Maharashtra Board Class 8 Science Chapter 14 Measurement and Effects of Heat PDF Download

Measurement and Effects of Heat

Can You Recall?

Which sources do we get heat from?

How is heat transferred?

Which effects of heat do you know?

Some effects of heat are shown in figure 14.1. What are they?

We have seen in previous standards that heat is a form of energy which flows from an object at high temperature to an object at low temperature. Temperature of an object tells us how hot or cold that object is. The temperature of a cold object is lower than the temperature 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 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.

Solved Examples

1. How much heat will be needed to raise the temperature of 1.5 kg of water from 15°C to 45°C? Give the answer in calories as well as in Joule.

Given: mass of water = 1.5 kg = 1500 gm.

Change in temperature = 45°C - 15°C = 30°C.

Heat required for temperature change = ?

Heat required for temperature change = mass of water (gm) x change in temperature (°C) cal.

= 1500 × 30 cal = 45000 cal

= 45000 × 4.18 J = 188100 J

2. If the temperature of water changes by 10°C on giving 300 cal of heat, what is the mass of water?

Given: Amount of heat given to water = 300 cal, change in temperature = 10°C, mass of water = m = ?

Amount of heat given (cal) = mass of water (gm) x change in temperature (°C)

300 = m × 10

m = 300 / 10 = 30 gm.

Teacher's Note

When your mother heats water for your bath, heat makes the water hot. This heat comes from the stove, just like sunlight heats water in a pot left outside.

Exam Trick

Remember: Heat = energy that flows. Temperature = how hot or cold something is. Heat flows from hot to cold, always!

Points to Remember

Heat is a form of energy.
Temperature tells us how hot or cold an object is.
Heat flows from high temperature to low temperature.
One calorie = 4.18 Joule.
Objects expand when heated and contract when cooled.

Sources of Heat

1. Sun: The Sun is the biggest source of heat received by the earth. A large amount of heat is generated due to the nuclear fusion taking place in its centre. In this process hydrogen nuclei fuse together to form helium nuclei, generating heat in the process. Some of it reaches the earth in the form of light and heat.

2. Earth: As the temperature at the centre of the earth is high, the earth is also a source of heat. This heat is called geothermal energy.

3. Chemical Energy: When fuels like wood, coal, petrol etc, burn, there is chemical reaction between the fuel and oxygen. Heat is generated in these reactions.

4. Electrical Energy: In your daily life, you have seen several equipments which produce heat with the help of electricity e.g. electric press, electric heater etc. Thus, electricity is a source of heat.

Teacher's Note

The sun gives us heat and light every day. Without the sun, there would be no life on Earth. The fire you see in your kitchen also gives heat when cooking food.

Exam Trick

Remember the four sources: Sun (biggest), Earth (hot inside), Chemical (burning things), Electrical (from wires). Sun is always the main answer!

Points to Remember

The Sun is the biggest source of heat for Earth.
Geothermal energy comes from inside the Earth.
Burning fuel creates heat through chemical reactions.
Electricity can produce heat in our homes.
All heat sources increase temperature of objects.

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.

Try This

1. Take three similar vessels. Let us call them 'A', 'B' and 'C'.

2. Fill A with hot water and B with cold water. Put some water from A and B in C.

3. Dip your right hand in A and left hand in B, and keep them immersed for 2 to 3 minutes.

4. Now dip both the 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 of some device for measuring temperature. Thermometer is a device for measuring temperature. You have read about thermometer in the previous class. In this lesson you are going to learn about the construction of a thermometer.

Teacher's Note

When you dip your hand in cold water after keeping it in hot water, the cold water feels even colder! This is because your hand feels the difference in temperature, not the actual temperature.

Exam Trick

Remember: Touching is not reliable for temperature. We need a thermometer to measure temperature accurately, just like we use a ruler to measure length!

Points to Remember

Our touch cannot measure temperature accurately.
Hot and cold are relative feelings.
We need a thermometer to measure temperature.
Touching very hot or cold things can hurt us.
Thermometer gives the exact temperature of an object.

Can You Recall?

What are potential and kinetic energies?

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.

Figure 14.3 'a' and 'b' show the velocities of atoms in a gas at high and low temperature, 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 temperature is larger than the velocity of atoms in the gas at lower temperature.

The velocities of atoms in a solid object are shown by arrows in figure 14.3(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. Higher the temperature of the solid, 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 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.

Try This

1. Take two steel vessels A and B of the same size.

2. Fill some water in A and double that amount in B. Make sure that the water in both vessels are at the same temperature.

3. 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.

Teacher's Note

Heat and temperature are different! A small cup of boiling water has the same temperature as a big pot of boiling water, but the pot has more heat in it. Heat is the total energy, temperature is just how fast atoms move.

Exam Trick

Remember: Temperature = average speed of atoms. Heat = total energy of all atoms. A large glass of hot water has MORE heat than a small cup of hot water even though both have the same temperature.

Points to Remember

Heat is the total kinetic energy of all atoms in an object.
Temperature is the average kinetic energy of atoms.
Two objects can have the same temperature but different heat content.
The object with more mass has more heat at the same temperature.
Atoms in solids vibrate in place while atoms in gases move freely.

Temperature Scales

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.

\[\frac{F-32}{9} = \frac{C}{5} \text{ --------(1)}\]

\[K = C + 273.15 \text{ -------(2)}\]

Some specific temperatures are given in the three scales in the following table. Verify that they satisfy the above relations and fill appropriate numbers in the blanks.

Solved Examples

1. How much will the temperature of 68°F be in Celsius and Kelvin?

Given: Temperature in Fahrenheit = F = 68, Temperature in Celsius = C = ?, Temperature in Kelvin = K = ?

According to formula (1), \[\frac{F-32}{9} = \frac{C}{5}\]

\[\frac{68-32}{9} = \frac{C}{5}\]

\[C = 5 \times \frac{36}{9} = 20 \text{ °C}\]

According to formula (2)

\[K = C + 273.15 = 20 + 273.15 = 293.15\]

Thus, the temperature in Celsius = 20°C and in Kelvin = 293.15 K

Teacher's Note

In India, we use Celsius. In America, they use Fahrenheit. Scientists everywhere use Kelvin. When you see temperature on TV, it is always in Celsius in India.

Exam Trick

Remember the formula: \(C = \frac{5(F-32)}{9}\). Always subtract 32 from Fahrenheit first, then multiply by 5, then divide by 9!

Points to Remember

Celsius is used in daily life in India.
Fahrenheit is used in America.
Kelvin is used in science.
Water boils at 100°C, 212°F, and 373.15 K.
Water freezes at 0°C, 32°F, and 273 K.

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