Heat

Class 6 | Science — Physics | Unit 1

Class 6 Unit 1 Science

We are all familiar with heat. We feel it on our body when the sun shines. We use heat for cooking our food, we reduce the heat by adding ice cubes while preparing fruit juice. Let us learn about sources of heat.

1.1 Sources of heat

Sun

We all know that the sun gives us light. Does it give us heat? After standing under the sun light for some time, touch your head. Does it feel hot? Yes, it feels hot because the sun gives out heat besides light. Now, you can understand why it is difficult to walk bare-footed on sunny days in the afternoon.

Combustion (Burning)

Heat energy can be generated by the burning of fuels like wood, kerosene, coal, charcoal, gasoline/petrol, oil, etc., In your home, how do you get heat energy to cook food?

Friction

Rub your palms for some time and then hold them to your cheeks. How do you feel? We can generate heat by rubbing two surfaces of some substances. In the past people used to rub two stones together to light fire.

Electricity

When electric current flows through a conductor, heat energy is produced. The water heater, iron box, electric kettle etc., work on this principle.

1.2 Heat

Molecules in objects are constantly vibrating or moving inside objects. We cannot see that movement with our naked eye. When we heat the object this vibration and movement of molecules increases and temperature of the object also increases. Thus, Heat is an energy that raises the temperature of a thing by causing the molecules in that thing to move faster. Heat is not a matter. It doesn't occupy space. It has no weight. Like light, sound and electricity, heat is a form of energy. In short, Heat is the total kinetic energy of constituent particles of objects. SI Unit of Heat is joule. The unit calorie is also used.

1.3 Hot and cold objects

In our day-to-day life, we come across a number of objects. Some of them are hot and some of them are cold. How do we decide which object is hotter than the other? We use the tip of our finger to find out whether the tea in a cup has enough heat to drink or whether milk has been cooled enough to set for making curds. We often determine heat by touching the objects. But is our sense of touch reliable?

Activity 1

Take three bowls. Pour very cold water in the first bowl. (you can also add ice cube for cooling). Place luke warm water in the second. Half fill the third with hot water (not hot enough to burn) Set them in a row on the table, with the lukewarm water in the center. Place your right hand in the cold water, and your left hand in the hot water. Keep them in for a few minutes. Then take them out, shake off the water and put both into the middle bowl. How do they feel?

Priya says,"My right hand tells me that the water in the bowl is hot and the left hand tells me that the same water is cold.

Write down in your own words what do you experience? Discuss in the class why this happens.

When you placed your left hand in the hot tub, the heat from the bowl made the molecules on your hand vibrate faster. When you keep the same hot hand in the second bowl the vibrations transferred from your hand to make the particles in the water vibrate. Therefore, you feel loss of heat and hence your hand feels cold. In the same way, your right hand which was placed in cold water, feels hot when you insert it into the lukewarm water. Because it takes heat energy from lukewarm water. So, the same lukewarm water gives your hands different feeling according to the temperature of your hand. Measuring temperature by touching is not correct. Thermometers are used to measure temperature accurately and quantitatively.

1.4 Temperature

Definition of Temperature

The measurement of warmness or coldness of a substance is known as its Temperature. SI unit of temperature is kelvin. Celsius and Fahrenheit are the other units used. Celsius is called as Centigrade as well. It determines the direction of flow of heat when two bodies are placed in contact.

Activity 2: The Temperature of Boiling Water

Take water in a vessel and place the vessel on a stove. Fix the thermometer as shown in figure (Caution: The thermometer should not touch the vessel in which the water is being heated. Otherwise the thermometer will be broken at high temperature.) All students have to read the temperature of the water and note the reading on the blackboard.

Do you notice that the temperature is raising? What is the temperature of water when it is boiling?

Does the temperature of the boiling water rise further after that?

When boiling water is heated for some time, the water continues to receive more heat, but it's temperature does not rise further. The point at which the water boils and temperature becomes stable is called the boiling point of water.

Exercise Guess and Write (Check your assumption with the help of a thermometer)

Approximate temperature of the tea when you drink:

Approximate temperature of cool lemon juice when you drink:

Normally, the room temperature of water is approximately 30°C. When we heat water, its temperature raises and it boils at 100°C. If we cool the water, it freezes at 0°C.

Is Neela correct?

Beaker A and B has water at 80°C. Then pour the water of A and B to an empty beaker C. Now, what is the temperature of the water in the beaker C? Neela says it will be 160°C.

What is your opinion? Does Neela say correctly? Make a guess and verify it experimentally.

Do You Know?

