Measurement and Effects of Heat Class 8 Science Notes Maharashtra State Board
We have seen in previous standards that heat is a form of energy flowing from an object at a high temperature to a low temperature. The temperature of an object tells us how hot or cold that object is. The temperature of a cold object is lower than that of a hot object. Thus, the temperature of ice cream is less than the temperature of tea.
We have also seen that when we give heat to an object it expands and it contracts on cooling. Also, the state of matter changes due to heat. The unit of heat in the SI system is Joule while that in CGS units is calorie. One calorie is equivalent to 4.18 Joule. One calorie heat is the heat required to increase the temperature of 1 gm of water through 1°C.
Sources of Heat
- Sun: The Sun is the biggest source of heat received by the earth. A large amount of heat is generated due to nuclear fusion taking place in its center. In this process hydrogen nuclei fuse to form helium nuclei, generating heat in the process. Some of it reaches the earth in the form of light and heat.
- Earth: As the temperature at the center of the earth is high, the earth is also a source of heat. This heat is called geothermal energy.
- Chemical Energy: When fuels like wood, coal, petrol, etc, burn, there is a chemical reaction between the fuel and oxygen. Heat is generated in these reactions.
- Electrical Energy: In your daily life, you have seen several pieces of equipment that produce heat with the help of electricity e.g. electric press, electric heater, etc. Thus, electricity is a source of heat.
- Atomic Energy: A huge amount of heat is produced in a very short time when the nuclei of some elements like uranium, thorium, etc undergo fission. This is used in atomic energy projects.
- Air: A large amount of heat is present in the air around us.
Temperature:
We can find out how hot or cold an object is by touching the object. However, our sense of ‘hot’ or ‘cold’ is relative. This can be understood from the following experiment.
Take three similar vessels. Let us call them ‘A’, ‘B’, and ‘C’ (see figure). Fill A with hot water and B with cold water. Put some water from A and B in C. Dip your right hand in A and left hand in B, and keep them immersed for 2 to 3 minutes. Now dip both hands in C. What do you feel?
Even though, both the hands are dipped in water in the same vessel i.e. water at the same temperature, your right hand will find the water to be cold while the left hand will find it to be hot. What is the reason for this? Think about it. You must have understood from the above activity that we cannot determine the temperature of an object accurately by simply touching it. Also, you may hurt yourself by touching very hot or cold objects. So we feel the need for some device for measuring temperature. The thermometer is a device for measuring temperature. You have read about thermometers in the previous class. In this lesson, you are going to learn about the construction of a thermometer.
Heat and Temperature:
What is the difference between heat and temperature? We know that a substance is made of atoms. The atoms in a substance are always in motion. The total kinetic energy of the atoms in a substance is a measure of the heat contained in that substance, while the temperature of a substance is related to the average kinetic energy of atoms. If the average kinetic energy of atoms in two objects is equal then their temperatures will also be equal.
Figures ‘a’ and ‘b’ show the velocities of atoms in a gas at high and low temperatures, respectively. The direction and the length of the arrows attached to the atoms show the direction and magnitude of the velocity of the atoms. The velocity of atoms in the gas at higher temperatures is larger than the velocity of atoms in the gas at lower temperatures. The velocities of atoms in a solid object are shown by arrows in Figure (c). The atoms in a solid object are tied to one another because of the forces acting between them. So they cannot be displaced from their places. Because of heat, they oscillate around their fixed position. The higher the temperature of the solid, the faster is their velocity of oscillation.
Suppose A and B are two objects made from the same substance. The mass of A is twice the mass of B which means that the number of atoms in A is twice the number of atoms in B. Even if the temperatures of A and B are equal, i.e. the average kinetic energy of atoms in A is the same as that in B, the total kinetic energy of atoms in A is twice that in B. Thus, the heat content of A is twice that of B even though, they both have the same temperature.
Take two steel vessels A and B of the same size. Fill some water in A and double that amount in B. Make sure that the water in both vessels is at the same temperature. Raise the temperatures of water in both vessels by 10°C using a spirit lamp. Did it take the same time to increase the temperature in the two vessels?
You must have required more time to raise the temperature of water in B. This means that for the same increase in temperature, you had to give more amount of heat to B. Thus, even though the water in A and B have the same temperature, the amount of heat in B is more than that in A. Temperature is measured in units of Celsius (°C), Fahrenheit (°F), and Kelvin (K). Kelvin is used in scientific experiments, while the other two are used in daily life. The relation between the three units is shown by the following formulae.
