Heat ice cal cal cal cal cal

SUB- TOPICS

 Calorie : It is defined as the amount of heat required to raise the temperature of 1 gm water by 1ºC

 International calorie : International caloire is the amount of heat required to raise the temperature of 1 gm water from 14.5 ºC to 15.5 ºC rise of temperature.

Hence, mechanical equivalent of heat is the work in producing 1 cal of heat

	When heat is given to a body and if its state does not change, then temperature of body rises and if heat is taken from a body its temperature falls, i.e. temperature can be regarded as the effect of cause ‘heat’. Two bodies are said to be in thermal equilibrium if and only if they are at same temperature.

x = physical properties

t = temperature

	Kelvin Scale (K) : The upper and lower fixed points of Kelvin scale are 373 K and 273K and the fundamental interval is 100º.

So following relation exists between different temperature scales.

∆C	=	∆F	=	∆R	=
5		9		4

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 Heat
Experiments show that when a body is heated, then the heat absorbed by a body depends upon the mass of the body ‘m’, specific heat s, and temperature difference ∆t, provided state of body is not changes.

Q= msθ

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Definition : The amount of heat needed for an unit increase in the temperature of unit mass of a (substance solid, liquid or gas) is called it’s specific heat
Unit : kilocalorie/kg-ºC or calorie/g-ºC
1 Kcal/kg-ºC = 1 cal/g-ºC
(i) Generally two types of specific heat are mentioned for a gas -
(a) specific heat at constant volume (Cv)
(b) specific heat at constant pressure (cp)
(ii) These specific heats can be molar or gram, depending on the amount of gas considered to define it

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 Water equivalent of a body
If m gram of a substance is given Q amount of heat which rises its temperature by ∆T. Now if on giving same amount of heat temperature of w gram of water is also increased by ∆T then w is called water equivalent of body of mass m.

The value of water equivalent of a body is same as it’s heat capacity. The difference is only in units. e.g. If heat capacity of a body is m caloire/ºC then it’s water equivalent will be m gram.

The heat absorbed or rejected during change of state is : Q = ML
The heat absorbed by the substance during change of state is used in increasing the distance between the atoms or molecules. During change of state the kinetic energy of the molecules remain constant but the potential energy increases.

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Heat given = Heat taken
Q1 = Q2
m1S1 (θ2 – θ) = msS2 (θ – θ1)
where m1, S1 and θ1 are the mass, specific heat and temperature of one material and m2, S2 and θ2 are the mass, specific heat and temperature of second material.

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
	(i)
	(ii)
	(iii)
	(iv)	Sublimation : Direct conversion of solid vapourstate is called sublimation.
	(v)	Condensation : The process of conversion from gaseous or vapour state to liquid state is known as condensation.

These materials again get converted to vapour or gaseous state on heating.

	Physics, Class : VIII

Illustrations –7: 5 g ice of 0ºC is mixed with 5 g of steam at 100ºC. What is the final temperature?

Sol. : Heat required by ice to raise its temperature to 100ºC,
Q1 = m1L1 + m1c1 ∆θ1 = 5 × 80 + 5 × 1 × 100 = 400 + 500 = 900 cal Heat given by steam when condensed,
Q2 = m2L2 = 5 × 536 = 2580 cal.

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(1) Conduction	(2) Convection	(3) Radiation

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 Effects and Uses of thermal conductivity of daily life
During, winter, iron seems colder and in summer seems warmer than wood
During summer evening when the outer temperature begins to fall, the temperature of the inner walls of the room becomes higher than that in day time
When hot water is poured in a beaker of thick glass, the beaker cracks
In winter wooden clothes, blankets and quilts filled with cotton are used
To prevent ice from melting its wrapped with blanket or felt

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e.g. Centre of earth, where g = 0, so No convection
Most of heat transfer that is taking place on earth is by convection, the contribution due to conduction and radiation is very small.

Sp. Example Convection :
Cold air flow from ocean to ground to earth.

		Physics, Class : VIII

Fastest mode of heat transfer.

Radiation which posses through any medium must be slightly absorbed by medium according to its absorptive power so temp. of medium slightly increases.

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 Type of thermal radiation
Two types of thermal radiation

Radiation which are incident
on a surface normally
Pressure on the surface (P)
P = 2u (Perfectly reflecting surface) P = u (Perfectly absorbing surface) where u = Energy density

Incident on the surface at
all angles (except N)

 Boyle’s radio micro meter

 Pyrometer – It’s measure only high temperature.

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	KEEYY PPO OIINNTTSS
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where J is called Joule’s mechanical equivalent of heat. The value of J = 4.186 joule/calorie.

