This presentation is a samle demonstration of the newton's law of cooling. The first part of the video defines the law and the second part designs an experiment to findout the specific heat of a given liquid by the method of cooling.
Newton’s law of cooling
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By Aditya Abeysinghe
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LAW: The rate of change of the temperature of an
object is proportional to the difference between its
own temperature and the temperature of its
dθ / dt = E A (θ – θr ) ; E- A constant that depends
upon the object , A – surface area, θ – A certain
temperature, θr – Room/ ambient temperature or the
temperature of the surroundings.
Newton’s law of cooling
Finding the specific heat capacity
of a liquid by the method of cooling
Let’s take the ,
Mass of the calorimeter = M
Mass of water = mw
Mass of the liquid = ml
The specific heat capacity of the calorimeter = C
The specific heat capacity of water = Cw
The specific heat capacity of the liquid = Cl
Also let’s assume that the times taken by water
and the liquid to cool from θ1 to θ2 is tw and tl.
median rate at which the calorimeter with water looses heat =
(MC + mw Cw) (θ1 - θ2 ) / tw
And the median rate at which the calorimeter with water looses
(MC + ml Cl) (θ1 - θ2 ) / tl
Under identical situations we can assume that the median rates
at which heat is lost by these two systems are equal.
(MC + mw Cw) (θ1 - θ2 ) / tw = (MC + ml Cl) (θ1 - θ2 ) / tl
If we conduct our experiment, by this equation, we can find the
specific heat capacity of the liquid.
1. Set up the apparatus as shown. Fill the empty space
between the outer vessel and the inner vessel with cold
2. Measure the mass of the calorimeter and pour water of at
most 70°C to about 1cm below the lid.
3. Then hang the calorimeter with the two strings, so that it’s
free in the air, without any contact with a physical surface.
4. Stir the calorimeter until the temperature of water comes
down to about 40°C. Record the temperature at regular
5. Measure the mass of the calorimeter with water.
6. Remove the cold water between the two vessels and refill it
with cold water.
7. Remove water inside the calorimeter and repeat the
experiment with a similar volume of the liquid.
8. Record the temperatures at regular intervals.
9. Draw the cooling curves for both the liquid and the water in the
same graph and within the same temperature range.
WaterLiquid Use these readings
and from the equation
we just got above, we
can find the specific
heat capacity of the
1. The calorimeter should be hanged between the two vessels
because during the experiment the environmental conditions can
change. Hence the flow of convection currents around the
calorimter might change.
2. When similar volumes of the liquid and water are used for the
experiment, the temperature variation across the surface of the
calorimeter at a certain temperature is identical. Sometimes,
when the volumes are different, although inside of the
calorimeter might have the same temperture, the temperature
distribution across the outer surface might be different.
3. When the calorimeter is stirred, the temperature is equally
distributed to all parts of the calorimeter. Thus, the temperature
absorbed by convection currents is the same regardless of the
4. When the calorimeter is closed at the top by a lid,
the heat loss due to convection and vaporization is
5. For both occasions, i.e. for the experiment of
water and the liquid, the same calorimeter should be
used. If we use two identical calorimeters,
sometimes the nature of the surface may change.
Thus, the readings of the practical and hence the
expected value for the specific heat of the gas may
7. It’s also better if the calorimeter and the stirer are
made of the same material.