Fig.1. We receive heat energy in 3 different ways. A better name for heat is ‘internal energy’ or ‘thermal energy’ because the energy arises from the motion of atoms and molecules. In solids the molecules do not move from place to place but they vibrate. Faster they vibrate hotter they would be. Temperature is a device we use to get an idea of the vibrational energy in molecules. Thermometer.
Fig.2. Principle of the thermometer. In making a thermometer it is necessary to use a large volume of the liquid and allow the expanding column to rise in a capillary ( hair thin) tube.H2O molecules are at a very high state of vibration. This is why gas bubbles form inside the liquid.. .Kinetic energy from these particles make the glass particles vibrate faster causing the the mercury particles also to vibrate faster.As mercury expands much more than solid glass, mercury level rises.Temperature is a very good measure to get an idea of how hot a substance is but it is not a measure of energy content.Heat flows from a hot body to a cooler body; that means from a high temperature to a lower temperature. ( It is not from one having a lot of energy to another having less energy) Generating heat
1. Conduction This can occur only in solids as the particles are closely packed. Faster vibrating particles influence the neighbouring molecules also to vibrate faster. All materials do not conduct very wee. Some do not conduct at all and then they are called insulators. An Experiment. Fig.3.Testing for conductivity. These two did an experiment to classify substances as conductors and insulators. They selected 4 materials of approximately the same length and thickness. They held one end of the 4 sticks and heated the other end.
This is their record sheet.
Fig.4 Matchstick experiment. A home experiment. Aim:- To select good conductors and bad conductors You need: A candle, a few match sticks. Rods of different materials. Warning: An adult is essential to help you. Method: Fig.2 .As the wax melt matches drop Select a rod of about 30 cm. Attach some match sticks on to that using candle wax. Then hold it with one hand and heat the other end by a flame. Count the number of matches that drop during a definite time. Repeat the experiment with different rods. Tabulate the results
.Tabulate the results in a chart like this.
1. Convection Fig.5. The shape of the candle flame depends on the convection current. Fig.5b. Convection currents in a liquid. In gases heated particles move faster. Due to the expansion , they become less dense. Then the hot molecules rise forming an upward draft. As this reduces the pressure cool air is drawn in. This can happen in liquids too. A home Experiment. Aim:-
Warning:- An adult is essential to help you. You need:- A box of matches Two tongs or tweezers A candle Fig.6 Which would ignite first. These two are holding two match sticks equidistant from a flame. If they do not light bring them closer to the flame very slowly. Observe which one will ignite first. Fig.3 Which will ignite first? Home Experiment 3. Aim:- To observe convection current. You need:- A glass jar Some thick paper Box of matches. Method:
Fig.7. Observing convection in air. 1. Radiation. All bodies above -273 °C radiate energy in the form of waves. These are known as electromagnetic waves. The quantity and the nature , (Wave length and frequency) depending on the temperature. Here are some waves of the electromagnetic spectrum.
Expansion This is another important effect of heating. Except for a very few cases, all materials increase in size as the temperature rises. This has been termed as ‘expansion’ This effect is mainly responsible for the changes in state also. Why do solids, liquids and gases expand when heated? Fig. 5 Why do molecules need more space at a higher temperature. The above diagram shows why materials expand when heated.The thick circle shows the molecule and the thin circle shows how far it can move on a vibration. Actually it is the amplitude of vibrations. As the amplitude increases molecules need more space. So they become far apart and less dense. Fig. 7 An experiment to observe the expansion of a liquid. Fig.9.An experiment to observe the lineear expansion of a metal wire. Change of state. What happens to the attraction between molecules as they become far apart? It has to decrease. This is why a blacksmith will heat a rod of iron before hammering it into a blade of a knife. Further heating will decrease the attractions to such an extent that the molecules will crumble down. This what we call melting. What happens if you raise the temperature a little more? With the complete loss of attractions the molecules will move faster and faster and some of them will escape and spread away in all directions. This the next change of state that we call evaporation. Quantity of heat energy Just like any other type of energy heat has to be measured in Joules. One aturto measure it is to note the increase in temperature of a pure substance such as water. Water needs 4.18 Joules of energy to increase the temperature of 1 gram by 1°C . This is known as the specific heat of water. Latent Heat
A Sensitive Thermometer
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HELP - Select the answers from these jumbled words and phrases. Metal cover, Vacuum, Prevents radiation, Prevents conduction. For drinking, Prevents convection, Protection, , Insulating lid, Double walled glass bottle. Metal cup , (2x10 = 20 Marks) 2.0 State the main method of heat transfer according to the pictures.. A- conduction, B- convection, C- radiation. D- Conduction.
3.0 Divide these into three groups, Good conductors, Poor conductors and insulators. Copper, glass , plastic, pyrex, ceramics, Brass, Stones (granite),water,wool Aluminium, lead, Concrete, wood, air, rubber. Iron.( 5x4 = 20 marks) 4.0 Test tube containing some water and ice is heated as shown.
2 What would happen to the water at the top and the ice at the bottom after a short time? ( 4 marks) 3 Give two important conclusions possible from the results of this experiment. ( 4 marks) 4 Name one liquid that may conduct heat well. ( 3 marks) (15 marks) 5.0 What could be having the nearest connection over these phenomenon. A – Conduction of heat B – Convection of heat C – Radiation D – Latent heat.
.1 Sea breezes and land breezes. 2. If you touch wood and iron at the same temperature iron feels cold.3. Sun heats the land without heating the air. 4. When the humidity is low we feel cooler.. 5. Even when you tilt a candle, the flame remains vertical. 6. Alcohol is cool when taken to a finger. 7. The temperature of Ice and water remains the same until all the ice melts. 8. What property of iron makes it unsuitable for a cooking pot handle. 9. People near a fire feels hot. 10. Double glazing of windows reduces heat loss. (3 x 10 = 30 Marks)(8 marks) 2 Mention one merit of using each of these as liquid in thermometers.
3 Which is the correct reason for a liquid column to go up in a thermometer when warm. A . All materials expand when heated. B - Liquids expand more than solids. C - Solids are good conductors while liquids are poor conductors. D - Convection currents cannot form in solids. (5 marks) For the solutions Click - Answer page - Science.
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