| Publisher | : Oriental Book Company Private Limited |
| Material | : Madhyamik Physical Science Solution |
| Subject | : Physical Science |
| Class | : 9 (Madhyamik) |
| Chapter Name | : Work, Power And Energy |
Very Short Answer Questions
Question 1
What is heat?
Answer
Heat is a form of energy that is transferred from one body to another due to a difference in temperature.
Question 2
What is SI unit of heat?
Answer
The SI unit of heat is joule (J).
Question 3
What is temperature?
Answer
Temperature is the thermal condition of a body that determines the direction of heat flow between two bodies in contact.
Question 4
What is expressed by °C?
Answer
The unit °C (degree Celsius) is used to measure temperature.
Question 5
What is SI unit of temperature?
Answer
The SI unit of temperature is Kelvin (K).
Question 6
What is the value of J?
Answer
The value of the mechanical equivalent of heat J is 4.2 joule/calorie or 4.2 × 10⁷ erg/calorie.
Question 7
What is latent heat?
Answer
Latent heat is the quantity of heat absorbed or released by a substance during a change of state (solid to liquid or liquid to gas) without changing its temperature.
Question 8
What is the value of latent heat of fusion of ice in CGS system?
Answer
The latent heat of fusion of ice in the CGS system is 80 cal/g.
Question 9
Does saturated vapour obey Boyle’s law?
Answer
No, saturated vapour does not obey Boyle’s law.
Question 10
Does unsaturated vapour obey pressure law?
Answer
Yes, unsaturated vapour obeys the pressure law.
Question 11
At what temperature is the density of pure water maximum?
Answer
The density of pure water is maximum at 4°C.
Question 12
Can a certain quantity of unsaturated vapour be made saturated?
Answer
Yes, a certain quantity of unsaturated vapour can be made saturated by cooling it or increasing its pressure.
Question 13
State two favourable conditions for the formation of dew.
Answer
The two favourable conditions for the formation of dew are:-
- High humidity in the air
- Clear sky and calm air, allowing heat loss from surfaces during the night
Question 14
What is dewpoint?
Answer
The dewpoint is the temperature at which air becomes saturated with water vapour, leading to the formation of dew.
Question 15
What is specific heat?
Answer
Specific heat is the amount of heat required to raise the temperature of unit mass of a substance by 1°C without a change in state.
Question 16
What is the unit of specific heat in CGS system?
Answer
The unit of specific heat in the CGS system is calorie per gram per degree Celsius (cal/g°C).
Question 17
How much joule is 1 calorie?
Answer
1 calorie = 4.18 joules.
Short Answer Questions
Question 1
What do you mean by heat and temperature? Explain.
Answer
- Heat is a form of energy that flows from a hotter body to a colder body due to a temperature difference. It is measured in joules (J) in the SI system.
- Temperature is a measure of the thermal state of a body and determines the direction of heat flow. It indicates how hot or cold a body is and is measured in Kelvin (K) in the SI system.
Example: A cup of hot water has a higher temperature than a bucket of warm water, but the bucket may contain more heat energy due to its larger mass.
Question 2
What is meant by calorimetry? State principle of calorimetry.
Answer
Calorimetry is the measurement of heat energy exchanged in physical and chemical processes. It is used to determine specific heat, latent heat, and heat of reactions.
Principle of Calorimetry:
According to the principle of calorimetry, when two or more bodies at different temperatures are mixed, the heat lost by the hotter body is equal to the heat gained by the colder body, assuming no heat is lost to the surroundings.
Mathematically,
Heat lost = Heat gained
Question 3
On what factors does quantity of heat depend? Define specific heat.
Answer
The quantity of heat required to raise the temperature of a substance depends on:
- Mass (m) of the substance – More mass requires more heat.
- Specific heat capacity (S) – Different substances require different amounts of heat for the same temperature change.
- Change in temperature (ΔT) – More heat is required for a greater temperature change.
Definition of Specific Heat:
The specific heat capacity (S) of a substance is the amount of heat required to raise the temperature of 1 gram of the substance by 1°C.
Formula:
Q = m × S × ΔT
Question 4
State principle of calorimetry. Mention the assumptions taken.
Answer
Principle of Calorimetry: When two bodies at different temperatures are brought into thermal contact, heat lost by the hotter body = heat gained by the colder body until thermal equilibrium is reached.
Assumptions:
- There is no heat loss to the surroundings.
- The system is perfectly insulated.
- There is no chemical reaction between the substances.
- The specific heat of the substances remains constant during the process.
Question 5
State the observation of Joule from his experiment on heat and work. What is meant by mechanical equivalent of heat?
