Q 4: Why is flash of lightening seen before the sound of the cloud thunder?

Answer: If you know that nothing can travel faster than the speed of the light that is 3 x 10⁸ m/s, then you know the answer. As light travels faster than the sound, therefore we see the flash of lightening before the sound of the thunder.

Q 5: Why a wet cloth is placed on the forehead of a patient having high fever?

Answer: You must have seen that when you sprinkle water on a hot thing then it evaporates and cooling occur. Same is the reason here.

When a patient is having high temperature, and when wet cloth is placed on the forehead of patient, then due to the body high temperature, water evaporates and so the cooling occurs which brings down the temperature of body.

Q 6: What is the frequency of the alternating current?

Answer: The frequency of alternating current can vary from country to country. For example, it is 50 cycles per second or 50 hertz in India.

Q 7: Why an electric bulb explodes when it is broken?

Answer: The bulb has a partial vacuum. When it is broken then air from outside enters it very fast and leads to a small explosion.