One day in 1922, the air temperature was measured at 59°C in the shade in Libya, Africa. The coldest temperature in the world was measured in the Antarctic continent. It was approximately -89°C. The minus sign (-) is used when the temperature falls below the freezing point of water, which is 0°C. If water becomes ice at 0°C, you can imagine how cold -89°C would be. Our normal body temperature is 37°C. Our body feels cool if the air temperature is around 15 to 20°C. Can you estimate the night temperature in your village or city during winter?

1.5 Heat and Temperature

Heat and temperature are not the same thing, they in fact mean two different things; Temperature is related to how fast the atoms or molecules move or vibrate within the substance. Heat not only depends on the temperature of the substance but also depends on how many molecules are there in the object. Temperature measures the average kinetic energy of molecules. Heat measures the total Kinetic Energy of the molecules in the substance

Activity 3

Take one litre water in a pan, and heat it on a stove. Calculate the time taken to start boiling. (that is, the time taken to thermometer reading goes up to 100°C). Take five litre water in another pan and heat it on the same stove. Calculate the time taken by the water to start boiling.

In which pan the water starts to boil earlier?

One litre water Five litre water

Both, however, show a temperature of 100°C at the boiling point. Five litre water takes more time to boil i.e. more heat is needed to boil the larger amount of water. So, five litre boiling water has more heat energy than one litre water.

Place an open can of lukewarm water in each pan. Observe their temperature to find out which can gets hotter.

In which can water shows quick rise in temperature?

Can in One litre boiled water Can in five litre boiled water

You can see that, five litre water pan will raise the can of water to a higher temperature. Though, both pans of boiling water have the temperature of 100°C the five litre water can give off more heat energy than one litre water. Because it has more heat energy, and gives more energy to the water in the can. Total heat is measured by calorie, the amount of heat needed to raise one gram of water by one degree centigrade.

Exercise Which has more heat energy in each pair? Put tick mark.

Let Us Think

Pavithra is having tea while watching the pond near her house. Surely, tea is in higher temperature than the water in the pond. Now, a question is arising in Pavithra's mind. Which one has more heat energy, a cup of tea or the water in the pond?

What do you think?

Even though the temperature of the tea is higher than that of pond water, the volume of the water in pond is very high, hence the number of molecules in the water in the pond is higher than the tea in the cup. So, pond has more heat energy than tea cup.

1.6 Flow of Heat

An analogy between temperature and water level

Water 'flows' when there is a difference in the 'levels' of water in different places. It does not matter if there is more water in one place or another. Water from a puddle can flow into a reservoir or the other way around. The 'temperature' of an object is like the water level – it determines the direction in which 'heat' will flow. Heat energy flows from higher temperature to lower temperature.

Thermal contact and Thermal equilibrium

Two objects at different temperature are put together Consider two bodies A and B. Let the temperature of A be higher than that of B. On bringing bodies A and B in contact, heat will flow from hot body A to the cold body B. Heat will continue to flow till both the bodies attain the same temperature. The temperature determines the direction of flow of heat.

Exercise Answer the following questions

1. You are holding a hot cup of coffee. Would the heat energy transfer from?

a. Your body to the coffee
b. The coffee to your body

2. You are standing outside on a summer day. It is 40°C outside (note that normal body temperature is 37°C). Would the Heat energy transfer from?

a. Your body to the air particles
b. The air particles to your body

3. You are standing outside on a winter day. It is 23°C outside. Would the heat energy transfer from?

a. Your body to the air particles
b. The air particles to your body

Two objects are said to be in thermal contact if they can exchange heat energy. Thermal equilibrium exists when two objects in thermal contact no longer affect each other's temperature. For example, if a pot of milk from the refrigerator is set on the kitchen table, the two objects are in thermal contact. After certain period, their temperatures are the same, and they are said to be in thermal equilibrium.

1.7 Expansion in solids

Sam is trying to open a tight jar, but he cannot open it. He asks his uncle to help. His uncle says that pour some hot water on the lid of the jar. Sam does so and tries to open it now. Wow! The jar is opened easily! Do you have such experience? How do you open a tightly closed cap of the pen which could not be opened by you normally?

Most substances expand when heated and contract when cooled. The change in length or area or volume (due to contraction or expansion) is directly related to temperature change.

Activity 4

Hammer a nail into a tin can. Ease the nail out. Put it in again to make sure that the hole is large enough for the nail. Then, holding the nail with a pair of pliers, scissors or forceps, heat the nail over a candle, in hot water, or over the stove. Try to put it into the hole in the can.

I see that:

You will see that, now it is hard to put the nail into the hole. Heat expands solids. The molecules in the solid move faster, spread apart and occupy more space. The expansion of a substance on heating is called, the thermal expansion of that substance.