Answer
Joule conducted experiments where mechanical energy was converted into heat energy (e.g., stirring water with a paddle). He found that the heat produced was directly proportional to the mechanical work done.
Mathematically, W = JH, where
- W = Work done
- H = Heat produced
- J = Mechanical equivalent of heat, which relates mechanical work to heat.
Mechanical equivalent of heat: It is the amount of work required to produce one unit of heat.
Value:
- SI system: J = 4.2 J/cal
- CGS system: J = 4.2 × 10⁷ erg/cal
Question 6
Define mechanical equivalent of heat. Give its values in SI and CGS system.
Answer
The mechanical equivalent of heat (J) is defined as the amount of mechanical work needed to produce one unit of heat energy when work is completely converted into heat.
Values:
- SI system: J = 4.2 J/cal
- CGS system: J = 4.2 × 10⁷ erg/cal
Question 7
What is latent heat? Explain this with the help of a simple experiment.
Answer
Latent heat is the amount of heat required to change the state of a substance without changing its temperature.
It is of two types:
- Latent heat of fusion – Heat required to convert solid to liquid.
- Latent heat of vaporization – Heat required to convert liquid to gas.
Experiment:
- Take ice in a beaker and place a thermometer inside.
- Heat the ice gradually.
- Initially, the temperature rises until 0°C is reached.
- After that, the temperature remains constant while ice melts into water.
This shows that the supplied heat is used for the change of state (melting), not for increasing temperature.
Question 8
What do you mean by saturated and unsaturated vapours?
Answer
- Saturated vapour: A vapour in equilibrium with its liquid at a given temperature is called saturated vapour. It cannot hold more vapour without condensation occurring.
- Unsaturated vapour: A vapour that can hold more moisture at a given temperature without condensation is called unsaturated vapour.
Example:
- Air at 100% humidity contains saturated vapour.
- Air with low humidity contains unsaturated vapour.
Question 9
Describe in short about formation of dew and mist.
Answer
Dew Formation:
- At night, the earth’s surface cools due to radiation.
- The air near the surface cools below the dew point.
- Water vapour in the air condenses on cool surfaces forming dew.
Mist Formation:
- If the air contains dust or smoke particles, water vapour condenses on these particles when the temperature drops.
- This forms tiny suspended droplets in the air, creating mist or fog.
Question 10
What do you mean by anomalous expansion of water? What is its importance in marine life?
Answer
Most substances expand when heated and contract when cooled. However, water behaves differently between 4°C and 0°C. When cooled from 4°C to 0°C, water expands instead of contracting. This is called anomalous expansion of water.
Importance in Marine Life:
In cold regions, as the surface water cools, it becomes denser and sinks. This continues until the water temperature reaches 4°C. Below 4°C, water expands and becomes lighter, so it stays at the surface. This prevents the entire water body from freezing solid, allowing marine life to survive in deeper layers at 4°C.
Numerical Questions
Question 1
A hot solid of mass 60g at 100°C is placed in 150g of water at 20°C. The final temperature recorded is 25°C. Calculate the specific heat capacity of the solid. [Specific heat capacity of water = 4200 J kg⁻¹°C⁻¹]
[Ans. 0.7 Jg⁻¹°C⁻¹]
Answer
Let the specific heat of the solid be S.
Heat lost by solid = mass × specific heat × change in temperature
= 60g × S × (100 – 25)
= 60 × S × 75
Heat gained by water = mass × specific heat × change in temperature
= 150g × 4200 × (25 – 20)
= 150 × 4.2 × 5
Since heat lost = heat gained,
60 × S × 75 = 150 × 4.2 × 5
or, S = (150 × 4.2 × 5) / (60 × 75)
S = 0.7 J/g°C
Question 2
200 g of hot water at 80°C is added to 300 g of cold water at 20°C. Calculate the final temperature of the mixture of water. Consider the heat taken by the container to be negligible. [Specific heat capacity of water is 4200 J kg⁻¹°C⁻¹]
[Ans. 38°C]
Answer
Let ‘t’ be the final temperature.
