Which of the following sources of energy is different from the other?

With the help of a labelled diagram,explain the construction and working of a solar water heater.

There are about $50$ teachers in a school and most of them come to school by their own cars. Many of the teachers come from the same place but everyone prefers to come to school by his/her own car.
Answer the following questions based on the above information:
$(a)$ Do you think that their practice of commuting to school is environmental-friendly? Give one suggestion. Justify your answer.
$(b)$ In what other two ways can we become environmental-friendly?

Name the agent which decomposes animal dung into biogas. Describe the steps involved in the process.

Name any two elements that are used in fabricating solar cells.

$A$ modern selenium solar cell can convert $25 \%$ of sunlight into electrical energy.