Cells & Batteries | EMF | Terminal Voltage

Electric Cell-

 

A cell is a device which supplies energy to charge carriers and thereby maintain their flow in the electric circuit.

 

• By using chemical reactions, a cell produces potential difference across its terminals.
• When the terminals of a cell are connected by a wire, the potential difference between the terminals of cell set up an electric field within the wire.
• Due to this electric field, charge carriers experiences the force and current starts flowing around the loop from higher potential to lower potential.

 

Internal Resistance of Cell-

 

The resistance offered by the electrolyte of a cell to the flow of current between the electrodes is called as internal resistance of the cell.

 

• The internal resistance is produced due to the collision between the ions of the electrolyte.
• A freshly prepared cell has low internal resistance but its value increases as we draw more and more current from it.
• An ideal cell has zero internal resistance but all practical cells that we use have some internal resistance.

 

Representation of Cell-

 

 

EMF of Cell-

 

EMF of a cell is the potential difference measured across the cell when it is not connected to any external circuit. OR EMF of a cell is the potential difference between its terminals in an open circuit i.e. when no current flows through the cell.

 

EMF of a cell is denoted by the symbol E.

Consider an electric cell as shown-

 

Using open loop rule from A to B, we have-

V_A – E = V_B

V_A – V_B = E

 

Thus, when cell is not connected to any external circuit, potential difference across its terminals is equal to the emf of the cell.

 

It is interesting to note that EMF stands for electromotive force but it does not really refer to a force rather it describes a potential difference in volts.

 

Terminal Voltage of Cell-

 

Terminal voltage of a cell is the potential difference measured across the cell when it is connected to an external circuit. OR Terminal voltage of a cell is the potential difference between its terminals in a closed circuit i.e. when current flows through the cell.

 

There are following two cases possible-

 

Case-01: When current comes out of the positive terminal of cell (Discharging of Cell)-

 

Consider the following electric circuit-

 

 

Using open loop rule from A to B, we have-

V_A – E + Ir = V_B

V_A – V_B = E – Ir

 

Thus, during discharging of cell, terminal voltage across the cell = E – Ir.

 

Important Notes   NOTE-01 During discharging, terminal voltage of cell is always less than its emf. This is because certain amount of voltage equal to Ir drops across its internal resistance (r).   NOTE-02 If the two ends of cell are connected using only a plane wire, then terminal voltage across the cell becomes zero. In this case, maximum current = E / r is drawn from the cell.

 

Case-02: When current goes into the positive terminal of cell (Charging of Cell)-

 

Consider the following electric circuit-

 

 

Using open loop rule from A to B, we have-

V_A – E – Ir = V_B

V_A – V_B = E + Ir

 

Thus, during charging of cell, terminal voltage across the cell = E + Ir.

 

Important Notes   NOTE-01 During charging, terminal voltage of cell is always greater than its emf.   NOTE-02 During charging, terminal voltage of cell cannot be zero.

 

Read the next article on-

Combination of Cells

 

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