Class 7 Science

Chapter 7: Heat Transfer in Nature

Fresh Notes, Easy Explanation, Activities and Questions with Short Solutions

Chapter at a Glance

Heat always transfers from a hotter region to a colder region. In nature and in everyday life, heat is mainly transferred in three ways:

Activity 7.1: Let us experiment

Caution — This activity should be carried out under the supervision of a teacher or an adult.

In this activity, a metal strip has pins attached to it with wax. One end of the strip is heated.

What will happen to the pins? Will they remain attached to the strip or will they fall?

Solution: The pins will fall one by one because heat travels through the metal strip and melts the wax.

Predict the order in which the pins will fall from the strip.

Solution: Pin I will fall first, followed by pins II, III and IV.

Record your observations in Table 7.1.

Good Conductors and Insulators

Materials such as metals allow heat to pass through them easily. They are called good conductors of heat.

Materials such as wood and glass do not allow heat to pass through them easily. They are called poor conductors or insulators of heat.

Clay and porcelain are also poor conductors of heat, which is why tea or coffee kept in such cups stays hot longer.

Image: Wikimedia Commons, Andrew Davidhazy, CC BY 4.0.

Key idea: In convection, particles of liquids and gases actually move from one place to another.

Image: Wikimedia Commons, public-domain sea-breeze diagram.

Conduction + Convection + Radiation Together

When water is heated in a pan:

• Heat travels from the flame to the pan mainly by conduction.
• Water gets heated and circulates by convection.
• Warmth felt around the flame and hot pan is due to radiation.

The Sun is the main source of heat for Earth. Heat from the Sun causes water to evaporate from oceans, rivers, lakes and other water bodies.

Image: USGS via Wikimedia Commons, public domain.

Simple Water Cycle

1. Evaporation: Water changes into water vapour due to heat from the Sun.
2. Transpiration: Plants release water vapour.
3. Condensation: Water vapour cools and forms clouds.
4. Precipitation: Water falls as rain, snow or hail.
5. Collection and seepage: Water returns to water bodies or enters the ground.

The continuous movement of water through these processes is called the water cycle.

7.4.1 Seepage of Water Beneath the Earth

Activity 7.5: Let us investigate

How does water seep through the surface of the Earth?

Solution: Water enters the spaces between soil and rock particles and moves downward. This process is called infiltration.

Water generally seeps fastest through gravel, more slowly through sand and slowest through clay because the spaces between gravel particles are wider.

Do your findings match with your predictions?

Solution: Yes, the expected result is that water seeps fastest through gravel, slower through sand and slowest through clay.

Important Terms

Groundwater is not unlimited. Excessive extraction and reduced infiltration can deplete groundwater. Rainwater harvesting and recharge pits help replenish groundwater.

Science and Society: Ice Stupa

In Ladakh, ice stupas are created during winter to store water in the form of ice. The ice melts slowly during spring and provides water for farming and other needs.

Image: Wikimedia Commons, Crb43, CC BY-SA 4.0.

In a Nutshell

• There are three ways in which heat is transferred: conduction, convection and radiation.
• Conduction transfers heat from a hotter part to a colder part.
• Good conductors allow heat to pass through them easily.
• Poor conductors or insulators do not allow heat to pass through them easily.
• In solids, heat is mainly transferred through conduction.
• In liquids and gases, heat is transferred by convection.
• Land and sea breezes are examples of convection.
• Heat from the Sun reaches Earth through radiation.
• Radiation does not require a material medium.
• The water cycle continuously moves water through evaporation, condensation, precipitation and other processes.
• Infiltration is the seepage of surface water through soil and rocks.
• Groundwater is stored beneath the Earth’s surface.
• Aquifers are underground layers that store groundwater.
• Rainwater harvesting and recharge pits help replenish groundwater.

Let Us Enhance Our Learning

1. Choose the correct option in each case.

(i) Your father bought a saucepan made of two diff erent materials, A and B, as shown in Fig. 7.14. The materials A and B have the following properties —

(a) Both A and B are good conductors of heat

(b) Both A and B are poor conductors of heat

(c) A is a good conductor and B is a poor conductor of heat

(d) A is a poor conductor and B is a good conductor of heat

Solution: (c) A is a good conductor and B is a poor conductor of heat.