\(\frac{(F-32)}{9}=\frac{C}{5}\) ………(1)
K = C + 273.15 ………(2)
Thermometer:
You must have seen the thermometer that is used when someone at home has a fever. That thermometer is called a clinical thermometer. Different thermometers are used for different purposes. Let us first learn about the working of a thermometer. A thermometer is shown in Figure (a). It has a narrow glass tube which has a bulb at one end. The bulb and part of the tube are filled with a liquid. Earlier, mercury was used but, as it is harmful for us, it has been replaced with alcohol. The rest of the volume of the tube has a vacuum and its other end is closed. The bulb is kept in contact with the object whose temperature is to be measured so that its temperature becomes the same as that of the object. Because of the increased temperature the alcohol inside it expands and its level in the tube rises. Using the properties of the expansion of alcohol (to be discussed below), the temperature can be obtained from the level of the alcohol. The tube of the thermometer is marked accordingly.
Figure (b) shows a clinical thermometer. As the body temperature of a healthy person is 37°C, clinical thermometers are designed to measure temperatures between 35°C and 42°C. These days, instead of the above type, digital thermometers are used for clinical purposes. One such thermometer is shown in Figure (c). This does not use the expansion of liquid due to heat. Instead, it has a sensor that can measure the heat coming out from the body directly and from that can measure the temperature of the body. The thermometers used in the laboratory are similar to the one shown in Figure (a) except that the range of temperatures that it can measure is large spanning 40°C to 110°C or even larger. A special type of thermometer is used to measure the minimum and maximum temperatures in a day. It is called the maximum-minimum thermometer. It is shown in Figure (d).
When a hot object is kept in contact with a cold object they both exchange heat. The hot object gives away heat while the cold object absorbs heat. Thus, the temperature of the hot object decreases, while that of the cold object increases. This means that the kinetic energy of atoms in the cold object goes on increasing while that in the hot object goes on decreasing. A time comes when the average kinetic energies of atoms in both objects become equal, which means that the temperatures of both objects become equal.
Specific Heat:
The specific heat of an object is the amount of heat required to increase the temperature of the unit mass of that substance through one degree. This is represented by the symbol ‘c’. Its unit in SI is Joule /(kg °C) and in CGS is cal/(gm °C). Suppose Q amount of heat is required to increase the temperature of an object of mass m and specific heat c, from Ti to Tf This amount depends on the mass and specific heat of the object and the increase in temperature and can be written as.
Q = m × c × (Tf – Ti) ……….(3)
Different substances have different specific heats. We are going to learn more about it in future classes. The specific heats of a few substances are given in the following table.
| Substances | Specific Heat cal/(gm °C) |
| Aluminium | 0.21 |
| Alcohol | 0.58 |
| Gold | 0.03 |
| Hydrogen | 3.42 |
| Iron | 0.11 |
| Copper | 0.09 |
| Mercury | 0.03 |
| Water | 1 |
Calorimeter:
We have seen that a thermometer is used to measure the temperature of an object. A calorimeter is used to measure the heat content of an object. Using this equipment, we can measure the heat produced or absorbed in a physical or chemical process. A calorimeter is shown in the figure. Similar to a thermo flask, a calorimeter has two vessels, an inner and an outer one. This way, no heat can be transferred from the inner to the outer vessel or from the outer to the inner vessel. Thus, the inner vessel is thermally isolated from the surroundings. The inner vessel is made of copper. A thermometer for measuring the temperature and a stirrer for stirring the liquid in the calorimeter are fitted in it.
Water at a fixed temperature is placed inside the calorimeter. This means that the temperature of the inner vessel and that of the water in it are the same. When a hot object is placed in water, heat is exchanged between the hot object, water, and the calorimeter and all three reach the same temperature. As the calorimeter is thermally isolated from the surroundings, the total heat lost by the hot object is equal to the total heat absorbed by the calorimeter and water inside it. Similarly, if we put a cold object in the calorimeter, the cold object will receive heat from the water and its temperature will increase, while water and calorimeter will lose heat and their temperature will decrease.
Suppose the mass of the inner vessel in the calorimeter is ‘mc’ and its initial temperature is ‘Ti’ and the mass of the water in the calorimeter is ‘mw’. The temperature of the water will also be ‘Ti’. Suppose we place an object of mass ‘mo’ and temperature ‘To’ in the calorimeter. If ‘To’ is higher than Ti the object will give away heat to the calorimeter and water. Soon the temperature of all three will become the same.
Let us call this final temperature ‘Tf’. The total heat given away by the object (‘Qo’) will be equal to the sum of the heat gained by the calorimeter (‘Qc’) and by water (‘Qw’). We can write this as Qo = Qc + QW ………..(4)
As seen above Qo, Qc, and Qw depend on mass, specific heat, and change in temperature ΔT. If the specific heats of the material of the calorimeter, water, and the object are cc, cw, and co respectively, we can write using formula (3),
Qo = mo × ΔTo × Co, ΔTo = To – Tf
Qw = mw × ΔTw × Cw, ΔTw = Tf – Ti
Qc = mc × ΔTc × Cc, ΔTc = Tf – Ti = ΔTw
Using (4), mo × ΔTo × co = mw × ΔTw × cw + mc × ΔTc × cc ………..(5)
We can measure all the masses and temperatures. If we know the specific heat of the material of the calorimeter i.e. copper and that of water, we can calculate the value of the specific heat of the object using formula (5). We will learn about this in more detail in higher standards.
