Heat and Temperature Notes PDF

HEAT AND TEMPERATURE. Introduction  All substances in our surrounding are made up of molecules.  These molecules are generally at motion and possess kinetic energy.  At the same time each molecule exerts a force of attraction on other molecules and so they poss...

HEAT AND TEMPERATURE. Introduction  All substances in our surrounding are made up of molecules.  These molecules are generally at motion and possess kinetic energy.  At the same time each molecule exerts a force of attraction on other molecules and so they possess potential energy.  The sum of the kinetic and potential energy is called the internal energy of the molecules.  This internal energy, when flows out, is called heat energy.  This energy is more in hot substances and less in cold substances and flows from hot substances to cold substances. Heat Heat is a form of energy that flows from a hot object to a cold object. Heat is the total energy of the motion of the molecules of a substance. It flows due to temperature difference. The heat is dependent on factors like the speed of the particles, the size of the particles and the number of particles. Its SI unit is joule, J. It can also be measured in calories. Heat is measured using calorimeter or joule meter. A calorimeter is an insulated container used to measure the heat exchanged in a chemical/physical process. Effects of Heat  When a substance is heated, the following things can happen. 1.Expansion:  When heat is added to a substance, the molecules gain energy and vibrate and force other molecules apart.  As a result, expansion takes place. You would have seen some space being left in railway tracks.  It is because, during summer time, more heat causes expansion in tracks.  Expansion is greater for liquids than solids and it is maximum in gases. 2.Change in State:  When you heat ice cubes, they become water and water on further heating changes into vapour.  So, solid becomes liquid and liquid becomes gas, when heat is added.  The reverse takes place when heat is removed. 3. Change in Temperature:  When heat energy is added to a substance, the kinetic energy of its particles increases and so the particles move at higher speed.  This causes rise in temperature.  When a substance is cooled, that is, when heat is removed, the molecules lose heat and its temperature falls. 4.Chemical changes:  Since heat is a form of energy it plays a major role in chemical changes.  In some cases, chemical reactions need heat to begin and also heat determines the speed at which reactions occur.  When we cook food, we light the wood and it catches fire and the food particles become soft because of the heat energy.  These are all the chemical changes taking place due to heat. Temperature. Temperature is a measure of hotness or coldness of an object. Temperature is a number that relates to the energy possessed by the molecules of a substance, which directly relates to the kinetic energy possessed by the particles of the substance. The temperature is directly related to the kinetic energy of the molecules, therefore if you double the temperature (Kelvin) of a substance, you double the average kinetic energy possessed by those molecules. It is measured kelvin (K). The temperature, is independent of the speed of the particles, the size of the particles and the number of particles. For example, let’s compare a tub of water with a cup of water. The water in the tub and the cup can be at the same temperature but by the virtue of the number of particles in the tub, the water in the tub has more thermal energy in it and thereby more heat even though they have the same temperature. Temperature scales. Temperature is measured using two primary scales : Celsius ( 0C) and Kelvin (K) Celsius is commonly used for everyday temperature measurements, while Kelvin is used in scientific contexts. Celsius scale is based on freezing point of pure water at standard atmospheric pressure (100°C) and Freezing point of pure water (0°C) Kelvin scale is the absolute temperature scale, where is 0 k represents zero(-2730C), the lowest possible temperature. To convert Celsius to Kelvin, add 273. K=0C+273 To convert Kelvin to Celsius, subtract 273. 0C=K-273. Common temperature measurements. Body temperature 370C Freezer temperature -180C Boiling point of water at sea level 1000C Freezing point of water 00C Room temperature 20-250C Hottest temperature recorded on Earth (in Furnace Creek Ranch, Death Valley, USA 56.70C Comparison of Heat and Temperature. Quantity Used Heat Temperature to Compare Temperature is the measure of The heat of an object is the total internal energy Definition the average kinetic energy of the of all the molecules inside that object. molecules in a substance. SI Unit Joule Kelvin Unit Joules, Calories Fahrenheit, Celsius, Kelvin Symbol Q T It is like a level which determines It’s a measure of the number of atoms the direction in which the heat Particles multiplied by the energy possessed by each will flow. It is related to how fast atom. the molecules within a substance are moving. It can be used to measure the Ability to do degree of heat. It has the ability to do work Work Heat is transferred from one object to another Temperature doesn't flow or due to a temperature difference. Heat transfer Transfer transfer between objects; it's a can occur through conduction (direct contact), mechanism property of an individual object or convection (through a fluid or gas), or radiation system (through electromagnetic waves). Heat is measured using calorimeter or joule Temperature is measured using Measurement meter thermometer