CBSE Notes Class 7 Science Chapter 3 Heat

Heat is a pattern of energy that is transferred over objects or systems due to a temperature difference. It moves from a realm of elevated temperature to a region of inferior temperature, and this transfer continues until thermal equilibrium is reached (i.e., both objects are at the same temperature). Heat plays a crucial role in various physical processes, such as the boiling of water, the melting of ice, and the warming of the Earth's surface.

1.0Temperature

• The appraise of degree of hotness or coldness of a body The measure of degree of hotness or coldness of a body.
• Thermometers are instruments designed to assess the temperature of an object or system.
• SI unit of temperature : Kelvin (K)
• Some other popular units are Celsius and Fahrenheit.

Note: The temperature of an object is the property that decides the direction of flow of heat between the object and its surroundings.

2.0Heat

• Heat is a pattern of energy that increases the temperature of a substance.
• It  is a form of energy transferred between two bodies due to a temperature difference between them.
• S.I. The unit of heat is Joules (J).
• Another commonly used unit is Calorie (cal).
• 1 calorie  = 4.18 J
• 1 kilocalorie (kcal) = 1000 calories = 4180 J
• If heat can flow between two objects or systems, the objects or systems are said to be in thermal contact.
• Heat is also called thermal energy.

3.0Temperature Scales

• Thermometers measure temperature by using some property of matter that is temperature-dependent.
• The familiar liquid-in-glass thermometer relies on thermal expansion, the mercury or alcohol expands as its temperature rises (or contracts as its temperature drops) and we read the temperature on a calibrated scale. This scale is called temperature scale.
• The calibrated scale made on a thermometer is called temperature scale.
• To measure temperature, two fixed points are taken on thermometers or temperature measuring devices.
• Lower Fixed Point-One fixed point is the melting point of ice (or freezing point of water) called ice point (lower fixed point). 
• Upper Fixed Point-The other fixed point is the boiling point of water called steam point (upper fixed point).
• Celsius Scale-The Celsius scale divides the interval between the icy and boiling points of water into 100 degrees.
• Fahrenheit Scale-On the Fahrenheit scale, water freezes at 32 degrees and boils at 212 degrees. There are 180 Fahrenheit degrees between the freezing point and the boiling point of water.
• Kelvin Scale-The Kelvin scale divides the interlude between the icy and boiling points of water into 100 divisions.Water freezes at 273 K and boils at 373 K.This scale is based on the existence of absolute zero, the minimum possible temperature.

4.0Thermometers

• Instruments that measure temperature are called thermometers. When a thermometer is in contact with a system, energy is exchanged until the thermometer and the system are in thermal equilibrium with each other.
• It consists of a mass of liquid, usually mercury or alcohol that expands into a glass capillary tube when its temperature rises and contracts when temperature falls.
• When the cross-sectional area of the capillary tube is constant, the change in volume of the liquid varies linearly with its length along the tube. 
• We can then define a temperature in terms of the length of the liquid column.

5.0Liquid In Glass Thermometer

A liquid-in-glass thermometer consists of following parts 

1. Glass Tube-A thermometer is a sealed glass tube marked with a scale on the outside.
2. Capillary Tube-Inside the glass tube there is another thinner tube, called the capillary tube.
3. Bulb-The capillary tube ends in a bulb made of thick glass. The bulb contains the thermometric liquid, alcohol or mercury.

6.0Selection of Thermometric Liquid

• Alcohol is used for study of atmosphere and weather conditions. This is because alcohol remains a liquid between -112 °C and 78 °C. Therefore, temperatures as low as -100 °C can be measured. 
• At such temperatures, mercury would freeze as the freezing point of mercury is -39 °C.
• For common use, mercury is preferred over alcohol for the following reasons: Mercury does not freeze or vaporize easily, i.e., it remains a liquid over a large temperature  range. Mercury freezes at  -39 °C and boils  at 357 °C.
• Since mercury is a good conductor of  heat, it quickly attains the temperature of the body with which it is in contact. Also, it has a higher density than water.
• Mercury does not wet the surfaces in contact thus it does not stick to the inner surface of the capillary tube.
• Mercury is an opaque liquid with a shining appearance like silver. Thus, it can be easily observed through the glass.

7.0Laboratory And Clinical Thermometer

1. The Laboratory Thermometer

• The thermometers available in the laboratory  consist of a long-sealed glass tube (also called stem) marked with a scale on the outside.
• Inside the glass tube there is another thin tube, called the capillary tube (or bore).
• At the end of which there is a bulb made of thick glass. The bulb is filled with a liquid at the other end.
• The most commonly used liquid is mercury.

