In case of a fire , before the fireman use the water hoses , they shut off the main electrical supply for the area. Explain why they do this.

Solar water heaters use cadmium black as a coating material on copper water pipes. Explain why

Name two metals that start floating after sometime when immersed in water and explain why they do so.

Give reason : Water is not used to control fire involving electrical equipment.

Two charged conducting spheres of radii a and b are connected to each other by a wire. What is the ratio of electric fields at the surfaces of the two spheres? Use the result obtained to explain why charge density on the sharp and pointed ends of a conductor is higher than on its flatter portions.

When calcium metal is added to water, the gas evolved does not catch fire but the same gas evolved on adding potassium metal to water catches fire. Explain why ?

a. When calcium metal is added to water, the gas evolved does not catch fire but the same gas evolved on adding potassium metal to water catches fire. Explain. b. Name a metal for each case: (i) It displaces hydrogen gas from nitric acid. (ii) It does not react with any physical state of water (iii) It reacts with neither cold nor hot water, but reacts with steam.