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Science [2 Mark Questions]

PHYSICS CH7 Heat · Tamil Nadu Board STD 9 (Tamil - English Medium). 21 questions.

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[2 Mark Questions]

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21 questions · timed · auto-graded

Question 12 Marks

Numerical Problem.

What is the heat in joules required to raise the temperature of 25 grams of water from 0°C to 100°C? What is the heat in Calories? (Specific heat of water $=\frac{4.18 J }{ g ^{\circ} C }$

Answer
Given $m =25 g , \Delta T =(100-0)=100^{\circ} C$
Or in terms of Kelvin $(373.15-273.15)=100 K$,
$C =\frac{4.18 J }{ g ^{\circ} C }$
Heat energy required, Q = m × C × ∆T = 25 × 4.18 × 100 = 10450 J
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Question 22 Marks
Define specific latent heat capacity
Answer
  • Specific latent heat is the amount of heat energy absorbed or liberated by unit mass of a substance during change of state without causing any change in temperature.
  • Specific latent heat is given as L = Q/m, where ‘Q’ is the amount of heat energy absorbed or liberated and ‘m’ is mass of a substance during its change of phase at a constant temperature.
  • The SI unit of specific latent heat is J/kg.
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Question 32 Marks
Define thermal capacity.
Answer
  • Heat capacity or thermal capacity is defined as the amount of heat energy required to raise the temperature of a body by 1°C. It is denoted by ‘C’.
  • C = Q/t, where C’ is the heat capacity, ‘Q’ is the quantity of heat required and ‘f’ is rise in temperature.
  • SI unit of heat capacity is J/K. It is also expressed in cal/°C, kcal/°C or J/°C.
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Question 42 Marks
What is specific heat capacity?
Answer
  • The specific heat capacity of a substance is defined as the amount of heat required to raise the temperature of 1 kg of the substance by 1°C or 1 K.
  • Q = mC∆T, where Q is the quantity of heat required to raise the temperature and m is the mass of the body and AT is the change in temperature of the body.
  • The SI unit of specific heat capacity is Jkg– 1 K– 1. The most commonly used units of specific heat capacity are J/kg°C and J/g°C.
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Question 52 Marks
Differentiate convection and radiation.
Answer
S.No. Convection Radiation
1. Flow of heat through a fluid from places of higher temperature to places of lower temperature by movement of the fluid itself. Flow of heat from one place to another by means of electromagnetic waves.
2. Convection needs matter to be present. Radiation can occur even in vacuum.
3.

Convection seen in daily life:

Hot air balloons, breeze, wind, chimney.

 Radiation in daily life:

