Torchlight, electric cell and battery — lesson. Physics CBSE Live product, Class 7 (2025-26).

A cell with terminals

Activity: To observe the functions of battery.

Step 1: Take a torch that uses two electric cells.

Step 2: Open the cell compartment of the torch and remove the cells.

Step 3: Place the cells back into the torch in a different order.

Step 4: Reverse the direction of one cell and insert them again.

Step 5: Slide the switch each time and check whether the lamp glows.

Step 6: Finally, place the cells in the correct order and observe the arrangement.

Observation:

The lamp does not glow when the cells are placed in the wrong order or when one cell is reversed. The lamp glows only when the positive terminal of one cell is connected to the negative terminal of the next cell.

Conclusion:

When two or more electric cells are connected such that the positive terminal of one cell is joined to the negative terminal of the next, they form a battery. Only in this proper arrangement can the battery supply electrical energy to make the lamp glow.

Battery:

A battery is formed when two or more electric cells are connected such that the positive terminal of one cell is joined to the negative terminal of the next, enabling it to supply electrical energy to a circuit.

Need for a battery:

Many electrical devices require more than one electric cell to operate. Therefore, two or more cells are connected together. Using multiple cells supplies electrical energy to the circuit for a longer duration and can also provide greater energy when required.

Use of the term “Battery”:

The word battery is often used even when there is only a single cell. For example, the power source used in mobile phones is commonly called a battery, although it may consist of just one cell.

Activity: To observe the function and parts of an incandescent lamp.

An incandescent lamp

Step 1: Take a torchlight that uses an incandescent lamp.

Step 2: Examine the lamp carefully without switching it on.

Step 3: Observe the glass bulb and look for a thin wire inside it.

Step 4: Switch on the torch and observe which part of the lamp glows.

Step 5: With your teacher’s help, remove the lamp from the torch and inspect it from all sides.

Step 6: Observe how the thin wire is fixed and how it is connected at the base of the lamp.

Observation:

A thin wire is present at the centre of the glass bulb. When the torch is switched on, this thin wire glows.
The thin wire is attached to two thicker supporting wires.
One thick wire is connected to the metal case at the base of the lamp, and the other is connected to the metal tip at the centre of the base.
These two connections act as the two terminals of the lamp and do not touch each other.

Conclusion:

The thin wire inside the glass bulb is called the filament. In an incandescent lamp, electric current passes through the filament, causing it to become very hot and glow, thereby producing light.

Incandescent lamp:

An incandescent lamp is a device in which electric current passes through a thin wire called a filament, causing it to become very hot and glow, thereby producing light.

Parts of an incandescent lamp

Activity: To observe the parts and function of an LED.

LED with the terminals

Step 1: Take an LED of any colour and observe it carefully.

Step 2: Look inside the LED and check whether you can see any filament.

Step 3: Observe the two wires attached to the LED.

Step 4: Compare their lengths and note whether one wire is longer than the other.

Observation:

No filament is seen inside the LED.
The LED has two terminals.
One terminal is connected to a longer wire, and the other to a shorter wire.
The longer wire represents the positive terminal, while the shorter wire represents the negative terminal.
A torch lamp may contain one or more LEDs, sometimes of different shapes.

Conclusion:

Unlike incandescent lamps, LEDs do not use filaments to produce light. They have two terminals with fixed polarity, and correct connection is required for them to glow.

LED:

An LED (Light Emitting Diode) is an electric device that produces light without the use of a filament. It has two terminals with fixed polarity, represented by a longer wire for the positive terminal and a shorter wire for the negative terminal.

An LED

1. Torchlight, electric cell and battery

Theory: