Class 7 (Science assignment)

 

 Heat and Light Energy

Heat and light are two important forms of energy that are essential for life and many activities around us. The Sun is the main natural source of both heat and light energy on Earth. Heat is a form of energy that flows from a hotter object to a colder object. It can cause changes in the temperature and physical state of matter. For example, when ice is heated, it melts into water, and when water is heated sufficiently, it changes into steam.

Heat can be transferred from one place to another in three main ways: conduction, convection, and radiation. Conduction is the transfer of heat through a substance without the actual movement of the substance, and it occurs mainly in solids. Metals such as copper and aluminium are good conductors of heat. Convection is the transfer of heat through the movement of fluids such as liquids and gases. Radiation is the transfer of heat in the form of waves and does not require a material medium. Heat from the Sun reaches the Earth mainly through radiation.

Light is a form of energy that enables us to see objects. Objects that produce their own light are called luminous objects, such as the Sun, stars, and electric bulbs. Objects that do not produce their own light but can be seen when light falls on them are called non-luminous objects, such as books, trees, and chairs. Light generally travels in a straight line. When light falls on a smooth and shiny surface, it is reflected. A mirror is a common example of a reflecting surface.

Materials can be classified as transparent, translucent, and opaque according to how they allow light to pass through them. Glass is generally transparent, frosted glass is translucent, and wood is opaque. Heat and light energy are closely related to our daily lives and are important for cooking, heating, vision, communication, plants' growth, and many other natural and human activities.

A. Short-Answer Questions

  1. What is heat? In which direction does heat normally flow?

  2. Differentiate between conduction and convection with one example of each.

  3. What is radiation? How does heat from the Sun reach the Earth?

  4. What are luminous and non-luminous objects? Give two examples of each.

  5. Differentiate between transparent, translucent, and opaque materials with one example of each.

B. Multiple-Choice Questions

1. Which is the main natural source of heat and light energy for the Earth?

A. Moon
B. Sun
C. Wind
D. Water

2. Heat normally flows from:

A. A colder object to a hotter object
B. A hotter object to a colder object
C. A smaller object to a larger object
D. A heavier object to a lighter object

3. Which method of heat transfer mainly occurs in solids?

A. Convection
B. Radiation
C. Conduction
D. Reflection

4. Which of the following is a good conductor of heat?

A. Wood
B. Plastic
C. Copper
D. Rubber

5. Heat from the Sun reaches the Earth mainly by:

A. Conduction
B. Convection
C. Radiation
D. Reflection

6. Which of the following is a luminous object?

A. Moon
B. Book
C. Sun
D. Chair

7. Light generally travels in:

A. A curved line
B. A straight line
C. A circular path
D. A zigzag path

8. Which of the following surfaces is commonly used to reflect light?

A. Mirror
B. Cloth
C. Paper
D. Wood

9. Which material allows most light to pass through it?

A. Wood
B. Brick
C. Clear glass
D. Metal sheet

10. Which of the following is an example of a non-luminous object?

A. Sun
B. Electric bulb
C. Candle flame
D. Tree

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