A crackling sound is heard by taking of a sweater during winter.why?
Sweater is made of wool and generally shirt we wear is made of cotton blended with some synthetic fibers. Constant friction between the shirt and sweeter lets transfer of electrons from one material to the other. This results in building of electric potential. When enough potential has been accumulated, while taking of the sweater it discharges and transfer of electrons take place. Passage of electrons through air in the forms of spark let out heat and sound energy. Thus we hear the crackling sound and in dark rooms we may see the spark lights as well.
Sweaters are usually made of wool or synthetic fibres, both of which are not conductive of electricity. Thus they are easily able to build up an electric charge when they are moved, and the small electrical discharges are what you hear. You hear them because some of the discharges are close to your ear.
A cotton tee shirt seldom produces the crackles.
And in winter, because the air is drier, and less conductive, hence allows these charges to build up.
The crackling sound which you refer is the static noise. Synthetic fibers tend to generate a little friction with your body heat, thereby generating a minute amount of electricity.
This can also be seen in places where people work too much on their system. Eventually when they touch each other, this static is felt with a piercing pain. Just like a prick of a needle.