Heat lost = mass × specific heat × change in temperature
= 200g × 4200 J/kg°C × (80 – t)
= 200 × 4.2 × (80 – t)
Heat gained = 300g × 4200 J/kg°C × (t – 20)
= 300 × 4.2 × (t – 20)
Since, heat lost = heat gained,
or, 200 × 4.2 × (80 – t) = 300 × 4.2 × (t – 20)
or, 200 (80 – t) = 300 (t – 20)
or, 16000 – 200t = 300t – 6000
or, 16000 + 6000 = 300t + 200t
or, 22000 = 500t
t = 44°C
Question 3
40g of water at 60°C is poured into a vessel containing 50 g of water at 20°C. The final temperature recorded is 30°C. Calculate the thermal capacity of the vessel. [Take specific heat capacity of water as 4.2 J g⁻¹°C⁻¹]
[Ans. 294 J°C⁻¹]
Answer
Heat lost by hot water = mass × specific heat × temperature change
= 40 × 4.2 × (60 – 30)
= 40 × 4.2 × 30
= 5040 J
Heat gained by cold water = mass × specific heat × temperature change
= 50 × 4.2 × (30 – 20)
= 50 × 4.2 × 10
= 2100 J
Heat absorbed by vessel = Heat lost – Heat gained
= 5040 – 2100
= 2940 J
Since thermal capacity (C) = \text{heat absorbed (Q)} / \text{temperature change (Δt)}
Thermal capacity of the vessel = 294 J°C⁻¹
Question 4
Some hot water was added to three times its mass of cold water at 10°C, and the resulting temperature was found to be 20°C. What was the temperature of the hot water?
[Ans. 50°C]
Answer
Let the mass of hot water be m and the temperature be t.
Mass of cold water = 3m
Heat lost by hot water = m × 4.2 × (t – 20)
Heat gained by cold water = 3m × 4.2 × (20 – 10)
Since heat lost = heat gained,
m × 4.2 × (t – 20) = 3m × 4.2 × 10
(t – 20) = 3 × 10
t – 20 = 30
t = 50°C
Question 5
Calculate the amount of heat released when 5g of water at 20°C is changed into ice at 0°C. [Specific heat capacity of water = 4.2 J/g°C ; specific latent heat of fusion of ice = 336 J/g]
[Ans. 2100 J]
Answer
Heat lost by water in cooling to 0°C = mass × specific heat × temperature change
= 5 × 4.2 × (20 – 0)
= 5 × 4.2 × 20
= 420 J
Heat released during freezing = mass × latent heat of fusion
= 5 × 336
= 1680 J
Total heat released = 420 + 1680
= 2100 J
Question 6
Calculate the heat energy that will be released when 5.0 kg of steam at 100°C condenses to form water at 100°C. Express your answer in SI unit. [Specific latent heat of vaporisation of steam = 2268 kJ/kg.]
[Ans. 11340000 J]
Answer
Heat released = mass × latent heat of vaporisation
= 5 × 2268 × 1000
= 11340000 J
Question 7
Water falls from a height of 50m. Calculate the rise in the temperature of water when it strikes the bottom. [Take g = 10 m/s² ; specific heat capacity of water = 4200 J/kg°C]
[Ans. 0.12°C]
Answer
Potential energy lost = heat gained
m × g × h = m × specific heat × ΔT
g × h = specific heat × ΔT
Solving for ΔT,
ΔT = (10 × 50) / 4200
= 0.12°C
Question 8
250 g of water at 30°C is present in a copper vessel of mass 50g. Calculate the mass of ice required to bring down the temperature of the vessel and its contents to 5°C. [Specific latent heat of fusion of ice = 336 × 10³ J/kg, specific heat capacity of copper vessel = 400 J/kg°C, specific heat capacity of water = 4200 J/kg°C]
[Ans. 74.9 g]
Answer
Heat lost by water = m₁ × s₁ × (t₁ – t₂)
= 0.250 × 4200 × (30 – 5)
= 0.250 × 4200 × 25
= 26250 J
Heat lost by copper = m₂ × s₂ × (t₁ – t₂)
= 0.050 × 400 × (30 – 5)
= 0.050 × 400 × 25
= 500 J
Total heat lost = Heat lost by water + Heat lost by copper vessel
= 26250 + 500
= 26750 J
Question 9
A piece of ice of mass 40g is dropped into 200g of water at 50°C. Calculate the final temperature of water after all the ice has melted.
[Specific heat capacity of water = 4200 J/kg°C ; specific latent heat of fusion of ice = 336 × 10³ J/kg]
[Ans. 28.3°C]
Answer
Heat required for melting = m₁ × L
= 0.040 × 336000
= 13440 J
Heat required = m₁ × S × (t – 0)
= 0.040 × 4200 × T
= 168t
Heat lost by hot water = m₂ × S × (50 – t)
= 0.200 × 4200 × (50 – t)
= 840 × (50 – t)
= 42000 – 840t
Heat lost by hot water = Heat gained by ice
42000 – 840T = 13440 + 168T
or, 42000 – 13440 = 840T + 168T
or, 28560 = 1008T
or, T = 28560 / 1008
T = 28.3°C
Question 10
A piece of iron of mass 2 kg has a thermal capacity of 966 J°C⁻¹.
