Electric Cell-

 

An electric cell may be defined as-

 

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

 

Working of Electric Cell-

 

• By using chemical reactions, a cell produces the potential difference across its terminals.
• When the terminals of 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, the charge carriers experiences an electric force.
• This starts the flow of electric current around the loop.

 

Internal Resistance of Electric Cell-

 

The internal resistance of an electric cell may be defined as-

 

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

 

• The internal resistance is produced due to the collisions 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 Electric Cell-

 

An ideal cell has zero internal resistance and is represented as-

 

Ideal Cell

 

A practical cell has some finite internal resistance and is represented as-

 

Practical Cell

 

EMF of Cell-

 

The emf of an electric cell may be defined as-

 

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.

 

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.

 

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.

 

Terminal Voltage of Cell-

 

The terminal voltage of an electric cell may be defined as-

 

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.

 

Following two cases are possible-

 

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

 

Consider the following electric circuit-

 

 

Here, current comes out of the positive terminal of the cell (E, r). In this case, the cell discharges.

 

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-

 

 

Here, current goes into the positive terminal of the cell (E, r). In this case, this cell charges.

 

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.

 

Test Your Concepts-

Quiz on Electric Cell | EMF & Terminal Voltage

 

Next Article-

Series Combination of Cells

 

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