25: Calorimetry : Neet-xii-physics

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NEET-XII-Physics

25: Calorimetry

with Solutions - page 2

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Qstn #1
The specific heat capacity of a body depends on
(a) the heat given
(b) the temperature raised
(c) the mass of the body
(d) the material of the body
digAnsr: d
Ans : (d) the material of the body
Heat capacity of a body is due to their material properties. Due to different molecular structures, different bodies have a different capacity to absorb heat. Therefore, specific heat of a body depends on the material of the body.
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Qstn #2
Water equivalent of a body is measured in
(a) kg
(b) calorie
(c) kelvin
(d) m³
digAnsr: a
Ans : (a) kg
Since water equivalent of a body is the mass of the water having the same heat capacity as the given body, the water equivalent is measured in kilogram.
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Qstn #3
When a hot liquid is mixed with a cold liquid, the temperature of the mixture
(a) first decreases then becomes constant
(b) first increases then becomes constant
(c) continuously increases
(d) is undefined for some time and then becomes nearly constant
digAnsr: d
Ans : (d) is undefined for some time and then becomes nearly constant
When hot liquid is mixed with cold liquid, the molecules collide and transfer heat. When heat is transferred, the temperature is undefined. Once the heat energy is shared by the molecules, the system reaches equilibrium and the temperature becomes nearly constant.
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Qstn #4
Which of the following pairs represent units of the same physical quantity?
(a) Kelvin and joule
(b) Kelvin and calorie
(c) Newton and calorie
(d) Joule and calorie
digAnsr: d
Ans : (d) Joule and calorie
One calorie is defined as the amount of heat needed to raise the temperature of 1 g of water from 14.5^o to 15.5^o at the pressure of 1 atm. Heat is a form of energy and the unit of energy is joule.
Therefore, joule and calorie represent energy.
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Qstn #5
Which of the following pairs of physical quantities may be represented in the same unit?
(a) Heat and temperature
(b) Temperature and mole
(c) Heat and work
(d) Specific heat and heat
digAnsr: c
Ans : (c) Heat and Work
As work done in raising temperature of a body is actually the heat supplied to the body, heat and work may be represented in the same unit.
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Qstn #6
Two bodies at different temperatures are mixed in a calorimeter. Which of the following quantities remains conserved?
(a) Sum of the temperatures of the two bodies
(b) Total heat of the two bodies
(c) Total internal energy of the two bodies
(d) Internal energy of each body
digAnsr: c
Ans : (c) Total internal energy of the two bodies
When two bodies at different temperatures are mixed in the calorimeter, heat flows from one body to the other due to the temperature difference. This results in change in the internal energy of the individual bodies. There is no exchange of heat with the surrounding in the calorimeter. Thus, the total internal energy of the bodies remain conserved as no external work is done on them.
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Qstn #7
The mechanical equivalent of heat
(a) has the same dimension as heat
(b) has the same dimension as work
(c) has the same dimension as energy
(d) is dimensionless
digAnsr: d
Ans : (d) is dimensionless
If the mechanical work done (W) produces the same temperature change as heat (H), then the mechanical equivalent of heat (J) is equal to W/H. Thus,
J = W/H
Since the unit of work and heat is the same, mechanical equivalent of heat is dimensionless.
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Section : iii

Qstn #1
The heat capacity of a body depends on
(a) the heat given
(b) the temperature raised
(c) the mass of the body
(d) the material of the body
digAnsr: c,d
Ans : (c) the mass of the body
(d) the material of the body
The bigger the body, the larger is its capacity to absorb heat. Therefore, the heat capacity of a body depends on the mass of the body. Also, different bodies have different heat capacities due to their material properties, i.e. due to their molecular structure, the heat capacity of a body depends on the material of the body.
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Qstn #2
The ratio of specific heat capacity to molar heat capacity of a body
(a) is a universal constant
(b) depends on the mass of the body
(c) depends of the molecular weight of the body
(d) is dimensionless
digAnsr: c
Ans : (c) depends on the molecular weight of the body
Specific heat capacity of a body, `` s=\frac{Q}{m\Delta \theta }``
Here,
Q = Heat supplied
m = Mass of body
Δθ = Change in temperature
Molar heat capacity of a body, `` C=\frac{Q}{n\Delta \theta }``
`` ``
Here,
Q = Heat supplied
n = Number of moles
Δθ = Change in temperature
∴ The ratio of the specific heat capacity and molar heat capacity is given by
`` \frac{s}{C}=\frac{\frac{Q}{m\Delta \theta }}{{\displaystyle \frac{Q}{n\Delta \theta }}}=\frac{n}{m}=\frac{n}{nM}=\frac{1}{M}``
`` ``
Here,
M = Molar mass related to number of moles
m = Mass
As the value of M is different for different bodies of different composition, the ratio cannot be a universal constant.
Also, the ratio is independent of the mass of the body.
The ratio of the specific heat and molar heat capacity depends on the molecular weight of the body.
Clearly, the unit of molecular weight is kg/mole. So, the ratio that depends only on the molecular weight cannot be dimensionless.
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Qstn #3
If heat is supplied to a solid, its temperature
(a) must increase
(b) may increase
(c) may remain constant
(d) may decrease
digAnsr: b,c
Ans : (b) may increase
(c) may remain constant
When heat is supplied to a solid, it is used up either to increase the temperature of the body or to change its state from one form to another by breaking the bonds between the molecules (without raising the temperature).
When heat is supplied to the solid, the internal energy of the solid increases, so the temperature does not decrease.
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Qstn #4
The temperature of a solid object is observed to be constant during a period. In this period
(a) heat may have been supplied to the body
(b) heat may have been extracted from the body
(c) no heat is supplied to the body
(d) no heat is extracted from the body
digAnsr: a,b
Ans : (a) heat may have been supplied to the body
(b) heat may have been extracted from the body
If there is no temperature change in a solid object, there is a possibility that the heat might have been supplied to the body that was used up in breaking the bond of the molecules, changing the state of the solid. This is why the temperature of the solid remians constant. Similar is the case when the heat is extracted from the body to change its state.
Since there is a possibility of supplying or extracting heat from the solid, we cannot say that heat is not supplied to the solid or is not extracted from the solid.
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Qstn #5
The temperature of an object is observed to rise in a period. In this period
(a) heat is certainly supplied to it
(b) heat is certainly not supplied to it
(c) heat may have been supplied to it
(d) work may have been done on it
digAnsr: c,d
Ans : (c) heat may have been supplied to it
(d) work may have been done on it
If the temperature of an object rises in a period, then there are two possibilities. The heat may have been supplied to it, leading to an increase of the internal energy of the object. That will increase the temperature of the body.
The second possibility is that some work may have been done on it, again leading to an increase of the internal energy of the body. That will also increase the temperature of the body.
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Qstn #6
Heat and work are equivalent. This means,
(a) when we supply heat to a body we do work on it
(b) when we do work on a body we supply heat to it
(c) the temperature of a body can be increased by doing work on it
(d) a body kept at rest may be set into motion along a line by supplying heat to it
digAnsr: c
Ans : (c) the temperature of a body can be increased by doing work on it
According to the statement "heat and work are equivalent", heat supplied to the body increases its temperature. Similarly, work done on the body also increases its temperature.
For example: If work is done on rubbing the hands against each other, the temperature of the hands increases. So, we can say that heat and work are equivalent.
When heat is supplied to a body, we do not do work on it. When we are doing work on a body, it does not mean we are supplying heat to the body. Also, a body at rest cannot be set in motion along a line by supplying heat to it. So, these statements do not justify the equivalence of heat and work.
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Section : iv