(i) How much heat is needed to warm it by 15°C?
(ii) What is its specific heat capacity in SI units?
[Ans. (i) 14490 J ; (ii) 483 J kg⁻¹°C⁻¹]
Answer
Mass of iron (m) = 2 kg
Thermal capacity of iron = 966 J/°C
(i) Heat required (Q) = Thermal capacity × Temperature change
= 966 × 15
= 14490 J
Thus, the heat needed to warm the iron by 15°C is 14490 J.
(ii) The specific heat capacity (S) is given by:
S = Thermal capacity / Mass
S = 966 / 2
S = 483 J/kg°C
Thus, the specific heat capacity of iron is 483 J/kg°C.
Objective Questions
Fill in the blanks:
Question (i)
In CGS system, heat is measured by unit called ___.
Answer
Calorie
Explanation:
In the CGS system (Centimeter-Gram-Second system), heat is measured in calories. 1 calorie is the amount of heat required to raise the temperature of 1 gram of water by 1°C.
Question (ii)
Temperature of a body depends on the ___ contained in it.
Answer
Heat
Explanation:
Temperature is a measure of the thermal energy of a body. The more heat energy a body has, the higher its temperature.
Question (iii)
The mechanical equivalent of heat is the ___ done to produce unit quantity of heat.
Answer
Work
Explanation:
The mechanical equivalent of heat (Joule’s Law) states that a certain amount of mechanical work can be converted into heat energy. The relation is given by:
W = JH (Work = Joule’s constant × Heat)
Question (iv)
In change of state, heat is ___ or ___.
Answer
Absorbed or Liberated
Explanation:
When a substance changes state (e.g., solid to liquid, liquid to gas), heat is either absorbed (for melting, vaporization) or liberated (for condensation, freezing).
Question (v)
Latent heat of fusion of ice is ___.
Answer
3.36 × 10⁵ J/kg
Explanation:
Latent heat of fusion of ice is the heat energy required to convert 1 kg of ice at 0°C into water at 0°C without a temperature change.
Question (vi)
Unsaturated vapour ___ Boyle’s law.
Answer
Obeys
Explanation:
Unsaturated vapours (not at their condensation point) follow Boyle’s Law, which states that pressure and volume are inversely proportional when temperature remains constant.
Question (vii)
The temperature at which dews are formed are called ___ point.
Answer
Dew
Explanation:
The dew point is the temperature at which air becomes saturated with moisture and water droplets form due to condensation.
Question (viii)
The density of water at 4°C is ___.
Answer
Maximum
Explanation:
Water has its maximum density at 4°C (1 g/cm³). Below 4°C, water expands instead of contracting, which is called anomalous expansion of water.
Question (ix)
SI unit of heat is ___.
Answer
Joule
Explanation:
In the SI system (International System of Units), the unit of heat is the joule (J). 1 calorie = 4.18 joules.
State whether true or false
Question (i)
Heat is a form of energy.
Answer
True (T)
Explanation:
Heat is a form of energy that flows from hotter to colder bodies due to a temperature difference.
Question (ii)
Calorie is the unit of temperature.
Answer
False (F)
Explanation:
Calorie is a unit of heat energy, not temperature. Temperature is measured in Celsius (°C), Kelvin (K), or Fahrenheit (°F).
Question (iii)
The heat lost by a body depends on the temperature of the body.
Answer
False (F)
Explanation:
The heat lost by a body depends on mass, specific heat capacity, and temperature change, not just temperature alone.
Question (iv)
In a steam engine, mechanical work is converted into heat.
Answer
False (F)
Explanation:
In a steam engine, heat energy is converted into mechanical work, not the other way around.
Question (v)
J = 4.2 erg/calorie.
Answer
False (F)
Explanation:
The correct value is J = 4.2 × 10⁷ erg/calorie or 4.2 J/calorie.
Question (vi)
The latent heat is related to the change of state of a substance.
Answer
True (T)
Explanation:
Latent heat is the heat required to change the state of a substance without changing its temperature (e.g., melting, boiling).
Question (vii)
During change of state, temperature remains constant.
Answer
True (T)
Explanation:
During melting, boiling, or freezing, temperature remains constant until the state change is complete, despite heat being added or removed.
Question (viii)
Saturated vapour obeys Boyle’s law.
Answer
False (F)
Explanation:
Saturated vapour does not obey Boyle’s law because it is at equilibrium with its liquid phase, and pressure affects both phases.
Question (ix)
At 4°C, volume of water is maximum.
