Experiment: To study the Newton’s law of cooling by plotting a graph between cooling time and temperature difference between calorimeter and surroundings.
The rate of loss of heat from a vessel presumably depends on the area of the exposed surface, the nature of that surface, the temperature of the surface and the temperature of the surroundings.
Newton’s law of cooling states that the rate of loss of heat of a body is directly proportional to the difference of temperature between the body and the surrounding. The law however holds good only for small difference of temperature.
Rate of cooling ∝ difference of temperature between body and surrounding
Consider a body of mass m, specific heat s, temperature T kept in a surrounding of temperature TR. If it looses an amount ΔQ of heat in time Δt and thus its temperature is lowered by ΔT, then
Calorimeter with stirrer, thermometer with ½o graduation, stop clock, heating device, water, oil (mustard or any other) about 100 ml, large metal box blackened inside and outside
1. Choose a clean, clear corner of the room.
2. Clean and dry the copper calorimeter but do not make its outside shining.
3. Note the room temperature and record it. Hold the thermometer and bring your eye in level with the upper tip of mercury thread to ensure correct reading.
4. Set up the apparatus as in figure.
5. In case a double walled Newton’s law apparatus is not available, use any large metal box blackened inside and outside. You can also just keep the copper calorimeter on the wooden table and work in any calm corner of the laboratory.
6. Heat water to about 80°C.
7. Fill the calorimeter about 2/3 with water. Place the thermometer in water.
8. Set the stop clock at zero and note its least count.
9. Start stirring the water in the calorimeter to make it cool uniformly.
10. Just when temperature of water is at about 70°C note it and start the stop clock.
11. Continue stirring and note temperature after every minute.
12. When fall of temperature becomes slower, note temperature at internal of two minutes, then five minutes and then ten minutes, till the temperature of water is close to room temperature.
13. Repeat the experiment with oil in the same way. Fill the calorimeter about 2/3 with oil, as in case of water.
Plot a graph between time and temperature of water. Take time along x-axis and temperature (Tw) along y-axis. It is called a cooling curve.
The temperature falls quickly in the beginning and then slowly as difference of temperature (Tw – TR) goes on decreasing. This is in agreement with Newton’s Law of Cooling.
Note: Cooling is fast if the surface area of the calorimeter is large. Cooling is fast from black, painted surface and slow from polished metallic surface. Cooling is faster from good conductor vessels and. slower from poor conductor ones. You can do simple changes in your apparatus and do the experiment to check this out as a project work.