Cell

A cell is a device that maintains the potential difference which exists between the two electrodes due to chemical reaction. A collection of two or more cells which are connected in series or parallel is called a Battery. Thus we will obtain the required voltage or current. A battery is an energy source that converts chemical energy to electrical energy. It is otherwise known as electrochemical cell. The energy is stored in the form of chemical energy inside a battery. Batteries give us a convenient source of energy for energizing devices without cables and wires. When it is connected to a circuit it produces electrical energy.

A battery consists of two terminals – A Positive and Negative Terminal. The Positive Terminal is called Cathode and the Negative Terminal is called Anode. They are also called as Electrodes of a Cell. These electrodes will be dipped in a solution called electrolyte. It is liquid which is ionic and conducts electricity. The output voltage of battery depends upon the elements used as electrodes, the size of the electrodes and the type of electrolyte used in it.

When the battery is about to charge, an external source is connected to it. The anode of the battery is connected to the negative terminal of the source and cathode is connected to the positive terminal of the source. As the external source is connected to the battery, electrons are inserted into the anode. When the cell or battery is connected to the circuit chemical reactions takes place. Thus chemical reactions take place within the two electrodes. Here oxidation and reduction reactions happen. Then reduction reaction occurs at cathode and oxidation process occurs at anode.

The cathode acts as the oxidizing agent by accepting electrons from the negative terminal anode. The anode acts as the reducing agent by losing the electrons. Thus due to these chemical reactions an electrical difference occurs between the terminals-anode and cathode. When there is no power the electrolyte prohibits the movement of electrons directly from anode to cathode. This is why we are using an external source or connecting to a circuit. Thus electrons travel from anode to cathode when the circuit is closed. Finally it gives power to the appliance which is connected to it. After a long time when the electrochemical process alters the anode and cathode materials it stop giving out electrons. Then the battery dies.

Emf of cell (E):

EMF or Electromotive force is defined as the potential difference which is developed between the two terminals of a battery in an open circuit. We know that anode has positive potential (V₊) and cathode has negative potential (V_-). So emf is the potential difference between the positive terminal anode and negative terminal cathode when there is no current flowing through it. The emf measures the energy which is transferred to the charge carries in the cell or a battery. It is the energy in joules divided by the charge in coulombs. The emf acts as the initiating force for the current to flow.

ε =

where

ε is the electromotive force

E is the energy

Q is the charge

The emf which is denoted by ε and the equation is given by ε = V+ - (-V_-) = V₊ + V_-. It is measured in volts.

Potential difference (V):

The voltage across the terminals of a cell when it is supplying current to external resistance is called potential difference or terminal voltage. Potential difference is equal to the product of current and resistance of that given part i.e. V = iR.

Internal resistance (r):

In case of a cell the opposition of electrolyte to the flow of current through it is called internal resistance of the cell.

The internal resistance of a cell depends on

Ø the distance between electrodes

Ø area of electrodes

Ø and nature, concentration

Ø and temperature of electrolyte

A cell is said to be ideal, if it has zero internal resistance.

Cell in Various Positions

Closed circuit:

Cell supplies a constant current in the circuit.

(i) Current given by the cell

(ii) Potential difference across the resistance

(iii) Potential drop inside the cell

(iv) Equation of cell

(v) Internal resistance of the cell

(vi) Power dissipated in external resistance (load)

Power delivered will be maximum when

Open circuit:

When no current is taken from the cell it is said to be in open circuit.

(i) Current through the circuit i = 0

(ii) Potential difference between A and B,

(iii) Potential difference between C and D,

Short circuit:

If two terminals of cell are join together by a thick conducting wire

(i) Maximum current (called short circuit current) flows momentarily

(ii) Potential difference V = 0

Important

1. It is important to note that during charging of a cell, the positive electrode of the cell is connected to positive terminal of battery charger and negative electrodes of the cell is connected to negative terminal of battery charger. In this process, current flows from positive electrode to negative electrode through the cell.

Therefore, V = ε + Ir

Hence, the terminal potential difference becomes greater than the emf of the cell.

2. The difference of emf and terminal voltage is called lost voltage as it is not indicated by a voltmeter. It is equal to Ir.

Grouping of Cells

In series grouping of cell’s their emf’s are additive or subtractive while their internal resistances are always additive. If dissimilar plates of cells are connected together their emf’s are added to each other while if their similar plates are connected together their emf’s are subtractive.

I= (nE)/(R+nr)

If R<<nr, then I = E/R

If R>>nr, then I = nE/R

Series grouping:

In series grouping anode of one cell is connected to cathode of other cell and so on. If n identical cells are connected in series:

· Equivalent emf of the combination

· Equivalent internal resistance

· Main current = Current from each cell

· Potential difference across external resistance

Parallel grouping:

In parallel grouping all anodes are connected at one point and all cathode are connected together at other point. If n identical cells are connected in parallel:

I= E/[R+(r/m)]

If R>>r/m, then I = E/R

If R>>r/m, then I = m(E/R)