Distinguish between heat and temperature. | Physics

THERMAL PROPERTIES OF MATTER

Very Short Question Answers

Q1. Distinguish between heat and temperature.
Ans.

Heat	Temperature
Heat is a form of energy Heat can be taken as the cause It is measured in calories or joules This is measured by using calorimeter	Temperature measures the hotness or coldness of a body. Temperature as its effect. Temperature is measured in centigrade (°c) (or) Fahrenheit (°F) This is measured by using thermometer.

Q2. What are the lower and upper fixing points in Celsius and Fahrenheit scales?
Ans.

Celsius scale: lower fixing point: 0 °C
Upper fixing point: 100°C

Fahrenheit scale: lower fixing point 32 °F
upper fixing point: 212°F

Q3. Do the values of coefficients of expansion differ, when the temperatures are measured on Centigrade scale or on Fahrenheit scale?
Ans.

Yes, the coefficient of expansion α, β and γ are not same in Celsius and in Fahrenheit scale.

Because, 1°C = 1.8°F

Q4. Can a substance contract on heating? Give an example.
Ans.

Yes, cast iron, rubber, and pure water (between 0°C to 4°C )

Q5. Why gaps are left between rails on a railway track?
Ans.

The gaps are left in between two successive rails to allow the free expansion of the rails during the summer season. (or) To allow the linear expansion of rails during summer.

Q6. Why do liquids have no linear and areal expansions?
Ans.

Liquid have no shape and size of their own. They take shape of the vessel. Hence, they does not have coefficients of linear and areal expansion.

Q7. What is latent heat of fusion?
Ans.

The amount of heat energy required to convert a unit mass of the solid substance into
liquid substance at 0°C is known as latent heat of fusion.

Q8. What is latent heat of vapourisation?
Ans.

The amount of heat energy required to convert a unit mass of liquid substance in to vapour at 100°C is known as latent heat of vaporization.

(or) Heat required to convert liquid into vapour.

Q9. Why utensils are coated black? Why the bottom of the utensils are made of copper?
Ans.

Black bodies are good absorbers. So, a black coated utensils will absorb more heat. Lower portion of the utensils is in contact with fire. Copper is a good conductor of that, so copper is used at the bottom of cooking utensils.

Q10. State Weins displacement law?
Ans.

The wave length ( λ_m) of maximum intensity of emission of black body radiation is inversely proportional to absolute temperature (T) of the black body

i.e. λ_mT = b(constant)

Q11. Ventilators are provided in rooms just below the roof. Why?
Ans.

Density of hot air is less. So in a room hot air goes to top layers i,e nearer to the roof. Hence to escape the hot air from the room ventilators are provided just below the roof.

Q12. Does a body radiate heat at 0 K? Does it radiate heat at O°C?
Ans.

No, the body does not radiate heat at 0°k.
Yes, the body radiates heat at 0°C

Q13. State the different modes of transmission of heat. Which of these modes require medium?
Ans.

Different modes of transmission of heat are i) conduction ii) convection iii) radiation.
Conduction and convection require medium

Q14. Define coefficient of thermal conductivity and temperature gradient.
Ans.

Coefficient of thermal conductivity: It is defined as the quantity of heat conducted per second per unit area of cross section per unit temperature gradient.

$\mathbf{k=\frac{Qd}{A(\theta_2 -\theta_1)t}}$

Temperature gradient: The change in temperature per unit distance moved in the directionof heat flow is called temperature gradient.

$\mathbf{Temperature gradient =\frac{Q_2-Q_1}{d}}$

Q15. Define emissivity.
Ans.

Emissivity: It is defined as the ratio of the emissive power of the body to that of black body at the same temperature.

Q16. What is greenhouse effect? Explain global warming.
Ans.

Greenhouse effect: Temperature of the earth increases due to the radiation emitted by the earth is trapped by atmospheric gases like Co₂, CH₄, N₂0, CFC etc. is called greenhouse effect.

Global warming: Due to human activities concentration of greenhouse gases enhanced and the Earth become warmer. This effect is called as Global warming. Because of Global warming ice caps are melting faster, sea level is rising and weather pattern is changing

Q17. Define absorptive power of a body. What is the absorptive power of a perfect black body.
Ans.