1.8 Linear and Cubical Expansion

A solid has a definite shape, so when a solid is heated, it expands in all directions i.e., in length, area and volume, all increase on heating. The expansion in length is called linear expansion and the expansion in volume is called cubical expansion.

Why is the iron rim of a bullock cart wheel heated before it is fitted onto the wheel? Why is a small gap left between two lengths of railway lines? We can perform an interesting experiment to find out an answer to these questions. All we need to do is to heat a cycle spoke.

Activity 5: Linear Expansion

Take a bulb, dry cell, candle, cycle spoke, coin (or broad - headed nail) and two wooden blocks. Place one end of the cycle spoke on a wooden block and connect an electric wire to it. Put a stone over the spoke to hold it firmly in place on the wooden block, as shown in Figure. The spoke should be parallel to the ground. Place the second wooden block under the free end of the spoke. Wrap some electric wire around the coin (or nail) and place it on the block. You may put a stone over the coin to hold it in place. Connect a bulb and dry cell to the free ends of the wires connected to the coin and the spoke and make the circuit shown in the figure. When the tip of the free end of the spoke touches the coin, the circuit is completed and the bulb lights up. Check to ensure this. If the bulb does not light up, it means the circuit is not complete, so check your connections properly. Now slide a page of your book between the coin and spoke and then slide it out. That way you would get a gap between the coin and spoke equal to the thickness of the sheet of paper.

Does the bulb light up? If it does not, what could be the reason?

You saw that the bulb does not light up when the spoke does not touch the coin. Now light the candle and heat the spoke with it.

Did the bulb light up after the spoke was heated for some time?

If it did, then explain how the spoke touched the coin after it was heated.

Why does the bulb go off some time after the candle is taken away from the spoke?

What happens to the length of the spoke when it is heated or cooled?

Activity 6: Cubical Expansion

Take a metal ring and metal ball of such size that the ball just passes through the ring. Heat the ball and check whether it passes through the ring.

Passed through Not passed through

Now let the ball cool down, and check whether it passes through the ring.

Passed through Not passed through

Solids expand due to heat and come back to the original state if heat is removed.

1.9 Uses of Thermal Expansion

Fitting the iron rim on the wooden wheel

The diameter of the iron ring is slightly less than that of the wooden wheel. Therefore, it cannot be easily slipped on from the rim of wooden wheel. The iron ring is, therefore, first heated to a higher temperature so that it expands in size and the hot ring is then easily slipped over to the rim of the wooden wheel. Cold water is now poured on the iron ring so that it contracts in size and holds the wooden wheel tightly.

Rivetting

Rivets are used to join two steel plates together. Hot rivet is driven through the hole in the plates. One end of the rivet is hammered to form a new rivet head. When cooled, the rivet will contract and hold the two plates tightly together.

1.10 Thermal Expansion Examples

Exercise Give Reasons for the following

1. Gaps are left in between rails while laying a railway track.

2. Gaps are left in between two joints of a concrete bridge.

Cracking of a thick glass tumbler

Glass is a poor conductor of heat. When hot liquid is poured into the tumbler, the inner surface of the tumbler becomes hot and expands while the outer surface remains at the room temperature and does not expand. Due to this unequal expansion, the tumbler cracks.

Electric wires

Electric wires between electric posts contract on cold days and sag in summers. To solve this problem, we leave wires slack so that they are free to change length.

Do You Know?

Glassware used in kitchen and laboratory are generally made up of Borosilicate glass (pyrex glass). The reason is that the Borosilicate glass do not expand much on being heated and therefore they do not crack. The photographs below show an expansion joint at the end of a bridge in winter and in summer.

Which season is shown in each picture? Explain how do you know?

1.11 Numerical problems

Exercise Solve the following problems

Point one: I put a kettle containing 1 litre of cold water on the gas stove, and it takes 5 minutes reach the boiling point. My friend puts on a small electric kettle, containing half litre of cold water, and it takes 5 minutes to get up to boiling point. Which gives more heat in 5 minutes?

a. the gas supply
b. the electricity supply

Point two: Can you say how many times as much?

One calorie heat energy is needed to raise the temperature of the water from 30°C to 31°C. How much heat energy is needed to raise the temperature of the water from 30°C to 35°C?