Precautions:

• The thermometer is delicate and should be handled with care to avoid breakage.
• It should be kept vertical and not tilted.
• The bulb should be completely surrounded by the substance whose temperature is being measured.
• While reading the thermometer, the level of mercury should be the same level as the eye.
• The thermometer should not be held by the bulb while reading the temperature.

2. The Clinical Thermometer

• Use : to measure body temperature
• Liquid : Mercury 
• Temperature Assortment : 35 °C  to  42 °C  (or 94 °F  to 108 °F)
• Normal body temperature : 37 °C  or 98.6°F

Features of Clinical Thermometer

• The capillary tube has a constriction or kink just above the bulb. It allows the mercury to rise but stops it from falling. 
• This is useful while taking a patient's temperature. The thermometer must be shaken or given a jerk to bring down the mercury level.
• The outer glass is made triangular or curved so that the thread of mercury is magnified and can be seen easily.
• The bulb is made of thick glass. This is done to make the bulb strong so that it does not break under the weight of mercury or when it is shaken. 
• To check the body temperature, a thermometer is placed in your mouth below the  tongue or in  your  armpit  for 2  - 3 minutes. 
• When  the thermometer is taken out of your mouth, the liquid in the bulb contracts and the mercury column breaks at the kink. 

Precautions:

• A thermometer should be cleaned before and after use, ideally with an antiseptic solution.
• Before use, make sure the mercury level is below 35°C.
• When reading the thermometer, ensure the mercury level is in line with your line of sight.
• Handle the thermometer carefully, as it may crack if it comes into contact with a hard object.
• Don’t hold the thermometer by the bulb while reading it. 

3. Maximum And Minimum Thermometer

• Maximum thermometer is a mercury-in-glass thermometer  and measures the highest temperature reached in a day.
• Minimum thermometer is an alcohol-in-glass thermometer and measures the lowest temperature attained in a day.

8.0Heat Transfer

• When heat is transferred, it always moves from warmer to cooler objects. The warmer object loses heat energy and becomes cooler as the cooler object gains heat energy and becomes warmer.
• This process of heat transfer can occur in three ways by conduction, radiation, or convection

1. Conduction

It is the transfer of heat by the direct contact of particles of matter. 

• Conduction usually occurs most easily in solids and liquids, where atoms and molecules are close together. 
• Then atoms and molecules need to move only a short distance before they bump into one another and transfer energy.
• As a result, heat is transferred more rapidly by conduction in solid and liquids than in gases.
• A conductor is any material that easily transfers heat. Metals are good conductors of heat. Metals have some electrons that are free to move from one place to another within the material. These free electrons help to transfer heat energy. 
• Heat Insulators-An insulator is a material in which heat doesn’t flow easily. Most pans have handles that are made from insulators. Liquids and gases are usually better insulators than solids. Air is a good Insulator,  and many insulating materials contain air spaces that reduce the transfer of heat by conduction within the material.
• Wool is a poor conductor of heat i.e. it is a heat insulator. Moreover, there is air trapped in between the wool fibers. Since air is also a heat insulator, It reduces the transfer of heat from our body to the colder surroundings, helping us feel warm.

2. Convection

It is a process by which heat is transferred by movement of heated fluid such as air or water. The circular movement of current that is set up in liquids and gases is called convection currents. The types of convection are:

(a) Natural convection-Natural convection is a type of heat transfer in which fluid motion occurs without the aid of any external source, such as a pump or fan. The heat flows only due to difference of densities within the fluid that occurs because of temperature differences in the different regions  of the fluid

• In natural convection, the fluid near a heat source absorbs heat, becomes less dense, and rises. Meanwhile, cooler surrounding fluid moves in to take its place.
• The cooler fluid moves in to take its place, gets heated, and the cycle repeats, creating a convection current.
• Natural convection happens when a warmer, less dense fluid is displaced by a cooler, denser fluid.

(b) Forced Convection-Forced convection is a mode of heat  transfer  in which  fluid motion is created by an outer source like a pump, fan, blower, etc. It is one of the main methods of transferring heat efficiently. Example-Central Cooling system, Air Conditioning

2. Radiation

Heat transfer by radiation occurs when energy is transferred in the form of waves. Heat waves are just like light but unlike light they are invisible. The exchange of heat energy by radiation can occur in empty space, as well as in solids, liquids, and gases. The Sun is not the only source of radiation.

• The waves which mainly carry heat from Sun to Earth are infrared rays. 
• The warmth you feel when you sit next to a fireplace is due to heat conveyed by radiation from the fire to your skin.
• The microwave oven is used to heat our food. 
• Black surfaces absorb almost all the thermal radiation that falls on them.
• For example, black coal tar road gets very hot in the summer because it effectively absorbs thermal radiation.