Absorptive power: At a given temperature and wavelength range, the ratio between the
radiant energy absorbed by the body and the energy incident on the body, is known as
absorptive power.
$\mathbf{ a_\lambda =\frac{amount \:of\: radiant\:energy\:abosrbed }{amount\: of \:radiant\:energy\:incident}}$
∴The absorptive power of a perfect black body = 1

Q18. State Newton's law of cooling.
Ans.

Newton’s law of cooling: The rate of loss of heat of the body is directly proportional to the difference of temperature of the body (T₂) and the surroundings(T₁).

$\mathbf{- \frac{\mathrm{d} Q}{\mathrm{d} t} =T_2-T_1}$

Q19. State the conditions under which Newton's law of cooling is applicable.
Ans.

The law holds good only for small difference of temperature.
Loss of heat is negligible by conduction and only when it is due to convection.
Temperature of the body is uniformly distributed over it.

Q20. The roof of buildings are often painted white during summer. Why?
Ans.

The roof of buildings are painted white during summer because white is a good reflector and poor absorber of heat. Hence in summer we feel cool in house.

Short Question Answers

Q1. Explain Celsius and Fahrenheit scales of temperature. Obtain the relation between Celsius and Fahrenheit scales of temperature.

Ans. Celsius scale of temperature: In this scale of temperature the freezing point of water is taken as lower fixed point $0°C$ and boiling point of water is taken as upper fixed point $100°C$ at atmospheric pressure. The interval between these two fixed points is divided into $100$ equal parts and each part is called $1℃$.
Fahrenheit scale of temperature : In this scale of temperature the freezing point of water is taken as lower fixed point $32°F$ and boiling point of water is taken as upper fixed point $212°F$ at atmospheric pressure. The interval between these two fixed points is divided into $180$ equal parts and each part is called $1°F$.

Relation between Celsius and Fahrenheit Scales. $$\begin{array}{}{\frac{ C-0}{100-0}=\frac{ F-32}{212-32}}\\ {\frac{ C}{100}=\frac{ F-32}{180}}\\ { C=\frac{5}{9}( F-32)} \end{array}$$

Q2. Two identical rectangular strips, one of copper and the other of steel, are riveted together to form a compound bar. What will happen on heating?

Ans. It bends copper on convex side. Because, the coefficient of linear expansion of copper is greater than that of iron. Therefore, when bimetallic strip made of copper and iron is heated, copper strip expands more than the iron strip.

Q3. Pendulum clocks generally go fast in winter and slow in summer. Why?

Ans. The time period of a pendulum clock $T= 2\pi \sqrt{{l}\over {g}}$
i) In winter, due to decrease in temperature, the length of the pendulum decreases. Thus time period decreases and therefore the pendulum clock gains time or go fast.
ii) In summer, due to increase in temperature the length of the pendulum increases. Thus the time period increases and therefore the pendulum clock looses time or goes slow.

Q4. In what way is the anomalous behaviour of water advantageous to aquatic animals?

Ans. The maximum density of water at $4°C$ . Hence , when temperature increase from $0°C$ to $4°C$ water contract instead of expansion. Thus, as atmospheric temperature decreases, the upper layers of the lakes cool, contract and sink to the bottom. This goes on until the whole of the water reaches the temperature of $4°C$. When the top layers cool further temperature falls below$ 4°C$ it expands and becomes lighter. It does not sink down wards and remains at the top with further cooling the top layer gradually from ice at the top. Ice and water are bad conductors of heat.This results in water remaining at the bottom at $4°C$. So that aquatic animals survive in those layers of water.

Rainbow

Q5. Explain conduction, convection and radiation with examples.

Ans. Conduction: Conduction is the process in which heat is transmitted from one part of the body to the other without the actual movement of the particles.

Ex: Heat transfer takes place from the hot end of the rod to the other end by conduction

Convention: It is the process in which heat is transferred from one point to another by the actual movement of the heated material particles.

Ex: Land and sea breezes and trade winds are due to convection.

Radiation: It is the process in which heat is transmitted from one place to another without any material medium.

Ex: Heat reaches the earth from the sun by the process of radiation. Heat radiations can pass through vacuum.

Essay Question Answers

Q1. Explain thermal conductivity and coefficient of thermal conductivity. A copper bar of thermal conductivity 401 W/(mk) has one end at 104°C and the other end at 24°C. The length of the bar is 0.10 m and the cross-sectional area is 1.0x 10 m² What is the rate of heat conduction, along the bar?