Methods of Heat Transfer - Class 7 Notes & Olympiad Questions

Conductors and Insulators

a) Conductors are materials that easily allow heat to flow through them. They include substances like aluminium, iron, copper, and brass. These materials are good at conducting heat, meaning they can quickly transfer heat from one place to another.
b) On the other hand, insulators are materials that prevent heat from passing through them. Examples of insulators are plastics, rubber, wood, and ceramics. These materials are not good at conducting heat, so they tend to retain heat rather than letting it escape or transfer to other objects.

Methods of Heat Transmission

a) Heat is a form of energy produced by the motion of atoms and molecules. All matter has this thermal energy.
b) Heat transfer is the process of moving heat from a hotter object to a cooler one. When there is a difference in temperature between two objects, heat can transfer between them.

There are three main ways heat can be transferred: Conduction, Convection, and Radiation.

Conduction, Convection and Radiation

I. Conduction

Conduction is a method of heat transfer that occurs through direct contact between objects or substances. It happens when there is a temperature difference between the objects, causing heat to flow from the hotter object to the cooler one.

Some types of conduction and some examples are:

1. Solid-to-Solid Conduction

a) This type of conduction occurs when heat is transferred between two solid objects in direct contact.
b) The heat energy is transferred from the higher-temperature object to the lower-temperature object.

For example:

i) Touching a hot stove and feeling the heat transfer to your hand.
ii) Heating a metal spoon in hot soup, where the heat is conducted from the hot soup to the spoon.

2. Solid-to-Liquid Conduction

a) In this case, heat is transferred from a solid object to a liquid substance.
b) The solid object heats up the adjacent liquid molecules, causing them to gain thermal energy.

For example:

i) Stirring a hot drink with a metal spoon, where the spoon transfers heat to the liquid.
ii) Placing a hot metal rod into a container of water, where the heat is conducted into the water.

Applications of Conduction

1. Cooking: Conduction helps heat move from a stove to a pot or pan, allowing us to cook food evenly.

2. Insulation: It helps in making materials that keep heat inside or outside, like insulation in homes or jackets.

3. Manufacturing: Conduction is used in processes like welding and metal casting to join or shape metals.

II. Convection

Convection is a way that heat can move through fluids like liquids and gases. When a fluid is heated, it becomes less dense and rises up, while the cooler fluid sinks down. This creates a circular motion called convection currents that help transfer heat.

There are two main types of convection:

1. Natural Convection: This happens when heat causes the fluid to move by itself. Natural convection occurs through the following steps:

a) Hot fluid rises: When a part of the fluid gets heated, it becomes lighter and rises up because it's less dense.
b) Cool fluid sinks: As the hot fluid moves up, the cooler fluid takes its place, sinking down because it's denser.
c) Continuous cycle: This creates a continuous flow where the hot fluid rises and the cool fluid sinks, forming convection currents.
d) Examples of natural convection include hot air rising from a heater or the movement of water in a pot when it's heated on a stove.

2. Forced Convection

a) In forced convection, an outside force is used to make the fluid move. This can be done by using a fan, pump, or some other mechanical method.
b) The force helps the fluid move faster and increases the transfer of heat.
c) Examples of forced convection include a fan blowing air over a hot surface, like in a convection oven, or a pump circulating coolant in a car's engine to remove heat.

Applications of Convection

1. Cooling: Convection is used in cooling systems like fans and air conditioners. The moving air creates convection currents that take away heat, making us feel cooler.

2. Wind

a) Wind is a result of convection in the atmosphere. The sun heats the Earth's surface, causing the air above it to warm up.
b) Warm air rises and creates an area of low pressure. Cooler air from surrounding areas then flows in to replace it, creating wind.

3. Sea and Land Breezes

a) During the day, the land heats up faster than water, causing the air above the land to become warmer. This warm air rises, creating a low-pressure area.
b) As a result, cooler air from the sea flows towards the land, creating a sea breeze.
c) At night, the process reverses, and a land breeze occurs.

4. Weather Patterns: Convection plays a role in weather patterns. It helps form clouds, rain, and thunderstorms when warm and moist air rises and cools in the atmosphere.

5. Cooking: Convection is used in ovens and grills. The hot air circulates inside, cooking food evenly and quickly.

III. Radiation

a) Radiation is the transfer of heat through electromagnetic waves. Unlike conduction and convection, radiation does not require a medium to transfer heat.
b) Heat energy is emitted in the form of electromagnetic waves, such as infrared radiation, visible light, or even higher-energy forms like X-rays.
c) Radiation can travel through empty space and is responsible for the warmth you feel from the sun, the heat emitted by a fire, or the heat generated by various electrical appliances.

Applications of Radiation

1. Energy from the Sun

a) Radiation from the sun, specifically in the form of sunlight, provides us with light and heat energy.
b) It allows plants to perform photosynthesis, providing us with oxygen and food.

2. Medical Imaging

a) Radiation, such as X-rays, is used in medical imaging to see inside the human body.
b) It helps doctors diagnose and treat illnesses or injuries by creating images of bones, organs, and tissues.

3. Cancer Treatment

a) Radiation therapy is a method used to treat cancer. It uses high-energy radiation to target and destroy cancer cells.
b) This helps in shrinking tumours and preventing the spread of cancer.

4. Sterilization

a) Radiation is used to sterilize medical equipment, such as syringes and surgical instruments.
b) It kills bacteria and other harmful microorganisms, making the equipment safe to use.

5. Food Preservation

a) Radiation can be used to preserve food. It kills bacteria, insects, and other pests that can spoil food, extending its shelf life.

Absorption of Heat and Clothing Choice

a) The heat from the sun reaches the Earth through a process called radiation. Unlike conduction and convection, which require a medium like air or water, radiation can travel through the vacuum of space.
b) When sunlight (radiation) reaches an object, some of the heat energy is absorbed by the object, causing it to become warmer.
c) The amount of heat absorbed or radiated by an object depends on its nature and colour. Dark-coloured objects tend to be good absorbers of heat, while light-coloured objects are poor absorbers.
d) In the summer, when we want to stay cool, we prefer to wear light-coloured clothes. Light colours reflect more sunlight and absorb less heat, helping us stay cooler in hot weather. On the other hand, in winter, we choose dark-coloured clothes. Dark colours absorb more heat from the sun, helping us stay warmer in colder temperatures.

Thermos Flask: Keeping Hot Things Hot and Cold Things Cold

A thermos flask is a special device designed to maintain the temperature of its contents, keeping hot things hot and cold things cold.
It is designed to minimize heat transfer through conduction, convection, and radiation.

It typically consists of three main components:

1. Outer shell: The outer shell is usually made of plastic or metal and provides insulation to minimize heat transfer through conduction and radiation.

2. Inner flask: The inner flask is made of glass or metal and holds the liquid. It is double-walled, with a vacuum between the walls to prevent heat transfer through conduction and convection.

3. Stopper: The stopper is an airtight lid that seals the flask to prevent air circulation and minimize heat transfer.

Working of the Flask

a) The construction of a thermos flask helps in its working principle. The vacuum between the inner and outer walls of the flask prevents heat transfer through conduction and convection because there is no air or gas to conduct or convect the heat.
b) The reflective coating on the inner surface of the flask reduces heat transfer through radiation by reflecting the heat back into the liquid.