Effects of Heat
In previous standards, we have studied two effects of heat on matter: 1. expansion and contraction and 2. change of state. In this lesson, we are going to learn more about expansion. You will learn about the change of state of matter in higher standards.
Expansion
When heat is given to any substance, its temperature increases and it expands. Its expansion depends on the increase in its temperature. Solids, liquids, and gases, all expand on receiving heat.
Expansion of Solids
Linear Expansion: The linear expansion of a solid is the increase in the length of a wire or a rod of a solid due to an increase in its temperature. When we increase the temperature of a rod of length l1 from T1 to T2, its length becomes l2. The change in length is proportional to the original length and the increase in temperature, (ΔT = T2 – T1). So we can write the change in length as follows.
Change in length ∝ original length × Change in temperature
l2 – l1 ∝ l1 × ΔT
l2 – l1 = λ × l1 × ΔT …………(6)
l2 = l1 (1 + λ ΔT) …………(7)
Here λ (lambda) is the constant of proportionality and it is called the coefficient of linear expansion of the solid substance.
The expansion coefficients of different substances are different. From the above formula we can see that if we take two rods of different substances, both having the same length, and increase their temperatures by the same amount, the rod of the substance having a higher expansion coefficient will expand more. Thus, the increase in its length will be larger. From formula (6), we can write the expansion coefficient as l = (l2 – l1)/(l1 ΔT) …………(8)
Thus, the expansion coefficient is the change in the length of a rod of unit length when its temperature is increased by 1 degree. From formula (8) we can see that the unit of expansion coefficient is the inverse of the unit of temperature, i.e. 1/°C. The expansion coefficients of some substances are given in the following table.
Coefficient of Expansion of Some Substances
| Solid | Coefficient of Linear Expansion × 106 (1/°C) | Liquid | Coefficient of Volume Expansion × 103 (1/°C) | Gas | Expansion Coefficient × 103 (1/°C) |
| Copper | 17 | Alcohol | 1 | Hydrogen | 3.66 |
| Aluminium | 23.1 | Water | 0.2 | Helium | 3.66 |
| Iron | 11.5 | Mercury | 0.2 | Nitrogen | 3.67 |
| Silver | 18 | Chloroform | 1.3 | Sulphur dioxide | 3.90 |
Volumetric Expansion of Solids:
Similar to a sheet, a three-dimensional piece of solid expands on all sides when heated and its volume increases. This is called the volumetric expansion of a solid. This is given by the following formula.
V2 = V1 (1 + β ΔT) …………… (10)
Here, ΔT is the temperature change, and V1 and V2 are the initial and final volumes of the solid. β is the volumetric expansion coefficient of the solid.
Have you seen Rails? They are not continuous. A small gap is kept between them at regular intervals. This is shown in the figure. This is kept to accommodate the change in the length of the rails with temperature change. If this gap is not kept, then the rails will get distorted due to expansion in summer which may lead to accidents.
Similar to rails, the length of bridges can also increase due to expansion in summer. The length of the 18 km long Great Belt Bridge in Denmark increases by 4.7 m in summer. Therefore, provision is made in the construction of the bridges to accommodate this expansion.
Expansion of Liquids
A liquid does not have a definite shape but it has a definite volume. So we can define a volumetric expansion coefficient for a liquid as follows.
V2 = V1 (1 + β ΔT) ………….(11)
Here, ΔT is the temperature change, and V1 and V2 are the initial and final volumes of the liquid. β is the volumetric expansion coefficient of the liquid.
Expansion of Gases
A gas does not even have a fixed volume. The gas expands on heating but if the gas is kept in a closed box, its volume cannot increase but its pressure increases. This is shown in Figure. Observe the Figure and find out answers to the questions.
1. Using the formula density = mass/volume, explain what will be the effect of heat on the gas kept in a closed bottle.
2. If the bottle is not closed but has a movable piston attached to its open end (see the figure), what will be the effect of heating the gas in the bottle?
Therefore, the expansion of a gas is measured by keeping its pressure constant. This volumetric expansion coefficient is called the constant pressure expansion coefficient and is given by the following formula.
V2 = V1 (1 + β ΔT) …………..(12)
Here, ΔT is the temperature change, and V1 and V2 are the initial and final volumes of the gas at constant pressure. β is the constant pressure expansion coefficient of the gas.
Maharashtra State Board Class 8 Science Notes Measurement and Effects of Heat can be used for revisiting and reinforcing previously learned content.