White or light coloured cloths, highly polished surface of airplane, helps to reflect most of the heat radiation from the sun.
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Question 62 Marks
A metal ball of heat capacity 50J/°C loses 2000 J of heat. By how much will its temperature fall?
Answer
Heat capacity of ball $=50 J ^{\circ} C ^{-1}$
Heat lost $Q=200 J$
Fall in temperature $t=$ ?
\[
\begin{aligned}
\text { Heat capacity } & =\frac{ Q }{t} \\
50 & =\frac{2000}{t} \\
t & =\frac{2000}{50} \\
t & =40^{\circ} C
\end{aligned}
\]
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Question 72 Marks
What will be the final temperature if 1,68,000 J of heat is absorbed by 2 kg of water at 30°C?
Answer
Heat absorbed $Q =1,68,000 J$
\[
m =2 kg
\]
Initial temperature $t_1=30^{\circ} C$
Let final temperature $t _2= x ^{\circ} C$
Rise is temperature $\Delta t=\left(t_2-t_1\right)$
\[
=(x-30)^{\circ} C
\]
Specific heat capacity of water $C =4200 J kg ^{-10} C ^{-1}$
We know that $Q = m \times c \times \Delta t$
\[
\begin{array}{l}
1,68,000=2 \times 4200 \times(x-30) \\
x-30=\frac{1,68,000}{2 \times 4200} \\
x-30=20 \\
x=30+20 \\
=50^{\circ} C
\end{array}
\]
So, the final temperature of water $=50^{\circ} C$
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Question 82 Marks
2875 J of heat is required to melt 115 g of lead at its melting point. Calculate the specific latent heat capacity of fusion of lead.
Answer
Mass of lead $m =115 g$
Heat required $Q =2875 J$
Specific latent heat of lead $L=$ ?
We know that $Q = m L$
\[
\begin{array}{l}
L =\frac{Q}{m}=\frac{2875}{115} \\
=25 Jg ^{-1}
\end{array}
\]
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Question 92 Marks
Calculate the amount of heat required to convert $200 g$ of ice at $0^{\circ} C$ into the water at $0^{\circ} C$ Specific latent heat of fusion of ice $=336 Jg ^{-1}$
Answer
Mass of ice $m=200 g$
Specific latent heat of fusion of ice $L =336 Jg ^{-1}$
Heat required $Q = mL$
\[
\begin{array}{l}
=200 \times 336 \\
Q=67,200 J
\end{array}
\]
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Question 102 Marks
What is the amount of heat required to raise the temperature of $5 kg$ of iron from $30^{\circ} C$ to $130^{\circ} C$ ? Specific heat capacity of iron $=483 Jkg ^{-1} C ^{-1}$.
Answer
Mass of iron $m =5 kg$
Initial temperature $t _1=30^{\circ} C$
Final temperature $t_2=130^{\circ} C$
Rise is temperature $\Delta t =\left( t _2- t _1\right)=130-30=100^{\circ} C$
\[
\begin{array}{l}
Q=m \times c \times \Delta t \\
=5 \times 483 \times 100 \\
=2,41,500 J
\end{array}
\] P 49 WORDS HTML Editor Code length: 354 1 Mass of iron $m=5 kg$
Initial temperature $t _1=30^{\circ} C$
Final temperature $t_2=130^{\circ} C$
Rise is temperature $\Delta t=\left(t_2-t_1\right)=130-30=100^{\circ} C$
\[
\begin{array}{l}
Q=m \times c \times \Delta t \\
=5 \times 483 \times 100 \\
=2,41,500 J
\end{array}
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Question 112 Marks
In winters, when the sun suddenly goes behind the clouds we feel cold, can you say why?
Answer
The clouds cut off the radiant heat from the sun.
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Question 122 Marks
Can convection take place in solids? Why?
Answer
No. The molecules in a solid are only free to vibrate about their fixed positions. For convection to take place, the molecules need to move to carry the heat with them. Hence, convection cannot take place in solids.
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Question 132 Marks
Give some practical applications of radiation.
Answer
    • White or light colored clothes are good reflectors of heat. They keep us cool during summer.
    • The bottom surface of cooking utensils is blackened because the black surface absorbs more heat from the surrounding.
    • The surface of the airplane is highly polished because it helps to reflect most of the heat radiation from the sun.
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Question 142 Marks
Give some practical applications of conduction in daily life.
Answer
    • Metals are good conductors of heat. So, aluminium is used for making utensils to cook food quickly.
    • Mercury is used in thermometers because it is a good conductor of heat.
    • We wear woolen clothes in winter to keep ourselves warm. Air, which is a bad conductor, does not allow our body heat to escape.
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Question 152 Marks
Define absolute zero.
Answer
The temperature at which the pressure and volume of a gas theoretically reaches zero is called absolute zero.
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Question 162 Marks
What do you mean by solidification or deposition?
Answer
Solidification is the process in which a gas directly condenses into its solid state without going into liquid state Ex : Carbon dioxide gas gets converted into dry ice.
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Question 172 Marks
Why are burns caused by steam more painful than those caused by boiling water at the same temperature?
Answer
    • When steam hits our skin, it condenses to water and then cools down to the temperature of skin.
    • Now, the energy released will be due to latent heat and fall in temperature.
    • Whereas when boiling water hits our skin, there is no phase transition but only fall in temperature and the heat transferred to skin will be only due to cooling.
    • Also, the loss of energy that is released from steam hitting our skin occurs quickly and in a small localized area, therefore causing damage to our cells.
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    Question 182 Marks
    Define latent heat of fusion?
    Answer
    Heat energy is absorbed by a solid during melting and an equal amount of heat energy ‘ is liberated by the liquid during freezing, without any temperature change. It is called latent heat of fusion.
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    Question 192 Marks
    What do you mean by thermal equilibrium?
    Answer

    • When two bodies at different temperatures are kept in contact with each other, then heat energy flows from the hot body to the colder one.

      It means that the hot body will lose heat and the cold body will gain heat till they reach a common temperature. This state is called thermal equilibrium.

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    Question 202 Marks
    Water is used as a coolant in car radiators. Why?
    Answer
    Water is used as a coolant in car radiators because it has high specific heat capacity. As a result, it can absorb large amounts of heat from the car engine without any substantial increase in its temperature.
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    Question 212 Marks
    Heat gained by a body depends upon which factors?
    Answer
    • Mass of the body
    • Change in temperature of the body
    • Nature of the material of the body.
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