Heat

Introduction

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.

Ø Thus, Heat is an energy that raises the temperature of a thing by causing the molecules in that thing to move faster.

Ø 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.

Ø 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?

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.

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.

Total heat is measured by calorie, the amount of heat needed to raise one gram of water by one degree centigrade.

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.

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.

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

a. Your body to the coffee, or

b. The coffee to your body?

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

a. Your body to the air particles, or

b. The air particles to your body?

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

a. Your body to the air particles, or

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 / area or volume (due to contraction / expansion) is directly related to temperature change.

· 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.

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.

❏ Riveting

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

❏ 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.

1.11 Numerical problems

1. I put a kettle containing 1 liter of cold water on the gas stove, and it takes 5 minutes to reach the boiling point. My friend puts on a small electric kettle, containing ½ liter 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; or

b. The electricity supply? Can you say how many times as much?

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

Points to remember

1. The main source of heat is sun, we can obtain heat from combustion, friction, and electricity.

2. Heat is an energy that raises the temperature of a thing by causing the molecules in that thing to move faster

3. Heat is the total Kinetic energy of constituent particles of objects.

4. SI unit of Heat is joule (J).

5. The measurement of warmness or coldness of a substance is known as its temperature.

6. SI unit of temperature is kelvin.

7. Temperature determines the direction of flow of heat when two bodies are placed in contact.

8. Two objects are said to be in thermal contact if they can affect each other’s temperature.

9. Thermal equilibrium exists when two objects in thermal contact no longer affect each other’s temperature.

10. Most substances expand when heated and contract when cooled. The expansion of a substance on heating is called the thermal expansion of that substance.

11. 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.