Internal resistance of a cell

Introduction

‘Internal resistance of a cell’ basically means the resistivity inside a cell. In electricity, we know that current flows in a circuit in a steady manner. To maintain the current in a steady manner, the electrolytic cell is required. There are two types of resistance: internal resistance and external resistance. 

Before understanding these terms, we have to know about the term ‘resistance’. What is resistance in electricity? Simply, it is the barrier that hinders the flow of current in an electric circuit. In other words, it is the force that slows down the flow of current. It is denoted by R. 

So, internal resistance is the resistance inside a cell whereas external resistance is the resistance due to external load. In this article, we will discuss the internal resistance of a cell.

Body

Let’s consider a simple device that is operated by cells, which has two electrodes – positive and negative terminals. Both the electrodes are immersed in an electrolytic solution. The electrodes start exchanging their charges in this electrolytic solution. The positive electrode has V+ potential difference with the electrolytic solution. Same as positive electrodes, the negative electrode also produces a negative potential difference that is V-. A steady current is maintained by the flow of charge from negative terminal to positive terminal,  whereas R is the resistance from external load.

So, the electrolyte through which the current flows has finite resistance r is called internal resistance.

What is an electrochemical cell?

It is a device that produces electrical energy from the chemical reaction occurring in the device or a device that uses electrical energy to cause chemical reactions. Those cells which generate electrical energy are called galvanic cells. On the other hand, those that use electricity to cause chemical reactions are called electrolytic cells.

Electrochemical cell represent the following,

1)  The anode and cathode terminals are separated by a salt bridge.

2)  The anode is on the left side whereas cathode is on the right side

Important terms

ELECTRODES: These are the terminals that are separated by the solution. Through these electrodes the current passes between metallic and non-metallic parts of an electric circuit.

HALF-CELL: Either of the two parts of the electrochemical cell which contain an electrode and electrolyte.

Internal resistance basically means the electrical resistance present inside the batteries or the cell that limits the potential difference that can be supplied to the external load. Through different circuit elements, the electric current flows from the positive terminal to the negative terminal. To maintain its continuity, the current has to flow from its negative terminal to the positive terminal. During this process, resistance is produced to the flow of current by the electrolyte of the cell. This is called internal resistance of a cell.

Let’s consider another circuit for better understanding. In this circuit the internal resistance is r, external resistance is R, potential difference between positive and negative terminals is V. The external resistance is connected in the circuite. The battery possesses electromotive force of a cell.

INTERNAL RESISTANCE is defined as the opposition in the flow of current that is offered by the batteries that results in generating the heat. Internal resistance is always measured in Ohms, and is denoted as omega.

Internal resistance shows relation with electromotive force (emf) of the cell and potential difference.

RELATION BETWEEN INTERNAL RESISTANCE AND EMF OF CELL

It is determined by the formula e = I ( r+R)

Where e is emf of the cell, I is current (measured in ampere and denoted by A), r is the internal resistance, and R is the external resistance or load resistance.

By the above equation, we get,

e = IR +Ir 

or we can say, e = V + Ir.

Where, V= IR is the potential difference across the cell when current flows throughout the circuit.

Note: The emf of the cell is always greater than the potential difference across the cell.

FACTS WHICH AFFECT INTERNAL RESISTANCE

A freshly prepared cell always has a low internal resistance. It increases with the age of the cell.

In simple words, the older the cell, the stronger the internal resistance will be. It also causes voltage drop when current flows through it. Clearly, it is a resistance which is present inside the cell and provided to the flow of current.

ELECTROMOTIVE FORCE

When there is no flow of current, the electrolyte has the same potential throughout the circuit , so that the potential difference between negative and positive is V+ – ( – V-) = (V+)+ (V-) . This difference is called emf or the electromotive force of a cell.

In simple words it is defined as the amount of energy given by the battery to each coulomb of charge (Q) passing through it . It is also defined as the amount of energy to drive a unit positive charge through an external circuit which is connected to a cell. When the circuit is open means there is no flow of current, the potential difference between the terminal of the cell is called its electromotive force. The SI unit of electromotive force is VOLTS.

Conclusion

From all of the above we conclude that the internal resistance is a resistance which is provided by the batteries and cells. It opposes the flow of current in the circuit. As we know, for the flow of current in the circuit in a steady manner an electrolytic cell is required. Also we understand that internal resistance shows relation with the electromotive force of the cell and potential difference of the cell. The unit of internal resistance is taken in ohms which come from omega. Also the internal resistance of a circuit can be neglected when the current I is e >> Ir. The actual value of internal resistance may vary from cell to cell. Always remember that the internal resistance of a dry cell is much higher than the common electrolytic cell.