THERMAL PROPERTIES MATTER
(i) Heat, (ii) Temperature, (iii) Scales of temperature, (iv) Thermometry, (v) Thermometers, (vi) Thermal expansion, (vii) Variation of density with temperature, (viii) Expansion of liquid, (ix) Effect of temperature on upthrust, (x) Anomalous expansion of water, (xi) Expansion of gases, (xii) Application of thermal expansion, (xiii) Thermal capacity and water equivalent, (xiv) Specific heats of solids, water, (xvi) Latent heats, (xvii) Principle of caloriemetry, (xviii) Heating curve
The energy associated with configuration and random motion of the atoms and molecules within a body is called internal energy and the part of this internal energy which is transferred from one body to the other due to temperature difference is called heats.
As heat is a form of energy it can be transformed into others and vice-versa.
e.g. Thermocouple converts heat energy into electrical energy, resistor converts electrical energy into heat energy. Friction converts mechanical energy into heat energy. Heat engine converts heat energy into mechanical energy.
Here it is important that whole of mechanical energy i.e. work can be converted into heats but the whole of heats can never be converted into work.
Generally, the temperature of a body rises when the heat’s supplied to it. However, the following two situations are also found to exist.
(i) When heat is supplied to a body either at its melting point or boiling point, the temperature of the body does not change. In this situation, heat supplied to the body is used up in changing its state.
(ii) When the liquid in a thermos flask is vigorously shaken or gas in a cylinder is suddenly compressed, the temperature of liquid or gas gets raised even without supplying heat. In this situation, work done on the system becomes a source of heats energy.
01. Temperature and Heat JEE Mains pattern MCQ Paper (Download here PDF.)
The solution of Temperature and Heat (Download here PDF.)
02. Temperature and Heat JEE Mains pattern MCQ Paper (Download here PDF.)
Solution of JEE Mains Papera (Download here PDF.)
An instrument used to measure the temperature of a body is called a thermometer.
The linear variation in some physical property of a substance with the change of temperature is the basic principle of thermometry and these properties are defined as a thermometric property (x) of the substance.
x may be (i) Length of liquid in the capillary
(ii) The pressure of gas at constant volume.
(iii) The volume of gas at constant pressure.
(iv) The resistance of a given platinum wire.
In old thermometry, two arbitrarily fixed points ice and steam point (freezing point and the boiling point at 1 atm) are taken to define the temperature scale. In Celsius scale freezing point of water is assumed to be 0°C while boiling point 100°C and the temperature interval between these is divided into 100 equation.
When matter is heated without any change in state, it usually expands. According to atomic theory of matter, asymmetry in potential energy curve is responsible for thermal expansion. As with rising in temperature the amplitude of vibration and hence the energy of atoms increases, hence the average distance between the atoms increases. So the matter as a whole expands.
(1) Thermal expansion is minimum in case of solids but maximum in case of gases because the intermolecular force is maximum in solids but minimum in gases.
(2) Solids can expand in one dimension (linear expansion), two dimensions (superficial expansion) and three dimensions (volume expansion) while liquids and gases usually suffer the change in volume only.
(1) When a substance changes from one state to another state (say from solid to liquid or liquid to gas or from liquid to solid or gas to liquid) then energy is either absorbed or liberated. This heat energy is called latent heat.
(2) No change in temperature is involved when the substance changes its state. That is, phase transformation is an isothermal change. Ice at 0°C melts into water at 0°C. Water at 100°C boils to form steam at 100°C.
Freezing mixture : If salt is added to ice, then the temperature of mixture drops down to less than 0°C. This is so because, some ice melts down to cool the salt to 0°C. As a result, salt gets dissolved in the water formed and saturated solution of salt is obtained, but the ice point (freezing point) of the solution formed is always less than that of pure water. So, ice cannot be in the solid state with the salt solution at 0°C. The ice which is in contact with the solution starts melting and it absorbs the required latent heat from the mixture, so the temperature of the mixture falls down.
The principle of Calorimetry:
When two bodies (one being solid and other liquid or both being liquid) at different temperatures are mixed, heat will be transferred from a body at higher temperature to a body at lower temperature till both acquire the same temperature. The body at higher temperature releases heat while the body at lower temperature absorbs it, so that
i.e. the principle of calorimetry represents the law of conservation of heat energy.
Conduction: Transfer of energy due to vibration and collision of medium particles without dislocation from their equilibrium position.