(ii) Pins are stuck to a metal strip with wax and a burning candle is kept below the rod, as shown in Fig. 7.15. Which of the following will happen?

(a) All the pins will fall almost at the same time

(b) Pins I and II will fall earlier than pins III and IV

(c) Pins I and II will fall later than pins III and IV

(d) Pins II and III will fall almost at the same time

Solution: (b) Pins I and II will fall earlier than pins III and IV.

(iii) A smoke detector is a device that detects smoke and sounds an alarm. Suppose you are fitting a smoke detector in your room. The most suitable place for this device will be:

(a) Near the floor

(b) In the middle of a wall

(c) On the ceiling

(d) Anywhere in the room

Solution: (c) On the ceiling.

2. A shopkeeper serves you cold lassi in a tumbler. By chance, the tumbler had a small leak. You were given another tumbler by the shopkeeper to put the leaky tumbler in it. Will this arrangement help to keep the lassi cold for a longer time? Explain.

Solution: Yes. The air trapped between the two tumblers acts as an insulator and slows heat transfer from the surroundings to the cold lassi.

3. State with reason(s) whether the following statements are True [T] or False [F].

(i) Heat transfer takes place in solids through convection. [ ]

Solution: False [F]. Heat is mainly transferred through conduction in solids.

(ii) Heat transfer through convection takes place by the actual movement of particles. [ ]

Solution: True [T].

(iii) Areas with clay materials allow more seepage of water than those with sandy materials. [ ]

Solution: False [F]. Water generally seeps faster through sand than clay.

(iv) The movement of cooler air from land to sea is called land breeze. [ ]

Solution: True [T].

4. Some ice cubes placed in a dish melt into water after sometime. Where do the ice cubes get heat for this transformation?

Solution: The ice cubes receive heat from the warmer surroundings.

5. A burning incense stick is fixed, pointing downwards. In which direction would the smoke from the incense stick move? Show the movement of smoke with a diagram.

Solution: The smoke will generally move upward because the hot gases and smoke are warmer and less dense than the surrounding air.

6. Two test tubes with water are heated by a candle flame as shown in Fig. 7.16. Which thermometers (Fig. 7.16a or Fig. 7.16b) will record a higher temperature? Explain.

Solution: The thermometer in Fig. 7.16(b) will record a higher temperature because it is positioned so that it is more directly affected by the heated water and convection currents.

7. Why are hollow bricks used to construct the outer walls of houses in hot regions?

Solution: Hollow bricks trap air. Air is a poor conductor of heat, so the trapped air reduces heat transfer through the walls.

8. Explain how large water bodies prevent extreme temperature in areas around them.

Solution: Water heats and cools more slowly than land. Large water bodies therefore reduce rapid temperature changes in nearby areas.

9. Explain how water seeps through the surface of the Earth and gets stored as groundwater.

Solution: Rainwater enters the spaces between soil and rock particles through infiltration. It moves downward and gets stored in the pore spaces of sediments and openings in rocks, forming groundwater.

10. The water cycle helps in the redistribution and replenishment of water on the Earth. Justify the statement.

Solution: Water continuously evaporates, condenses and returns through precipitation. Some water flows into rivers and lakes while some infiltrates the ground and recharges groundwater. Thus, the water cycle redistributes and replenishes water.

Exploratory Projects

Society: Visit a site of water harvesting or a recharge pit. Find out from people how they are constructed and how they work. Prepare a report with illustrations.

Suggested work: Record the location, type of structure, materials used, source of rainwater, method of collecting water and how the structure helps recharge groundwater.

Activity: Tightly wrap a thin paper strip around a metallic rod. Try to burn the paper with a candle while rotating the rod continuously. Does the paper burn? Explain your observations.

Solution: The paper may not burn easily because the metal rod conducts heat away from the heated region. Continuous rotation also spreads the heat over the rod.

Activity: Take a sheet of paper. Draw a spiral on it, as shown in Fig. 7.17a. Cut the paper along the spiral. Suspend the paper as shown in the Fig. 7.17b above a burning candle. Observe what happens. Provide an explanation for your observation.

Solution: The spiral paper moves upward because the candle heats the surrounding air. The warm air rises and produces convection currents that make the spiral move.