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	∆Q	=	KA	 ∆T	
	∆t			∆x	

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	AASSSSIIG GNNM MEENNTT –– II
1.
	(A)		50					(B)								80
	(C)							(D)

	(A)							(B)								80
	(C)	180						(D)

	(A)							(B)
	(C)	farenheit						(D)								all of these

	(A)												(B)
	(C)						K	−	273.16					(D)		C		=		K		−	273.16
	(C)						K	−	273.16					(D)		C		=		K

	(A)			F	−	32	=	K		−	273.6	(B)				F	−		32		=		K		−
	(A)				9		=	K		−	5	(B)				F					=		K		−
	(C)	F			−	32	=	K		−					(D)	F		−	32			=		K		+
6. 7.	(C)	F					=	K		−					(D)	F		9				=		K		+
	At what temperature Farenheit and Celsius represent the same measurement
	(A)							(B)
	(C)							(D)
	Absolute zero corresponds to
	(A)		–273°C					(B)								273°C
	(C)	273°R						(D)

	(A)							(B)
	(C)	37°F						(D)								38°K

	(A)							(B)
	(C)	300 K						(D)
	One calorie is equal to

	Physics, Class : VIII
14. 15. 20.	(A)		1 J 4.2


	(A)		JKg–1°C–1	(B)	JKg–1°K–1
	(C)			(D)

	(A)			(B)	1 cal g-1°C-1
	(C)	0.5 cal g–1°C–1		(D)

	(A)			(B)
	(C)	latent heat and volume of water		(D)	mass and avogadro number

	(A)			(B)
	(C)			(D)	none of these
	Heat flow as a result of difference of
	(A)			(B)
	(C)			(D)
	Energy are needed to heat 1 kg of water to change the temperature by 2°C
	(A)		40,000 J	(B)
	(C)			(D)

	(A)		temperature	(B)	amount of heat
	(C)			(D)

	(A)			(B)	2000 cal
	(C)	1500 cal		(D)
	1000 cal energy change the temperature of water from 10°C to 20°C the mass of water is
	(A)			(B)
	(C)			(D)	200 g
	5000 cal energy change the temperature of water is
	(A)			(B)
	(C)			(D)

Heat	Physics, Class : VIII
	AASSSSIIG GNNM MEENNTT –– IIII
5. 6.
	(A)			(B)
	(C)	cal/kg		(D)	J/g

	(A)			(B)
	(C)			(D)	400 cal/g
	Latent heat of fusion of water is
	(A)			(B)
	(C)			(D)
	The amount of heat energy required to change the 250 g water to steam at 100°C is
	(A)		14 × 106 cal	(B)
	(C)			(D)

	(A)		1°C	(B)	50°C
	(C)			(D)

	(A)			(B)	80 cal of heat is absorbed
	(C)	no heat is required		(D)

(C) 716 cal (D) 800 cal

8. Which of the following is the best heat insulator

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Straight Objective Type
This section contains multiple choice questions. Each question has 4 choices (A), (B), (C), (D), out of which ONLY ONE is correct. Choose the correct option.

1.
	(A) 215°F	(B) –297°F
	(C) 329°F	(D) 361°F

		(B) 2.46 K
		(D) 7 K

	(A) 32°F	(B) 39.2°F
	(C) 42°F	(D) 4°F

	On which of the following scales of temperature, the temperature is never negative
	(A) Celsius	(B) Fahrenheit
		(D) Kelvin

		(B) 77°F
		(D) 45°F
	The gas thermometers are more sensitive than liquid thermometer because

(A) Gases expand more than liquids (C) Gases are much lighter

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(B) Greater than its latent heat of (D) less than its latent heat of fusion

14. How much heat energy is gained when 5 kg of water at 20°C is brought to its boiling point

18. 19.		(B) 1700 kJ
		(D) 1740 kJ

		(B) 0.01 kilocalorie
	(C) 716 colorie	(D) 1 kilocalorie
	Work done in converting one gram of ice at –10°C into steam at 100°C is
		(B) 6056 J
		(D) 616 J

		(B) 2 : 3
	(C) 1 : 1	(D) 4 : 3
	The thermal capacity of 40 gm of aluminimum (specific heat = 0.2 cal/gm/°C)
		(B) 160 cal/°C
		(D) 8 cal/°C

Heat	Physics, Class : VIII
20.	(A) 1 gm	(B) 2 gm
		(D) 8 gm

		(B) 16°C
	(C) 8°C	(D) 24°C

22. Identify the correct statements from the following:
(A) when hot water is poured in a thick walled glass tumbler suddenly it develops cracks because of unequal expansions of the surfaces.

(B) for the use of hot liquids thin walled glass containers are preferred.

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Linked Comprehension Type

This section contains paragraphs. Based upon each paragraph multiple choice questions have to be answered. Each

25.

(A) 25 × 103 cal (B) 35 × 105 cal

(C) 30 × 103 cal (D) 35 × 103 cal

(i) The tmeperature of both in celsius is

(C) –200oF (D) –125oF

(iii)The tmeperature of both in Kelvin’s scale is

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Matrix Match Type
This section contains Matrix-Match Type questions. Each question contains statements given in two columns which have to be matched. Statements (A, B, C, D) in Column–I have to be matched with statements (p, q, r, s) in Column–II. The answers to these questions have to be appropriately bubbled as illustrated in the following example.

If the correct matches are A-p, A-s, B-q, B-r, C-p, C-q and D-s, then the correctly bubbled 4  4 matrix should be as follows:



(A) Heat lost = Heat gain	(p)	S	= ∆Q
	(p)	S
	(q)	L	=
	(q)	L	=	M
(C) Latent heat

	Column-B
Column-A	Column-B


(C) Water equivalent	(r) JKg–1K–1

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