Points to remember

• The main source of heat is sun, we can obtain heat from combustion, friction, and electricity.
• Heat is an energy that raises the temperature of a thing by causing the molecules in that thing to move faster
• Heat is the total Kinetic energy of constituent particles of objects.
• SI unit of Heat is joule (J).
• The measurement of warmness or coldness of a substance is known as its temperature.
• SI unit of temperature is kelvin.
• Temperature determines the direction of flow of heat when two bodies are placed in contact.
• Two objects are said to be in thermal contact if they can affect each other's temperature.
• Thermal equilibrium exists when two objects in thermal contact no longer affect each other's temperature.
• Most substances expand when heated and contract when cooled.
• The expansion of a substance on heating is called the thermal expansion of that substance.
• A solid has a definite shape, so when a solid is heated, it expands in all directions i.e., in length, area and volume, all increase on heating.

ICT CORNER - Heat

Through this activity you will be able to understand the 'Thermal Energy Transfer'.

Step 1: Use the given URL in the browser. 'THERMAL ENERGY TRANSFER activity page will open.

Step 2: Click the = icon on the top left of the activity window, a list will drop down, from the list select a title.

Step 3: A small flash video window will open, click the play icon to play the video and observe.

Step 4: From the list select any title under the 'Example" list, a small flash activity window will open, click anyone of the tab given under the window to know the process of thermal transfer. Repeat the activity with different titles from the menu.

THERMAL ENERGY TRANSFER URL: http://d3tt741pwxqwm0.cloudfront.net/WGBH/conv16/conv16-int-thermalenergy/index.html#/intro

Evaluation

Exercise I. Choose the appropriate answer

1. When an object is heated, the molecules that make up the object

a. begin to move faster b. lose energy c. become heavier d. become lighter

2. The unit of heat is

a. newton b. joule c. volt d. Celsius

3. One liter of water at 30°C is mixed with one liter of water at 50°C. The temperature of the mixture will be

a. 80°C b. More than 50°C but less than 80°C c. 20°C d. around 40°C

4. An iron ball at 50°C is dropped in a mug containing water at 50°C. The heat will

a. flow from iron ball to water b. not flow from iron ball to water or from water to iron ball c. flow from water to iron ball d. increase the temperature of both Exercise II. Fill in the blanks

1. Heat flows from a body to a body.

2. The hotness of the object is determined by its

3. The SI unit of temperature is

4. Solids on heating and on cooling.

5. Two bodies are said to be in the state of thermal if there is no transfer of heat taking place.

Exercise III. True or False. If False, give the correct statement 1. Heat is a kind of energy that flows from a hot body to a cold body. 2. Steam is formed when heat is released from water. 3. Thermal expansion is always a nuisance. 4. Borosilicate glass do not expand much on being heated. 5. The unit of heat and temperature are the same. Exercise IV. Give reasons for the following

1. An ordinary glass bottle cracks when boiling water is poured into it, but a borosilicate glass bottle does not.

2. The electric wire which sag in summer become straight in winter.

3. Rivet is heated before fixing in hole to join two metal plates.

Exercise V. Match the following

1. Heat
2. Temperature
3. Thermal Equilibrium
4. Ice cube
5. Boiling water

Options: 0°C, 100°C, kelvin, No heat flow, joule

Exercise VI. Analogy

1. Heat: Joule :: Temperature:

2. Ice cube: 0°C :: Boiling water:

3. Total Kinetic Energy of molecules: Heat :: Average Kinetic Energy:

Exercise VII. Give very short answer

1. Make a list of electrical equipments at home which we get heat from.

2. What is temperature?

3. What is thermal expansion?

4. What do you understand by thermal equilibrium?

Exercise VIII. Give short answer

1. What difference do you think heating the solid will make in their molecules?

2. Distinguish between heat and temperature.

Exercise IX. Answer in detail

1. Explain thermal expansion with suitable examples.

Exercise X. Questions based on Higher Order Thinking Skills

1. When a window is accidentally left open on a winter night, will you feel uncomfortable because the cold is getting in, or because the heat is escaping from the room?

2. Suppose your normal body temperature were lower than what it is. How would the sensation of hot and cold change?

3. If you heat a circular disk with a hole, what change do you expect in the diameter of the hole? Remember that the effect of heating increases the separation between any pair of particles.

📝 Summary

1. Heat has four main sources: Sun (provides light and heat energy), Combustion (burning of fuels like wood, kerosene, coal), Friction (rubbing surfaces together), and Electricity (current flowing through conductors).
2. Heat is a form of energy that raises the temperature of objects by causing molecules to move faster - it is the total kinetic energy of constituent particles and has no weight or space.
3. The SI unit of heat is joule, and calorie is also used as an alternative unit for measuring heat energy.
4. Our sense of touch is not reliable for determining hot and cold objects, as demonstrated by the three-bowl experiment where the same lukewarm water feels different to hands at different temperatures.
5. When hands at different temperatures are placed in lukewarm water, heat energy transfers between the hand and water - making cold hands feel warm and warm hands feel cool in the same water.