Chapter 14: THERMAL ENERGY
Chapter 14: Thermal Energy Class 10 Science and Technology (212) NIOS Textbook Solutions
Chapter 14. THERMAL ENERGY
INTEXT QUESTIONS 14.1
State whether the following statements are true or false:
(i) Heat can be measured in kelvin.
(ii) –30° F is a lower temperature than –30° C.
(iii) The numerical value of temperature of any hot body measured on kelvin’s scale is always higher than the value on Fahrenheit scale.
(iv) Thermal energy can be measured either in calories or in joules.
(v) Pure alcohol can also be used as thermometric liquid.
(vi) A body is felt cold when heat flows from our body to that body.
Answer: (i) Heat can be measured in kelvin. → False
[ Kelvin is a unit of temperature, not heat. Heat is measured in joules (J) or calories (cal). ]
(ii) –30° F is a lower temperature than –30° C. → False
[ Converting –30°C to Fahrenheit: 
So, –30°F is actually colder than –30°C. ]
(iii) The numerical value of temperature of any hot body measured on kelvin’s scale is always higher than the value on Fahrenheit scale. → True
[For any temperature above absolute zero, the Kelvin value is always numerically higher than the Fahrenheit value because Kelvin starts at -273.15°C, while Fahrenheit starts at -459.67°F (e.g., 0 K = -459.67°F).]
(iv) Thermal energy can be measured either in calories or in joules. → True
[ Thermal energy (heat) is measured in calories or joules.]
(v) Pure alcohol can also be used as thermometric liquid. → True
[ Alcohol is used in thermometers because it expands uniformly and works at low temperatures.]
(vi) A body is felt cold when heat flows from our body to that body. → True
[ We feel cold when heat leaves our body and flows to another object.]
INTEXT QUESTIONS 14.2
Fill in the blanks with the correct choice
1. A bimetallic strip is used as a thermostat in the electrical device named …………….. (electric bulb, T.V., refrigerator).
2. Melting point of 1 kg wax will be …………….. the melting point of 2 kg wax. (double, half, same as).
3. Latent heat of evaporation is measured in …………….. (J, J/K, J/kg).
4. 1 kg steam at 100 °C has 2260 J …………….. heat than water at 100 °C (more, less).
5. The cubical expansivity of a substance is …………….. its linear expansivity (equal to, two times, three times)
6. The expansivity of …………….. is maximum. (solids, liquids, gases).
Answer: 1. A bimetallic strip is used as a thermostat in the electrical device named refrigerator.
2. Melting point of 1 kg wax will be same as the melting point of 2 kg wax.
3. Latent heat of evaporation is measured in J/kg.
4. 1 kg steam at 100 °C has more heat than water at 100 °C. (Steam contains latent heat of vaporisation.)
5. The cubical expansivity of a substance is three times its linear expansivity.
6. The expansivity of gases is maximum.
INTEXT QUESTIONS 14.3
Choose the correct alternative
1. Two iron balls of radii r and 2r are heated to the same temperature. They are dropped in two different ice boxes A and B respectively. The mass of ice melted
(a) will be same in the two boxes.
(b) in A will be twice than in B.
(c) in B will be twice than that in A.
(d) in B will be four times than that in A.
Answer: (d) in B will be four times than that in A.
[ Heat given by each iron ball depends on its mass, and mass is proportional to volume (∝ r³). For radii r and 2r, masses are in ratio 1 : 8. Hence ball B melts eight times more ice than A.
So none of the given options is correct; the actual answer should be 8 times.]
2. An iron ball A of mass 2 kg at temerature 20°C is kept in contact with another iron ball B of mass 1.0 kg at 20°C. The heat energy will flow
(a) from A to B only
(b) from B to A only
(c) in neither direction
(d) Initially from A to B and then from B to A.
Answer: (c) in neither direction
[ Heat flows only when there is a difference in temperature. Since both iron balls A and B are at 20°C, no heat transfer takes place between them.]
3. When solid ice at 0°C is heated, its temperature
(a) rises (b) falls
(c) does not change until whole of it melts.
(d) first rises then falls back to 0°C.
Answer: (c) does not change until whole of it melts.
[ When ice at 0°C is heated, the heat supplied is used as latent heat of fusion to change ice into water. Therefore, the temperature remains constant at 0°C until all the ice melts.]
4. When steam at 100°C is heated its temperature
(a) does not change. (b) increases
(c) decreases (c) none of these
Answer: (b) increases
[ Steam at 100°C has already changed into gas. When more heat is supplied, there is no change of state, so its temperature starts increasing.]
5. Specific heat of aluminium is almost two times the specific heat of copper. Equal amount of heat is given to two pieces of equal masses of copper and iron respectively. Rise in temperature of
(a) Copper will be equal to that of aluminium.
(b) Copper will be twice the rise in temperature of aluminium.
(c) Copper will be half the rise in temperature of aluminium.
(d) Copper will be four times the rise in temperature of aluminium.
Answer: (b) Copper will be twice the rise in temperature of aluminium.
[ Using Q=mcΔθ, for same heat Q and same mass m, rise in temperature (Δθ) is inversely proportional to specific heat (c).
Since aluminium’s specific heat is twice that of copper, copper’s temperature rise will be twice that of aluminium.]
6. Equal heat is given to three pieces of copper A, B, and C having masses in the ratio 1:2:3. The rise in temperature will be in the order
(a) A > B > C (b) B > C > A
(c) C > B > A (d) A > C > B
Answer: (a) A > B > C
[ Using 
, for the same heat Q and same material (same c), rise in temperature (Δθ) is inversely proportional to mass (m).
