These DAV Class 7 Science Notes and DAV Class 7 Science Chapter 5 Notes – Heat act as excellent revision resources, particularly in preparation for board exams.
Heat Class 7 DAV Notes
→ Temperature: The degree of hotness of a body is expressed in temperature.
→ Heat: Heat is a form of energy which is transferred from one body to another due to the existence of a temperature difference between them. Heat is also known as heat energy or thermal energy.
→ Transfer of heat energy always takes place from the body at a higher temperature to that at a lower temperature.
→ Difference between heat and temperature: Heat is the energy of a body due to the motion of its particles. Temperature is just an indicator of this energy.
→ Effects of Heat:
1. Physical Changes:
→ Change in temperature: Addition of heat to a body usually raises its temperature, while removal of heat from a body usually lowers its temperature.
→ Change of state: Under proper conditions, addition or removal of heat to or from a body can also bring a change in its state. A solid changes to liquid and a liquid changes to gas on addition of heat. On the contrary, a gas changes to liquid and a liquid changes to solid on removal of heat.
→ Thermal expansion: Most of the materials expand on heating. This phenomenon is called thermal expansion.
2. Chemical Changes: When reactants are heated up, many chemical changes take place.
→ Measurement of Temperature: Temperature is measure with an instrument called thermometer.
→ Structure of Thermometer: The most common thermometer is composed of mercury in a glass tube. It consists of a narrow capillary tube which is made of glass. The tube is closed at the upper end and has a bulb at its lower end. The bulb is filled with mercury. The mercury can be seen as a shining thread inside the capillary tube. The mercury; in the bulb; expands with increase in temperature. This results in the shining thread of mercury appearing longer; which gives the reading of temperature. Proper scales are made on the thermometer.
→ Scale on Thermometer: A thermometer usually has Celsius scale or Fahrenheit scale.
Table 1
→ Range of laboratory thermometer: From – 10° C to 110°C.
→ Clinical Thermometer: This thermometer is specially designed and calibrated to measure our body temperature. The range of clinical thermometer is from 35°C to 42°C; because the body temperature of humans remains within this range.
→There is a kink near the bulb in clinical thermometer. The kink prevents mercury from falling down on its own.
→ Note: In laboratory thermometer, the kink is absent.
→ Transfer of Heat: There are following three methods of transfer of heat.
→ Conduction: When transfer of heat takes place from one molecule to the next molecule, it is called conduction of heat. Transfer of heat in solids takes place through conduction. Good Conductors: Materials through which heat can easily get transferred are called good conductors of heat. Examples: iron, copper, tin, aluminium, etc.
→Bad Conductors or Insulators: Materials through which heat does not get transferred easily are called bad conductors or insulators. Examples: wood, asbestos, plastic, etc. Significance of conductors and insulators in our daily life:
- Cooking utensils are usually made up of metals. Metals are good conductors of heat and hence are ideal for making cooking utensils.
- Handles of cooking utensils are usually made of wood or plastic. This ensures that one can safely hold these utensils from their handles.
- Houses are made from bricks or mud. These are insulators and hence shield the house against temperature variations.
- Air is a bad conductor of heat. Air trapped inside sweaters and blankets help in maintaining the warmth of the body.
- Ice boxes are often made of double wall. The air between the two walls does not allow transfer of heat from outside to inside and thus, prevents the ice from melting.
(b) Convection: When transfer of heat takes place because of movement of molecules, it is called convection. Transfer of heat in liquids and gases takes place because of convection.
→ Significance of Convection: Wind movements on earth happen because of convection currents. The air near the earth’s surface heats up and rises up; leaving an area of low pressure near the ground. This low pressure area is filled in colder air moving from the surrounding.
→ Ventilators are usually made in walls near the ceiling so that warm air can escape through them. Windows are usually made at a lower level so that cool air can come inside the room.
→ Convection is also responsible for creating ocean currents and storms and typhoons.
→ Land Breeze and Sea Breeze: These phenomena are observed in coastal areas. During daytime, the land heats up faster than the ocean surface. Warm air from land rises up; leaving an area of low pressure near the ground. This low pressure area is filled in by cooler air moving from the ocean. This is called sea breeze. During night, the land cools down faster than the ocean surface. The warm air from ocean surface moves up; leaving an area of low pressure near the ocean surface. This low pressure area is filled in by cooler air moving from land. This is called land breeze.
→ Radiation: When transfer of heat takes place without the involvement of any media, it is called radiation. Transfer of heat from the sun to the atmosphere of the earth takes place because of radiation. A white body reflects most of the radiation. A black body absorbs most of the radiation.
→Significance of Radiation:
- The bottom of cooking utensils is usually made black. This helps in absorbing the heat in a better way.
- Light coloured clothes are preferred during summer because light colour reflects most the radiation. Thus, light coloured clothes are comfortable during summer.
- Dark coloured clothes are preferred during winter because dark colour absorbs most of the radiation. Thus, dark coloured clothes are comfortable during winter.
- The helmet of firemen is made of shining brass. The shining surface reflects most of the radiation and thus helps firemen remain comfortable; while fighting the fire.
- Outer walls of houses are usually coloured in light colours so that most of the heat can be reflected off. This helps in keeping the temperature of walls down.
- Thermos flask has shining walls which helps in radiating less energy and thus in maintaining the temperature of items kept in it.
→ Thermal: Anything related to heat is given a prefix ‘thermal’.
→ Thermodynamics: Change in heat is studied under thermodynamics.
→ Solar radiation: The heat energy from sun is called solar radiation.
→ Thermos flask: A specially designed container in which hot or cold things are kept to maintain the temperature for a longer time.
→ Thermal expansion: Most of the materials expand on heating. This phenomenon is called thermal expansion.
→ Conduction of heat: When heat transfer takes place from one molecule to the next. Convection of heat: When heat transfer takes place because of movement of molecules. Radiation of heat: When heat transfer takes place in without the involvement of a medium. Insulator of heat: A material which does not allow heat to flow through it.
→ Temperature: Measurement of degree of hotness or coldness of a substance.