Answer
False (F)
Explanation:
Water has its minimum volume (maximum density) at 4°C. Below this temperature, it expands, which is why ice floats on water.
Match the following
| Column-I | Column-II |
|---|---|
| (i) cal/g°C is a unit of | (a) water equivalent |
| (ii) S.I. unit of heat | (b) principle of calorimetry |
| (iii) mass of the body × specific heat | (c) specific heat capacity |
| (iv) heat lost = heat gained | (d) 4.2 J/cal |
| (v) mechanical equivalent of heat | (e) joule |
Answer
| Column-I | Column-II |
|---|---|
| (i) cal/g°C is a unit of | (c) specific heat capacity |
| (ii) S.I. unit of heat | (e) joule |
| (iii) mass of the body × specific heat | (a) water equivalent |
| (iv) heat lost = heat gained | (b) principle of calorimetry |
| (v) mechanical equivalent of heat | (d) 4.2 J/cal |
Multiple Choice Questions
Question (i)
Calorimetry relates to the measurement of:
(a) heat
(b) temperature
(c) mechanical energy
(d) none of these
Answer
(a) heat
Explanation:
Calorimetry is the science of measuring heat transfer during physical and chemical processes. It helps in determining specific heat, latent heat, and heat of reactions.
Question (ii)
Quantity of heat of a body depends on its:
(a) temperature
(b) mass
(c) material
(d) all of these
Answer
(d) all of these
Explanation:
The heat content of a body depends on:
- Mass (m): More mass means more heat is required.
- Specific heat capacity (material property): Different materials require different amounts of heat.
- Temperature change (ΔT): Greater temperature change requires more heat.
Question (iii)
Quantity of heat is given by:
(a) ms / t
(b) mst
(c) mt / s
(d) st / m
Answer
(b) mst
Explanation:
The formula for heat is:
Q = m × S × ΔT
where:
- m = mass of the body
- S = specific heat capacity
- ΔT = temperature change
Question (iv)
Work done W and heat produced H are related to each other as (where J is the mechanical equivalent of heat):
(a) WH = J
(b) H = JW
(c) W = JH
(d) WJH = 1
Answer
c) W = JH
Explanation:
According to Joule’s Law, mechanical work W is proportional to the heat produced H, and the proportionality constant is J (mechanical equivalent of heat):
W = JH
This means 1 calorie of heat is equivalent to 4.2 joules of work.
Question (v)
The value of mechanical equivalent of heat (in erg/calorie) is:
(a) 4.2 × 10⁷
(b) 4.2 × 10³
(c) 4.2 × 10⁴
(d) 4.2
Answer
(a) 4.2 × 10⁷
Explanation:
The mechanical equivalent of heat in the CGS system is:
J = 4.2 × 10⁷ erg/calorie
This means 1 calorie = 4.2 × 10⁷ ergs.
Question (vi)
The latent heat of fusion of ice is:
(a) 80 cal g⁻¹
(b) 3.36 × 10⁵ cal g⁻¹
(c) 80 joule kg⁻¹
(d) 3.36 cal g⁻¹
Answer
(a) 80 cal g⁻¹
Explanation:
The latent heat of fusion of ice is 80 cal/g in the CGS system. This means 80 calories of heat are needed to convert 1g of ice into water at 0°C without changing temperature.
Question (vii)
Saturated vapour obeys:
(a) Boyle’s law
(b) Charles’ law
(c) Pressure law
(d) none of these laws
Answer
(d) none of these laws
Explanation:
Saturated vapour is at equilibrium with its liquid, meaning it does not follow Boyle’s, Charles’ or Pressure laws because additional pressure or volume changes cause condensation or evaporation.
Question (viii)
Dews are formed at a temperature:
(a) greater than dew point
(b) less than dew point
(c) equal to dew point
(d) independent of dew point
Answer
(c) equal to dew point
Explanation:
The dew point is the temperature at which air becomes saturated with moisture, leading to condensation and forming dew.
Question (ix)
The volume of water is minimum at a temperature:
(a) – 4°C
(b) 4°C
(c) 0°C
(d) 1°C
Answer
(b) 4°C
Explanation:
Water has maximum density (minimum volume) at 4°C. Below this temperature, water expands instead of contracting, due to anomalous expansion of water.
Question (x)
Thermal capacity of a body of mass m and specific heat s is:
(a) m / s
(b) s / m
(c) 1 / ms
(d) ms
Answer
(d) ms
Explanation:
Thermal capacity (also called heat capacity) is defined as:
Thermal Capacity = mass × specific heat
Thus, the correct expression is ms.
