Heat Notes 8th Science Lesson 4 Notes in English

Heat Notes 8th Science Lesson 4 Notes in English

All the substances in our surrounding are made up of atoms and molecules. These atoms and molecules are always at vibratory motion. Due to this motion, substances have an energy known as heat energy. This energy flows from hot substances to cold substances or from hot region to cold region of a substance. When heat energy is supplied to any substance it increases the energy of the atoms and molecules in it and so they start to vibrate. These atoms and molecules which vibrate make other atoms and molecules to vibrate. Thus, heat energy is transferred from one part of the substance to other part. We can see this heat energy transfer in our daily life also. Heat energy brings about lot of changes. You will learn about them in this lesson. You will also study about transfer of heat and measurement of heat change.

Effects of Heat:

When heat energy is supplied to any substance, it brings about many changes. There are three important changes that we can see in our daily life. They are:

• Expansion
• Increase in temperature
• Change in state

Expansion:

Why didn’t the ball go through the ring initially but went through it after some time? When the ball is heated the atoms in the ball gain heat energy. They start vibrating and force each other apart. As a result an expansion takes place. That’s why the ball did not go through the ring. After some time, as the ball lost the heat

energy to the surrounding it came back to its original size and it went through the ring. This shows that heat energy causes expansion in solids. This expansion takes place in liquids and gases also. It is maximum in gases.

Rise in Temperature:

When the water is heated, water molecules receive heat energy. This heat energy increases the kinetic energy of the molecules. When the molecules receive more energy, the temperature of the water increases. This shows that heat energy causes increase in temperature.

Change of State:

In ice cubes the force of attraction between the water molecules is more. So they are close together. When we heat them the force of attraction between the molecules decreases and the ice cubes become water. When we heat the water, the force of attraction decreases further. Hence they move away from one another and become vapour. Since water vapour escape to the surrounding, water level decreases further. From this we understand that heat energy causes change in the state of the substances. When heat energy is removed, changes take place in reverse direction.

If heat energy is supplied to or taken out from a substance, it will undergo a change from one state of matter to another.

One of the following transformations may take place due to heat energy.

• Solid to Liquid (Melting)
• Liquid to Gas (Vapourisation)
• Solid to Gas (Sublimation)
• Gas to Liquid (Condensation)
• Liquid to Solid (Freezing)
• Gas to Solid (Deposition)

Change of state in Water

Transfer of Heat:

If heat energy is supplied to any substance, it will be transferred from one part of the substance to another part. It takes place in different ways depending on the state of the substance. Three ways of heat transfer are:

• Conduction
• Convection
• Radiation

Conduction:

How did the other end of the spoon become hot? It is because heat in the hot water is transferred from one end to other end of the spoon. In solid substances such as silver spoon, atoms are arranged very closely. Hot water molecules which are vibrating transfer the heat energy to the atoms in the spoon and make them vibrate. Those atoms make other atoms to vibrate and thus heat is transferred to the other end of the spoon.

In conduction heat transfer takes place between two ends of the same solid or through two solid substances that are at different temperatures but in contact with one another. Thus, we can define conduction as the process of heat transfer in solids from the region of higher temperature to the region of lower temperature without the actual movement of atoms or molecules.

Conduction in Solids

Conduction in daily life:

• We cook food in vessels made up of metals. When the vessel is heated, heat is transferred from the metal to the food.
• When we iron dresses, heat is transferred from the iron to the cloth.
• Handles of cooking utensils are made up of plastic or wood because they are poor conductors of heat.
• The temperature inside igloo (snow house) is warm because snow is a poor conductor of heat.

Convection:

When water in the vessel is heated, water molecules at the bottom receive heat energy and move upward. Then the molecules at the top come down and get heated. This kind of heat transfer is known as convection. This is how air in the atmosphere is also heated. Thus, the form of heat transfer from places of high temperature to places of low temperature by the actual movement of molecules is called convection. Convection takes place in liquids and gases.

Convection in daily life:

• Formation of land breeze and sea breeze is due to convection of air.
• Wind flows from one region to another region by convection.
• In hot air balloons heat is transferred by convection and so the balloon raises.
• In refrigirators, cool air moves downward and replaces the hot air because of convection.

Radiation:

Radiation is the third form of heat transfer. By conduction, heat is transferred through solids, by convection heat is transferred through liquids and gases, but by radiation heat can be transferred through empty space even through vacuum. Heat energy from the Sun reaches the Earth by this form of heat transfer. Radiation is defined as the way of heat transfer from one place to another in the form of electromagnetic waves.

