Heat - Part I

We have seen that energy comes in various forms. Heat, light, sound, chemical energy, mechanical energy, electrical energy, etc. are all different forms of energy. Energy can be converted from one form into another.

Temperature of a body measures the heat contained within the body. Temperature is a measure of hotness or coldness of a body. Temperature is measured in ƒC (degrees Celcius or degrees centigrade) or ƒF (degrees Fahrenheit). The MKS unit for measuring temperature is K (Kelvin). A thermometer can measure temperature when brought in contact with the body whose temperature has to be measured Click here for more information on how to measure temperature.

What we will study in this chapter :

1. Quantity of heat, specific heat, thermal capacity, thermal equilibrium
2. Change of state and latent heat  
3. Evaporation and  boiling
4. How heat is transmitted  
5. Sources of heat

Quantity of heat, specific heat, thermal capacity, thermal equilibrium :  
To find out what factors affect the amount of heat taken up by a body, take a beaker and add 250 ml of water. Take a burner and try to heat the water. Measure the change in temperature by dipping a mercury thermometer that is calibrated correctly. Note the time taken by the water to go from room temperature to boiling point of water which is 100 ƒC.

Now take 500ml of water and do the same experiment. You will notice that it takes longer time to boil.

If you take 250 ml of oil and try and heat it, you will notice that oil boils at lower temperature than water.

Do another experiment : take 250 ml of hot water from a domestic geyser and try and boil it. You will see that this time also, the water boils much faster than the first experiment.

From the four experiments done, we can conclude that the quantity of heat required to change temperature of a body depends on :

• Mass of the body

• Nature of the substance the body is made of

• The initial temperature of the body.

Heat energy is measured in Joules. But more commonly, heat is measured in calories or in kilocalories. One kilocalorie is defined as the amount of heat needed to raise the temperature of 1 kg of water by 1ƒC.

1 calorie = 4.18 Joules 

From the comparison of heating water and oil, we can say that different substances require different amounts of heat for the same increase (or decrease) in temperature.  On a cold morning, you will see that a door knob is much colder than the door. This means that the knob has lost more heat than the door, although both are exposed to the same surrounding or ambient temperature. This is because the materials have different specific heats and thermal capacities.

Specific heat of a substance is defined as the amount of heat required to raise the temperature of 1 kg of the substance through 1ƒC. The specific heat is denoted by s and its unit of measurement is J/kg ƒC (Joules per kilogram per degree Centigrade). The specific heat of water is 4180 J/kg ƒC.

The quantity of heat taken in or given out by a body depends on the mass of the body, specific heat of the body and the change in temperature that occurs because of the heat taken in or given out.

Thus Q = quantity of heat taken in or gained = m x s x T  (T : rise in temperature)

Or  Q = quantity of heat given out = m x s x T                  (T : fall in temperature )

Thermal capacity of a substance is defined as the amount of heat required to raise (or lower) its temperature by 1ƒC.

Thermal capacity  = mass x specific heat

Thermal capacity of water is very large, This is the reason why it is used as a coolant in many applications such as in car radiators. It can store within its mass more heat energy before it starts to change its state to steam.

Heat energy flows from a hotter body to a colder body when the two bodies are brought in contact.  The heat will continue to flow till the temperatures are equalized. No heat flows from one body to the other when they are at equal temperatures. This is called the thermal equilibrium.

A few examples will make the concept clearer.

Example 1 :  100 gm of water at 100ƒC is mixed with 100 g of water at room temperature ( 25 ƒC). What is the final temperature of the mixed water ?

Let the final temperature be  =   T ƒC

The hot water looses heat. Let its fall in temperature be =  (100-T) ƒC

The water at room temperature gains heat.
Its gain in temperature can be written as   = (T-25) ƒC

Heat lost by the hot water at 100ƒC  =  Heat gained by the water at room temperature (25ƒC)

As discussed earlier Q (lost)  = Q (gained)

m₁ x s x (100-T) =  m₂ x s x (T-25)

m₁ = mass of hot water = 100 gm

m₂ =  mass of water at room temperature = 100 gm

s = specific heat of water = 4180 J/kg/ƒC

Substituting all the values in the equation, we get

(100-T) ƒC =   (T-25) ƒC

2 T  =  125 ƒC

Thus T = 62.5 ƒC

So the final temperature of 200 gm of water will be 62.5 ƒC.

Example 2 : A copper cooking vessel contains 50 gm water. How much energy is required to boil the water in the vessel ? The mass of the vessel is 0.5 kg. Specific heat of copper is 390 J/kg/ƒC. 

The temperature of boiling water is = 100 ƒC

Heat energy needed to take the copper vessel at room temperature (25 ƒC) to 100 ƒC is

Q (gained by copper vessel ) =  m_c x s_c x (100-25) ƒC

                                          =   0.5 kg x 390 J/kg/ƒC x (100-25) ƒC

                                           =  14625 J

Heat energy needed to take the water at room temperature (25 ƒC) to 100 ƒC is 

Q (gained by water ) = m_w x s_w x (100-25) ƒC

                              = 15675 J 

Next        Main        Previous