Resistance 

We know that when the current flows in the circuit, it is proportional to the voltage difference in the circuit. The current is defined as the ratio between the potential difference (V) measured in Volts to the resistance (R) in Ohms (Ჲ). It is given by the formula. 

R = V/I 

Resistance is the measure of the opposition of electric current in a circuit. And, its SI unit is Ohm (Ჲ). 

Similarly, how wires have resistance, even the electrolyte solutions in batteries oppose the flow of electric current, leading to a finite value of resistance. This is called the internal resistance of the electric cell.

Its Derivation and Formula 

The electromotive force can be written down as the sum of the potential difference between the two electrodes, and then, we further subtract the product of current through the circuit, and the internal resistance experienced due to the electrolytic cell as follows, 

V = V+ + V− − Ir 

V = ξ – Ir (ξ = V+ + V−)

Here r is internal resistance.  This internal resistance is usually negligible in real life; the electromotive force, also known as EMF, is relatively more prominent than the internal resistance. Moreover, when a piece of external electrical equipment is attached to the circuit, its resistance (R) is also added.

V = IR

Substituting this into,

V = ξ – Ir

IR = ξ – Ir

I = ξ / (R + r)

Thus, the cell’s Internal Resistance can be written as r = ξ – IR/I in ohms. 

Conclusion

To summarize what we learnt in this article, here are a few key points:

We first learnt about the electric cell, which acts as a power supply device in a circuit and produces electricity by converting chemical energy into electric energy.

Second, we learnt about the components of an electrochemical cell. It consists of two electrodes, which are anode and cathode, and an electrolytic solution that acts as an electrically connecting medium for the electrodes.

Thirdly, we saw the working mechanism of an electrolytic cell. We learn about the reduction and oxidation processes taking place at the electrodes and how they are responsible for the flow of electric current.

Next, we learnt about the electromotive force, which is used in determining if the cell is galvanic or not. Then, we learned about the internal resistance of the cell or electrochemical cell and learnt how to derive its formula.