Heat transfer by radiation

Radiation in daily life:

• Heat energy from the Sun reaches the Earth by radiation.
• While standing near fire we feel the heat which is transferred as radiation.
• Black surfaces absorb heat radiation. So that the bottom of the cooking vessels are painted black.
• White colour reflects heat radiation. That’s why we are advised to wear white cloth during summer.

Calorimetry:

We studied about the effects of heat energy. When heat energy is supplied to substances, physical changes take place in them. Solid form of water (ice) is changed to liquid form, and liquid form of water is changed to gaseous form. These are all the physical changes due to heat energy. Similarly, heat energy produces chemical changes also. To know more about the physical and chemical changes that take place in substances, we need to measure the amount of heat involved. The technique used to measure the amount of heat involved in a physical or a chemical process is known as calorimetry.

Temperature:

Temperature is a physical quantity which expresses whether an object is hot or cold. It is measured with the help of thermometer. There are three scales to measure the temperature. They are:

• Celcius scale
• Fahrenheit scale
• Kelvin scale

Among these three scales, Kelvin scale is the most commonly used one. You will study about this elaborately in Standard IX.

Unit of Heat:

We know that heat is a form of energy. The unit of energy in SI system is joule. So, heat is also measured in joule. It is expressed by the symbol J. The most commonly used unit of heat is calorie. One calorie is the amount of heat energy required to raise the temperature of 1 gram of water through 1°C. The relation between calorie and joule is given as, 1 calorie = 4.186 J.

Heat capacity:

In general, the amount of heat energy gained or lost by a substance is determined by three factors. They are:

• Mass of the substance
• Change in temperature of the substance
• Nature of the material of the substance

Different substances require different amount of heat energy to reach a particular temperature. This nature is known as heat capacity of a substance. Heat capacity is defined as the amount of heat energy required by a substance to raise its temperature by 1°C or 1 K. It is denoted by the symbol C’.

Heat capacity

= Amount of heat energy required (Q) / Raise in temperature (ΔT)

C’ = Q / ΔT

The unit of heat capacity is cal / °C. In SI system, it is measured in JK^-1.

Specific heat capacity:

When the heat capacity of a substance is expressed for unit mass, it is called specific heat capacity. Specific heat capacity of a substance is defined as the amount of heat energy required to raise the temperature of 1 kilogram of a substance by 1°C or 1 K. It is denoted by the symbol C.

Specific heat capacity

= Amount of heat energy required (Q) / Mass x Raise in temperature (ΔT)

C = Q / m x ΔT

The SI unit of specific heat capacity is J kg^-1 K^-1.

Calorimeter:

A calorimeter is a device used to measure the amount of heat gained or lost by a substance. It consists of a vessel made up of metals like copper or aluminium which are good conductors of heat and electricity.

The metallic vessel is kept in an insulating jacket to prevent heat loss to the environment. There are two holes in it. Through one hole a thermometer is inserted to measure the temperature of the contents. A stirrer is inserted through another hole for stirring the content in the vessel. The vessel is filled with liquid which is heated by passing current through the heating element. Using this device we can measure the heat capacity of the liquid in the container.

Thermostat:

A thermostat is a device which maintains the temperature of a place or an object constant. The word thermostat is derived from two Greek words, ‘thermo’ meaning heat and ‘static’ meaning staying the same. Thermostats are used in any device or system that gets heated or cools down to a pre-set temperature. It turns an appliance or a circuit on or off when a particular temperature is reached. Devices which use thermostat include building heater, central heater in a room, air conditioner, water heater, as well as kitchen equipments including oven and refrigerators. Sometimes, a thermostat functions both as the sensor and the controller of a thermal system.

Thermostat Thermos flask (Vacuum flask):

The thermos flask (Vacuum flask) is an insulating storage vessel that keeps its content hotter or cooler than the surroundings for a longer time. It is primarily meant to enhance the storage period of a liquid by maintaining a uniform temperature and avoiding the possibilities of getting a bad taste.

Working of Thermos flask:

A thermos flask has double walls, which are evacuated. It is silvered on the inside. The vacuum between the two walls prevents heat being transferred from the inside to the outside by conduction and convection.

Thermos flask

With very little air between the walls, there is almost no transfer of heat from the inner wall to the outer wall or vice versa. Conduction can only occur at the points where the two walls meet, at the top of the bottle and through an insulated support at the bottom. The silvered walls reflect radiated heat back to the liquid in the bottle.

