MAK LABS — Std 8 General Science Chapter 14 Measurement and Effects of Heat

1A. Whom should I pair with?

Group A	Group B
a. Temperature of a healthy human body	98.6 °F
b. Boiling point of water	212 °F
c. Room temperature	296 K
d. Freezing point of water	0 °C

2B. Who is telling the truth?

(a) The temperature of a substance is measured in joules. False

Temperature is measured in °C, °F or K.

(b) Heat flows from an object at higher temperature to an object at lower temperature. True

Heat always flows from a hot object to a cold object.

(c) Joule is the unit of heat. True

Joule is the SI unit of heat.

(d) Objects contract on heating. False

Objects expand on heating.

(e) Atoms of a solid are free. False

Atoms of a solid are held by strong forces. They only oscillate at their places.

(f) The average kinetic energy of atoms in a hot object is less than the average kinetic energy of atoms in a cold object. False

The average kinetic energy of atoms in a hot object is more.

3C. You will find if you search.

(a) A thermometer is used to measure temperature.

(b) The apparatus used to measure heat is called a calorimeter.

(c) Temperature is the measure of the average kinetic energy of the atoms in a substance.

(d) The heat contained in a substance is the measure of the total kinetic energy of atoms in the substance.

4Nishigandha kept a vessel containing all the ingredients for making tea in a solar cooker. Shivani kept a similar vessel on a stove. Whose tea will be ready first and why?

Shivani’s tea will be ready first.

A stove gives a large amount of heat in a short time. A solar cooker gets less heat from sunlight. So it heats slowly.

5Write brief answers.

(a) Describe a clinical thermometer. How does it differ from the thermometer used in laboratory?

A clinical thermometer is used to measure the temperature of the human body. It has a narrow glass tube with a bulb at one end. The bulb has a liquid (alcohol). When the bulb touches the body, the liquid expands and rises in the tube.

Clinical thermometer	Laboratory thermometer
It measures body temperature.	It measures the temperature of substances.
Its range is 35 °C to 42 °C.	Its range is about −10 °C to 110 °C.
It is short.	It is long.

(b) What is the difference between heat and temperature? What are their units?

Heat	Temperature
Heat is a form of energy.	Temperature tells how hot or cold an object is.
It is the total kinetic energy of atoms.	It is related to the average kinetic energy of atoms.
It is measured by a calorimeter.	It is measured by a thermometer.
Units: Joule, calorie	Units: °C, °F, K

(c) Explain the construction of a calorimeter. Draw the necessary figure.

A calorimeter has two vessels, an inner and an outer one.
The inner vessel is made of copper.
There is an air cavity between the two vessels. So heat does not pass in or out.
The vessels are separated by a heat resistant ring.
A heat resistant lid covers the calorimeter.
A thermometer and a stirrer are fitted through the lid.

Construction of a calorimeter.

(d) Explain why rails have gaps at specific distances.

Rails are made of iron. Iron expands on heating. In summer, the length of rails increases. If there are no gaps, the rails will bend. Hence, gaps are kept between rails.

(e) Explain with the help of formulae the expansion coefficients of liquid and gas.

Liquid: A liquid has a definite volume. So its volumetric expansion is measured.

V₂ = V₁ (1 + β ΔT)

Here, β is the volumetric expansion coefficient of the liquid.

Gas: The volume of a gas changes with pressure. So its expansion is measured at constant pressure.

V₂ = V₁ (1 + β ΔT)

Here, β is the constant pressure expansion coefficient of the gas.

6Solve the following examples.

(a) What must be the temperature in Fahrenheit so that it will be twice its value in Celsius?

Let Celsius temperature = C. So, F = 2C
(F − 32) / 9 = C / 5
(2C − 32) / 9 = C / 5
10C − 160 = 9C
C = 160 °C, F = 2 × 160 = 320 °F

The temperature is 320 °F.

(b) A bridge is made from 20 m long iron rods. At temperature 18 °C, the distance between two rods is 0.4 cm. Up to what temperature will the bridge be in good shape?

Given: l₁ = 20 m, Δl = 0.4 cm = 0.004 m, λ = 11.5 × 10⁻⁶ /°C
Δl = λ × l₁ × ΔT
0.004 = 11.5 × 10⁻⁶ × 20 × ΔT
ΔT = 17.4 °C
Final temperature = 18 + 17.4 = 35.4 °C

The bridge will be in good shape up to 35.4 °C.

(c) At 15 °C the height of the Eiffel tower is 324 m. If it is made of iron, what will be the increase in length in cm at 30 °C?

Given: l₁ = 324 m, ΔT = 30 − 15 = 15 °C, λ = 11.5 × 10⁻⁶ /°C
Δl = λ × l₁ × ΔT
Δl = 11.5 × 10⁻⁶ × 324 × 15 = 0.0559 m

The increase in length is 5.59 cm.

(d) Two substances A and B have specific heats c and 2c respectively. If A and B are given Q and 4Q amounts of heat respectively, the change in their temperatures is the same. If the mass of A is m, what is the mass of B?

For A: Q = m × c × ΔT
For B: 4Q = m_B × 2c × ΔT
Dividing: 4 = 2 m_B / m
m_B = 2m

The mass of B is 2m.

(e) When a substance having mass 3 kg receives 600 cal of heat, its temperature increases by 10 °C. What is the specific heat of the substance?

Given: m = 3 kg = 3000 gm, Q = 600 cal, ΔT = 10 °C
c = Q / (m × ΔT)
c = 600 / (3000 × 10) = 0.02 cal/(gm °C)

The specific heat is 0.02 cal/(gm °C).

The textbook answer 0.0033 cal/(gm °C) is a misprint. The correct answer is 0.02 cal/(gm °C).

14. Measurement and Effects of Heat

Exercises