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We know that the conductor contains large number of electrons. The electrons near the positive terminal of the battery are attracted by it and start to move towards positive terminal. As a result, the amount of positive charge on this plate decreases. So the electric force Fe becomes weaker than chemical force Fc and chemical force pulls negative ions from the positive plate (anode) and makes them move towards the negative plate (cathode). The negative terminal pushes one electron into the conductor because of stronger repulsion between negative terminal and negative ion. Hence, the total number of electrons in the conductor remains constant during the current flow. The above said process continuous till equilibrium is attained between the forces Fe and Fc

Electromotive force(emf)

becomes weaker than chemical force Fc and chemical force pulls negative ions from the positive plate (anode) and makes them move towards the negative plate (cathode). The negative terminal pushes one electron into the conductor because of stronger repulsion between negative terminal and negative ion. Hence, the total number of electrons in the conductor remains constant during the current flow. The above said process continuous till equilibrium is attained between the forces Fe and Fc . Electromotive force (emf) When the ends of the conductor are connected to the terminals of a battery, the electrons in the conductor move with a drift speed from negative terminal to positive terminal because of the electric force acting on them. At the same time negative ions of equal amount of charge move from positive terminal to negative terminal against the electric force (Fe ) because of the chemical force acting on them within battery. Thus, some chemical energy is spent to move ions in the battery. It means that an amount of work is done by the chemical force (Fc ).

Let us assume that work done by the chemical force to move a negative charge q from positive terminal to negative terminal against the electric force Fe be ‘W’ and also assume that the magnitude of chemical force Fc is equal to magnitude of electric force (Fe ).

The work done on negative charge q by the chemical force, W = Fc d where ‘d’ is the distance between the terminals. Then work done by the chemical force to move 1 Coulomb charge from positive terminal to negative terminal is given by, W/q = Fc

d/q. We know that Fc = Fe , then we get W/q = Fe d/q.

This W/q is the work done by the chemical force on unit negative charge to move it from positive terminal to negative terminal. This is called emf (ε). ε = W/q = Fe d/q

Generally, emf is defined as the work done by the chemical force to move unit positive charge from negative terminal to positive terminal of the battery.

• How can we measure potential difference or emf?


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