Points to Remember:

• Heat is a form of energy which is transferred from one part to another part of a substance.
• Heat transfer causes expansion, increases temperature and changes the state of the substance.
• When thermal energy is supplied to a solid, the atoms or molecules present in it gain energy and vibrate more vigorously about their fixed positions, forcing each other further apart.
• Melting, vapourisation, sublimation, condensation, freezing and deposition are the change of states that take place due to heat energy.
• Heat transfer takes place in three ways: conduction, convection and radiation.
• Conduction occurs in solids, convection in liquids and gases, and radiation takes place in vaccum.
• Capacity of substances to gain or loose heat energy is determined by three factors: mass of the substance, change in temperature and nature of the substance.
• There are three scales to measure temperature: Celcius scale, Fahrenheit scale and Kelvin scale.
• Calorimeter measures the heat capacity of water.

Glossary:

Calorimeter – A device which measures the heat capacity of liquids.

Calorimetry – The technique used to measure the amount of heat involved in a physical or a chemical process.

Conduction – The process of heat transfer in solids from a region of higher temperature to a region of lower temperature without the actual movement of molecules.

Convection – The form of heat transfer from places of high temperature to places of low temperature by the actual movement of liquid or gas molecules.

Heat capacity – Amount of heat energy required to raise the temperature of a substance by 1°C or 1 K.

Radiation – The form of heat transfer from one place to another place in the form of electromagnetic waves.

Specific heat capacity – Amount of heat energy required to raise the temperature of 1 kilogram of a substance by 1°C or 1 K.

Temperature – Physical quantity which expresses whether an object is hot or cold.

Thermos flask – An insulating storage vessel that keeps its content hotter or cooler than the surroundings for a longer time.

Thermostat – A temperature sensing device that turns an appliance or circuit on or off when a particular temperature is reached in it.

Do You Know?

Electric wires used for long distance transmission of electricity will expand during day time and contract at night. That is why they will not be set very tightly. If they are set very tightly they will break when they cool at night.

Water is the only matter on the Earth that can be found naturally in all three states – Solid, Liquid and Gas.

All metals are good conductors of heat. The substances which does not conduct heat easily are called bad conductors or insulators. Wood, cork, cotton, wool, glass, rubber, etc are insulators.

Heat transfer by radiation is visible to our eyes. When a substance is heated to 500°C the radiation begins to become visible to the eye as a dull red glow, and it is sensed as warmth by the skin. Further heating rapidly increases the amount of radiation, and its perceived colour becomes orange, yellow and finally white.

The amount of energy in food items is measured by the unit kilo calorie.

1 kilo calorie = 4200 J (Approximately).

Water has higher heat capacity than most other substances. This accounts for the use of water as common coolant. 100 g of water can take away more heat than 100 g of oil.

Problem 1:

The temperature of a metal ball is 30° C. When an energy of 3000 J is supplied, its temperature raises by 40° C. Calculate its heat capacity.

Solution:

Heat capacity, C’ = Q / ΔT

Here, Q = 3000 J

ΔT = 40°C – 30°C = 10°C or 10 K

C’ = 3000 / 10 = 300 JK^-1

The heat capacity of the metal ball is 300 JK^-1.

Problem 2:

The energy required to raise the temperature of an iron ball by 1 K is 500 JK-1. Calculate the amount of energy required to raise its temperature by 20 K.

Solution:

Heat capacity, C’ = Q / ΔT

Q = C’ x ΔT

Here, C’ = 500 JK^-1

ΔT = 20 K

Q = 500 x 20 = 10000 J

The amount of heat energy required is 10000 J.

Problem 3:

Energy of 84000 J is required to raise the temperature of 2 kg of water from 60° C to 70° C. Calculate the specific heat capacity of water.

Solution:

Specific heat capacity, C = Q / m x ΔT

Here, Q = 84000 J

m = 2 kg

ΔT = 70° C – 60° C = 10° C or 10 K

C = 84000 / 2 x 10 = 4200 J kg^-1 K^-1

The Specific heat capacity of water is 4200 J kg^-1 K-¹.

Problem 4:

The specific heat capacity of a metal is 160 J kg^-1K^-1. Calculate the amount of heat energy required to raise the temperature of 500 gram of the metal from 125° C to 325° C.

Solution:

Specific heat capacity, C = Q / m x ΔT

Q = C x m x ΔT

Here, C = 160 J kg K^-1

m = 500 g = 0.5 kg

ΔT = 325° C – 125° C = 200° C or 200 K

= 160 x 0.5 x 200 = 16000 J.

The amount of heat energy required is 16000 J.

The world’s first ice-calorimeter was used in the year 1782 by Antoine Lavoisier and Pierre-Simon Laplace, to determine the heat generated by various chemical changes.

The vacuum flask was invented by Scottish scientist Sir James Dewar in 1892. In his honour it is called as Dewar flask. It’s also known as Dewar bottle.