Chapter 1 Physical Environment Force Active Without Contact Experiments
Let us carry out the following three experiments:
Experiment-1
Let us take an ebonite rod and rub it with wool. It is then suspended freely from some support by a silk thread. Another ebonite rod, which has also been rubbed with wool, is then brought near it.
It is observed that the suspended ebonite rod is repelled (i.e. moves away) as
Experiments on force without contact for Class 8
Experiment-2
Let us take a glass rod. It is rubbed with silk and then it is suspended from a support by a silk thread. Another glass rod, which has been rubbed with silk, is now brought nearer to it. It is found that the suspended glass rod is repelled, as
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Experiment -3
let us take a glass rod. It is rubbed with silk and then suspended freely from a support by a silk thread. An ebonite rod, which has been rubbed with wool, is now brought near to it. It is found that the glass rod is attracted by the ebonite rod as
WBBSE Class 8 Force Active Without Contact notes
From experiments 1 and 2, it is found that when two objects have like charges or similar charges on them, they repel each other.
That is why two charged glass rods or two charged ebonite rods repel each other. From experiment 3,
we can understand that charges produced on the ebonite rod and charges produced on the glass rods are not similar. When they come closer, they attract each other.
The above experiments were carried out by Dr Gilbert. From the experiments, he came to the conclusion that like charges repel and unlike charges attract each other.
These are static charges produced by friction. When something is not moving, it is said to be static. Similarly, static electricity is the electric charge at rest.
Dr Gilbert was also able to establish that various bodies which get electrified on rubbing either acquired charges similar to those acquired by glass rod or that acquired by ebonite rod.
We can perform another experiment to show that equal charges are produced on two different rubbing surfaces.
Force and pressure experiments for Class 8 students
Experiment -4
A glass rod is rubbed with a piece of silk. The rod and the silk piece are separately held over some small pieces of paper. Each is found to attract paper pieces.
This proves that each is electrically charged. Now, both the glass rod and the silk piece are held over the paper pieces, keeping the rubber parts of the two objects close to each other.
This time no paper piece is attracted by either of the two. This is due to the fact that equal charges of opposite nature are developed when the two different bodies are rubbed.
So, we can conclude that Like charges (or similar charges) repel each other.
Unlike charges attract each other. It will be convenient for us if we assign specific symbols to two different types of charges produced on the objects due to rubbing.
Renowned scientist Benjamin Franklin named the two different charges as positive charge (+) and negative charge (-).
The charge produced on the glass rod (when it is rubbed with silk) is positive and the charge produced on the ebonite rod (when it is rubbed with wool) Is negative.
How can we decide the nature of charges produced on the two objects when they are rubbed against each other? This can be decided using the following table
| Wool | Metal |
| Glass | Ebonite |
| Paper | Lac |
| Silk | Amber |
| Wood | Resin |
| Human body | Celluloid |
Understanding non-contact forces for Class 8
If any two objects listed in the table are rubbed with each other, then the object that lies above in the list will acquire a positive charge and the one that lies below acquires a negative charge.