CBSE Class 9 Physics Effect of Heat Notes Set 05

Heat and Temperature

Heat is a form of energy that flows from a body at higher temperature to one at lower temperature; the direction of flow depends only on the temperatures, not on masses or specific heats. Temperature measures the degree of hotness or coldness.
1. The SI unit of heat is the joule (J); 1 calorie = 4.2 J and 1 kcal = 4200 J.
2. Heat gained or lost: \( Q = mc\,\Delta T \), where c is the specific heat capacity (J/kg K) of the substance. For water c = 4200 J/kg K = 1 cal/g °C.
3. Thermal (heat) capacity of a body = mc, in J/K. In a mixture, heat lost by the hot body = heat gained by the cold body and the calorimeter (principle of calorimetry).
4. Mechanical energy that is completely converted into heat raises the temperature by \( \Delta T = \dfrac{E}{mc} \).

Thermometers

1. A thermometric substance must expand uniformly with temperature. Mercury is used because it expands uniformly, has a high boiling point, does not wet glass and is a good conductor so it responds quickly; its freezing point is fairly low (-39°C).
2. In very cold regions alcohol is used because its freezing point is very low; it can be coloured to make the level easy to read.
3. Platinum resistance thermometers are used for accurate scientific work, since the resistance of platinum varies steadily with temperature.
4. A thermostat (for example a bimetallic strip) automatically maintains a steady temperature by switching the heat supply off and on.

Expansion of Solids and Liquids

1. A solid rod expands in length by \( \Delta L = L\alpha\,\Delta T \); the expansion depends on the original length, the rise in temperature and the material. The coefficient of linear expansion \( \alpha \) is in the order invar < steel < brass < aluminium.
2. Areal expansion is about \( 2\alpha \) and volume expansion about \( 3\alpha \) (for the coefficient of cubical expansion \( \gamma = 3\alpha \)).
3. A hole in a heated metal sheet expands as though it were filled with the same metal, so the hole becomes larger.
4. Liquids expand more than solids; they need a container, so we observe the apparent expansion, which is less than the real expansion by the expansion of the vessel: \( \gamma_{real} = \gamma_{apparent} + \gamma_{vessel} \).
5. The density of a substance falls on heating: \( \rho_t = \dfrac{\rho_0}{1 + \gamma\,\Delta T} \).
6. A steel ball weighed in alcohol shows a larger apparent weight at a higher temperature, because the liquid expands more than steel and its buoyant force decreases.

Worked Example

Example. A body of energy 840 J falls into 500 g of water and all the energy becomes heat. Find the rise in temperature (c = 4200 J/kg K).
\( \Delta T = \dfrac{Q}{mc} = \dfrac{840}{0.5 \times 4200} = 0.4 \)°C.