ICSE Class 8 Physics Chapter 03 Heat

Heat

Learning Outcomes

Heat - form of energy

Effects of heat

Specific heat capacity

Heat capacity

Change of state

Latent heat

When Summit was feeling very cold during a chilly winter evening, he took a steaming cup of coffee and his body warmed up. When Rashmi was sweating badly on a sunny afternoon, she drank cold orange juice after which she felt cool and refreshed. How did the hot coffee warm up Summit who was feeling cold? How did Rashmi, who was feeling hot, start feeling cool and refreshed after taking a glass of cold orange juice?

The answer to these questions lies in a term called heat. In class 7 you studied how heat is transferred in substances and how it causes expansion in them. Here in this chapter, you will study how heat can change the temperature and state of a substance and how heat gained/lost by a substance can be calculated.

Touch a few things like an ice cube, a freshly cooked chapati, a steaming bowl of soup, a fruit taken from the refrigerator, etc. Are they hot or cold? You will find that the cooked chapati and the bowl of soup are hot, while the ice cube and the fruit are cold.

The degree of hotness or coldness of a substance is determined by a property known as temperature.

Heat - Form Of Energy

Just like mechanical energy and chemical energy, heat is also a form of energy. What happens when you boil water on a stove with the lid on? The lid moves because of the energy of the heat. Now, how does this heat energy flow? Heat energy flows between two bodies if there is a difference in temperature between them. Thus, heat is a form of energy that is transferred by a difference in temperature. We must note that the heat energy always flows from a body at high temperature to a body at low temperature.

We know that temperature is a measure of average kinetic energy of molecules. So, we can also say that when there is a difference in the average kinetic energies of two bodies, which are in contact, heat energy flows.

Can you now explain why we feel hot when we touch hot coffee and cold when we touch ice? Hot coffee is at a higher temperature than our body, so when we touch it heat flows from the hot coffee to our body and we feel hot. On the other hand, ice is at a lower temperature than our body, so when we touch ice, heat flows from our body to ice and we feel cold. Thus, we say that heat is a form of energy only when it flows.

Let us understand this with a simple example. In Figure 3.1, two beakers A and B are connected to each other with a pipe. We see that since the level of water in tank A is higher than that in tank B, water flows from tank A to tank B, irrespective of the size of the tank and the quantity of water in it. Once both the tanks attain the same level, there is no more flow of water.

Similarly, heat flows from one body to another only if the temperatures of the two bodies are different. When the two bodies have attained the same temperature, no further flow of heat takes place, and we say that the two bodies have reached thermal equilibrium.

You would have learnt in class 7 that all bodies absorb and emit heat radiations all the time. Hence, we can say that there is no net flow of heat energy at thermal equilibrium.

Units Of Heat

Do you remember the SI unit of energy? It is joules. Since heat is a form of energy, its unit will be the same as the unit of energy, i.e., joules.

Thus, the SI unit of heat is joules. Another important unit of heat is calorie. One calorie is defined as the amount of heat required to raise the temperature of 1 g of water by 1°C.

One more unit of heat is called kilocalorie, which is defined as the amount of heat required to raise the temperature of 1 kg of water by 1°C.

1 kilocalorie = 1,000 calories

The calorie is a bigger unit than joule.

1 calorie = 4.2 joules (J)

Quantity Of Heat

We know that when a body is heated, its temperature increases due to gain of heat, and when a body is cooled, its temperature decreases due to loss of heat. The quantity of heat gained or lost by a body, to increase or decrease its temperature, depends on many factors. Let us observe the following figures to study the factors that affect the amount of heat gained or lost by a body.

When different quantities of water taken in two different beakers are heated uniformly, water in beaker A (200 g) boils faster and takes less time than water in beaker B (400 g). Thus, heat energy depends on the mass of the substance. More the mass taken more will be the heat required, less the mass taken less will be the heat required.

When equal quantities of water in two beakers are heated from room temperature to 50°C in beaker A and to 70°C in beaker B, the time taken to reach 70°C is more than the time taken to reach 50°C. Thus, heat energy depends on the change in temperature. More the temperature change, more will be the heat required.

When equal quantities of water and oil in two different beakers are heated uniformly, oil takes less time to reach a higher temperature than water. Thus, heat energy depends on the nature of the substance (called the specific heat capacity of the substance).

Thus, we conclude that the heat gained or lost by a body depends on

1. the mass of the body;

2. the rise or fall in temperature of the body; and

3. the nature of the body.

