Comparing EMF of Two Primary Cells

The primary cell’s electromotive force is the amount of energy created by the battery and after one usage, these cells are discarded. The potentiometer is a piece of equipment to find the difference between the electromotive force of two primary cells and create a comparison between the two. Let us understand more about this through this article. 

Definitions and concepts 

Primary cells are a battery that recharges quickly after one use and are disposed off after that one usage. They are also considered dry cells because all the electrolytes consist of an absorbent substance or a separator which means no presence of free or loose electrolytes. 

Electromotive force (EMF) is the quantity of energy provided by the cell’s battery. EMF and terminal potential difference (V) of all cells are equal when there is no electric current. These quantities are calculated and estimated in terms of Volts, yet both these are not the same type of quantity. It is also referred to as voltage. Although, it is not a true force. 

A potentiometer is an apparatus used to compute the difference between the e.m.f. (electromotive force) of two cells and measure the internal resistance and potential difference across the resistor. 

It consists of a 10 m long cable and a uniform cross-sectional area throughout the length. The cable used for the potentiometer must have high resistivity and less temperature coefficient. In the potentiometer, both the wires are arranged parallel on a wooden block and copper bands are used to join the wires. The wooden block also has a metre scale for the readings.

Working principle of potentiometer 

The principle linked with the potentiometer states that the potential difference between two junctures of any cable is directly proportional to the height of the cable if the steady current flows through the cable with a uniform cross-sectional area. 

Through experimentation the emf of two cells can be determined and compared, is mentioned below: 

​Required equipment 

• Potentiometer
• Leclanche cell
• Daniel cell
• Ammeter
• Voltmeter
• Galvanometer
• Battery 
• Rheostat 
• Resistance box 
• Keys (one way and two way)
• Jockey 
• Connecting cables and sandpaper  

Step-by-step instructions 

• Organise the equipment according to a standard circuit. 
• Using sandpaper eliminates the insulation from the edges of the connecting copper wires. 
• Calculate and compare the e.m.f. of the battery (E) and cells (E1 and E2) and observe that E should be greater than E1 and E2. 
• Use a one-way key to attach the positive and negative pole of the battery (battery should have steady e.m.f.) with the zero-edge (P) of the potentiometer and connect an ammeter and low resistance rheostat to the other edge (Q) of the potentiometer.  
• Join the positive pole side of the cells (E1 and E2) with the zero-edge (P) of the potentiometer and negative poles with the terminals a and b of the two-way key. 
• Now, join the galvanometer (G) and resistance box (R.B.) with the jockey (J) by using the common terminal c of the two-way key. 
• Keep the resistance in the rheostat zero and take maximum current from the battery. 
• Then, set the plugin the one-way key and between the edges of two-way keys a and c. 
• Keep the resistance around 2,000 ohms in the resistance box (R.B.).
• Keep the jockey at the zero ends and observe the direction of reflection in the galvanometer. 
• Then, again press the jockey at the other edge of the potentiometer cable. Suppose the deflection is in opposite directions as per the first case. In that case, the connections are accurate (in the wrong situation, the deflection direction will be in the same direction as the e.m.f. of the battery is very low).
• Move the jockey over the potentiometer cables till you get no deflection in the galvanometer. 
• Keep the 2000 ohms resistance back into the resistance box and find a null point state with accuracy. 
• Write down the measurement of the cables for cell E1. Also, write down the current shown by the ammeter. 
• Now, remove the plug of cell E1 and connect E2. 
• Follow the same conditions for the E2 cell as well. 
• Take the observations alternatively for each cell again and again with the same amount of current. 
• You can also increase the current by modifying the rheostat and similarly getting three or four observations. 
• Write down all the observations. 

Precautions while experimenting 

• There should be a neat and clean joining of cables and tight also.
• While making observations, plug and keys should be tightly joined. 
• The zero terminal sides of the cables must be joined with the positive poles of battery E and cells E1 and E2. 
• There should not be an interaction between the jockey key and cable. One should handle the cable wisely. 
• Keep the ammeter reading steady till the specific observations. You can even use the rheostat for this. 
• The value of the electromotive force of the battery must be higher than the electromotive forces of two cells

Conclusion

Electromotive force and potentiometer are crucial to compare the energy associated with the two cells. The potentiometer works on the principle of potential difference between two junctures of any cable, which is directly proportional to the height of the cable in case of a steady flow of current through the cable with a uniform cross-sectional area.