Electric cells and batteries — lesson. Physics CBSE Live product, Class 8 (2025-26).

Electric cells are devices that produce electrical energy using chemical reactions. Different types of cells have been developed to meet various needs, from simple laboratory cells to batteries used in everyday appliances.

Voltaic cell:

A Voltaic cell, also called a Galvanic cell, is a device used to produce electricity from chemical reactions.

Construction of a voltaic cell:

It consists of two metal plates made of different materials placed inside a glass or plastic container. The container holds a liquid known as an electrolyte.

Electrodes and electrolyte:

The metal plates, called electrodes, are partially dipped into the electrolyte, which is usually a weak acid or a salt solution. The electrolyte allows the chemical reaction to take place.

A simple voltaic cell

Working of a voltaic cell:

A chemical reaction between the electrodes and the electrolyte produces electrical energy. When the cell is connected to an external circuit, electric current flows from the positive terminal to the negative terminal through the circuit.

Life of a voltaic cell:

Over time, the chemicals involved in the reaction get used up. When this happens, the cell stops producing electricity and is then called a ‘dead’ cell, as it can no longer supply electrical energy.

Activity: To demonstrate the working of a lemon cell.

A lemon cell setup

Step 1: Take a fresh, juicy lemon, two different metal electrodes (such as a copper strip/wire and an iron nail), connecting wires, and a digital multimeter.

Step 2: Insert the copper electrode and the iron electrode into the lemon, keeping them apart so that they do not touch each other.

Step 3: Connect the copper electrode to the positive (red) probe of the multimeter using a connecting wire.

Step 4: Connect the iron electrode to the negative (black) probe of the multimeter.

Step 5: Set the multimeter to measure voltage and observe the reading displayed on the screen.

Observation:

The multimeter shows a small voltage reading.This indicates that an electric potential difference is produced between the two metal electrodes inserted in the lemon.

Conclusion:

The voltage reading on the multimeter confirms that the lemon cell is working.
The copper wire and iron nail act as electrodes, while the lemon juice acts as the electrolyte that allows the chemical reaction to take place.
This activity demonstrates that chemical energy can be converted into electrical energy.
Multiple lemons connected in series can produce a higher voltage capable of lighting an LED.

Dry cells:

Voltaic cells were a major scientific advancement, but they are not practical for daily use. For this reason, dry cells are commonly used as a convenient source of electric power in everyday devices.

Dry cells

Why they are called dry cells:

Dry cells are known as “dry” because the electrolyte inside them is not a free-flowing liquid. Instead, it is present in the form of a thick, moist paste, which makes the cell safer and easier to handle.

Construction of a dry cell:

A dry cell consists of a zinc container that serves as the negative terminal. At the centre, there is a carbon rod fitted with a metal cap, which acts as the positive terminal. The carbon rod is surrounded by the paste-like electrolyte.

Parts of a dry cell

Uses and limitations:

A dry cell is a single-use cell; once it is exhausted, it cannot be reused and must be discarded. For many modern applications, rechargeable batteries are now preferred because they can be used multiple times.

Rechargeable batteries:

Advantages:

Rechargeable batteries can be charged and used repeatedly. This reduces waste and helps save money over a long period of time.

LiIonbatteriesformobilephonesw600 (1).jpg

Rechargeable mobile batteries

Types and applications:

There are many types of rechargeable batteries designed for different purposes. Small rechargeable batteries are used in devices such as watches and mobile phones, while larger ones power laptops and tablets. Even bigger rechargeable batteries are used in inverters and electric vehicles.

Rechargeable car battery

Limitations of rechargeable batteries:

Although rechargeable batteries can be reused many times, they do not last forever. After repeated charging and discharging, their performance gradually decreases and they eventually wear out.

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