Effect Of Heat

Take a little water in a pan and heat it for sometime. Can you list the changes that you observe? You will notice that (i) the temperature of the water rises; and (ii) the water changes its state and becomes vapour. Thus, we see that heating a body can have two effects:

1. the body can experience a rise in temperature; and

2. the body can change its state, e.g., from liquid to vapour.

Although you have learnt about four effects of heat in class 7, we will only study the two mentioned above in detail.

Specific Heat Capacity

We have seen earlier that if equal quantities of water and cooking oil are given the same amount of heat, their temperature goes up by varying degrees. Oil heats up much faster than water, i.e., it requires less heat than water for the same rise in temperature. This difference is because of the specific heat capacity of substances, which depends on the nature of the substance.

The specific heat capacity of a substance is defined as the amount of heat required to raise the temperature of unit mass (1 g or 1 kg) of a substance by 1°C.

The SI unit of specific heat capacity is J/kg-°C. It can also be expressed in J/g-°C, when we consider 1 g of that substance. To convert J/g-°C to J/kg-°C, the value should be multiplied by 1,000.

Table 3.1 shows the specific heat capacities of some substances. From the table, we see that the specific heat capacity of water is more than oil, and that is why it requires more amount of heat to raise its temperature by 1°C. Thus, substances having high specific heat capacity require more time to increase or to decrease their temperature. We also see that water has the highest specific heat capacity.

This high specific heat capacity of water renders it a lot of advantages. Some of them are shown below.

Plants can be protected from frost if fields are filled with water as wet soil takes a long time to cool. Also, vegetables, fruits, etc. are kept under water in cold countries because water can store large amounts of heat, which prevents the eatables from freezing.

Because of the high specific heat capacity of water, it can store lot of energy for a small temperature rise and so, retain heat for a very long time. Hence, it is used in hot water bottles.

Since water can absorb large amounts of heat without rising to a high temperature, it can be used as a coolant in car radiators, power stations, and mills and factories to keep the engine and other parts cool.

The temperature change of sea water is much less than the temperature change of land. This explains why coastal areas have a cooler summer and a milder winter than inland areas. For the same reason, a swimming pool remains cool even in peak summer.

So far, we have learnt that the amount of heat gained/lost by a body depends on the mass of the body (m), the rise/fall in the temperature of the body (ΔT), and the nature or the specific heat capacity of the body (C). Keeping all these together in an equation, the amount of heat gained/lost by a body (Q) is

\[Q = m \times C \times \Delta T\]

or \[Q = mC\Delta T\], where \[\Delta T = \text{higher temperature} - \text{lower temperature}\].

Let us see some solved examples based on this formula.

Solved Examples

1. How many joules of energy is needed to raise the temperature of 2 kg of water from 15°C to 55°C? Sp. heat capacity of water is 4,200 J/kg-°C.

Mass of water (m) = 2 kg

Sp. heat capacity of water (C) = 4,200 J/kg-°C

Rise in temperature (ΔT) = 55 - 15 = 40 °C

Heat energy required (Q) = m × C × ΔT

Q = 2 × 4200 × 40

= 336,000 J

2. How many joules of energy is given out by 500 g of iron, when it is cooled from 400 °C to 30 °C? Sp. heat capacity of iron is 440 J/kg-°C.

Mass of iron (m) = 500 g = 0.5 kg

Sp. heat capacity of iron (C) = 440 J/kg-°C

Fall in temperature (ΔT) = 400 - 30 = 370 °C

Heat energy required (Q) = m × C × ΔT

Q = 0.5 × 440 × 370

= 81,400 J

3. If 24,000 J of heat energy is given out by 200 g of oil at 80°C, what will be the final temperature of oil? Sp. heat capacity of oil is 2,000 J/kg-°C.

Mass of oil (m) = 200 g = 0.2 kg

Sp. heat capacity of oil (C) = 2,000 J/kg-°C

Heat energy given out (Q) = 24,000 J

We know that,

\[Q = m \times C \times \Delta T\]

Therefore, \[\Delta T = \frac{Q}{m \times C}\]

Therefore, \[\Delta T = \frac{2400}{0.2 \times 2000}\]

\[\Delta T = 60°C\]

Now, \[\Delta T = 80 - x\] (if x is the final temperature)

60 = 80 - x

Thus, x = 80 - 60 = 20 °C

Teacher's Note

When you heat water for tea, you can feel how water absorbs a lot of heat before boiling, demonstrating why water has such a high specific heat capacity compared to metals.

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