Mass ratio = 1 : 2 : 3, so temperature rise = A > B > C. ]
TERMINAL EXERCISE
1. Distinguish clearly between heat and temperature.
Answer: The difference between Heat and Temperature are :
|
Heat |
Temperature |
|
Heat is a form of energy transferred from a hotter body to a colder body. |
Temperature is the measure of degree of hotness or coldness of a body. |
|
It depends on the quantity of substance present. |
It does not depend on the quantity of substance. |
|
It is measured using a calorimeter. |
It is measured using a thermometer. |
|
SI unit is joule (J). |
SI unit is kelvin (K). |
2. During change of state: (i) Is there a rise in temperature of the material on heating it? and (ii)What happens to the heat we supply?
Answer: (i) No, there is no rise in temperature of the material during change of state even when heat is supplied.
(ii) The heat supplied is used to change the state of the substance . This heat is called latent heat. It is used to overcome intermolecular forces and not to increase temperature.
3. Name the factors on which the thermal expansion of a wire depends.
Answer: The thermal expansion of a wire depends on,
(i) Original length, (ii) Rise in temperature, and (iii) Nature of material .
4. Give any two uses of a bimetallic strip.
Answer: Two uses of a bimetallic strip are:
(i) In thermostats: Used to control temperature in devices like refrigerators and electric irons.
(ii) In fire alarms: Used to detect increase in temperature and activate the alarm.
5. If you have a uncalibrated mercury thermometer how will you calibrate it into a
(a) celsius thermometer (b) fahrenheit thermometer.
Answer: (a) Celsius thermometer:
(i) Place the thermometer in melting ice and mark the mercury level as 0°C.
(ii) Place it in steam above boiling water at normal atmospheric pressure and mark it as 100°C.
(iii) Divide the distance between these marks into 100 equal parts. Each division represents 1°C.
(b) Fahrenheit thermometer:
(i) Mark the mercury level in melting ice as 32°F.
(ii) Mark the level in steam above boiling water as 212°F.
(iii) Divide the distance between the two marks into 180 equal parts. Each division represents 1°F.
6. Explain the following:
(i) Why is mercury used as a thermometric liquid?
(ii) Why does a bimetallic strip bend on heating?
(iii) Why does steam at 100°C gives more severe burns than water at 100°C?
(iv) Why do we use ice for cooling our drinks and not water at 0°C.?
Answer: (i) Mercury is used because it expands uniformly on heating, is clearly visible, does not stick to glass and can measure a wide range of temperatures.
(ii) A bimetallic strip consists of two different metals having different rates of expansion. On heating, one expands more than the other, causing the strip to bend.
(iii) Steam contains latent heat of vaporisation. When it condenses on the skin, it releases this extra heat, causing more severe burns.
(iv) Ice absorbs latent heat of fusion while melting, so it removes more heat from the drink than water at 0°C and cools it more effectively.
7. Why is the heat given at the time of change of state called latent heat?
Answer: The heat supplied during change of state is called latent heat because it remains hidden and does not increase the temperature of the substance. It is used only to change the state of the substance.
8. A certain amount of water is heated at a constant rate. The time to bring it to boiling is t1 and the time required from beginning of boiling to boiling off the whole amount is t2. Which is greater t1or t2? Why?
Answer: t₂ is greater than t₁.
During t₁, heat only raises the temperature of water.
During t₂, heat supplies latent heat of vaporization , which is much larger than the heat required for the same temperature rise. Hence, boiling off takes longer.
9. At what temperature the numerical value of temperature on fahrenheit scale will be double the value on celsius scale.
Answer: Let Celsius = C, Fahrenheit =F=2C.
We have, 
A/Q, 
⇒2C=9C+1605 
⇒10C=9C+160 
⇒10C-9C=160 
⇒C=160 
Thus, the temperature is 160°C (or 320°F).
10. A 50 cm silver bar shortens by 1.0 mm when cooled. How much was it cooled?
(Given: Coefficient of linear expansion of silver =18×10-6
per degree celsius)
Answer: Given:
Length of silver bar, L0= 50 cm = 0.5 m
Decrease in length, ΔL = 1.0 mm = 0.001 m
Coefficient of linear expansion, α = 18 × 10⁻⁶ 
per degree Celsius
Using the formula,
ΔL=L0αΔT 
⇒∆T=∆LL0α
⇒∆T=0.0010.5×18×10-6=1×10-39×10-6 
⇒∆T=19×103=0.1111×1000
⇒∆T=111.1°C 
Therefore, the silver bar was cooled by 111°C.
11. How much heat energy is required to change 200 g of ice at –20°C to water at 70°C ?
(Given: Latent heat of fusion of ice = 335 kJkg-1
, and specific heat of ice = 2100 Jkg-1°C-1
, specific heat of water = 4.2 kJkg-1°C-1
)
Answer: Given, Mass of ice, m = 200 g = 0.2 kg , Initial temperature = –20°C , Final temperature = 70°C
Latent heat of fusion of ice, L = 335 kJ/kg = 335000 J/kg
Specific heat of ice, c₁ = 2100 J/kg°C
Specific heat of water, c₂ = 4.2 kJ/kg°C = 4200 J/kg°C
Heating ice : We have,
Q1=mc1ΔT
Q1=0.2×2100×20 =8400
J
Melting ice : We have,
Q2=mL 
Q2=0.2×335000 =67000
J
Heating water : We have,
Q3=mc1ΔT
Q3=0.2×4200×70=58800 
J
Total heat required
Q=Q1+Q2+Q3
=8400+67000+58800 =134200J=134.2 kJ 
Therefore, heat energy required = 134.2 kJ
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