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4

# According to Newton's law of cooling, the heat transfer from a hot body to a cold body is

Directly proportional to the surface area

Directly proportional to the difference of temperatures between the two bodies

Either (A) or (B)

Both (A) and (B)

### Correct Answer :

D. Both (A) and (B)

### Related Questions

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# Heat transfer takes place as per

Zeroth law of thermodynamics

First law of thermodynamic

Second law of the thermodynamics

Kirchoff's law

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# According to Prevost theory of heat exchange

It is impossible to transfer heat from low temperature source to t high temperature source

Heat transfer by radiation requires no medium

All bodies above absolute zero emit radiation

Heat transfer in most of the cases takes place by combination of conduction, convection and radiation

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4

# A grey body is one whose absorptivity

Varies with temperature

Varies with wavelength of the incident ray

Is equal to its emissivity

Does not vary with temperature and. wavelength of the incident ray

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4

# Fourier's law of heat conduction is valid for

One dimensional cases only

Two dimensional cases only

Three dimensional cases only

Regular surfaces having non-uniform temperature gradients

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# A grey body is one whose absorptivity

Varies with temperature

Varies with the wave length of incident ray

Varies with both

Does not vary with temperature and wave length of the incident ray

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4

# The time constant of a thermocouple is

The time taken to attain the final temperature to be measured

The time taken to attain 50% of the value of initial temperature difference

The time taken to attain 63.2% of the value of initial temperature difference

Determined by the time taken to reach 100°C from 0°C

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4

# Log mean temperature difference in case of counter flow compared to parallel flow will be

Same

More

Less

Depends on other factors

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4

# 40% of incident radiant energy on the surface of a thermally transparent body is reflected back. If the transmissivity of the body be 0.15, then the emissivity of surface is

0.45

0.55

0.40

0.75

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4

# The process of heat transfer from one particle of the fluid to another by the actual movement of the fluid particles caused by some mechanical means, is known as

Conduction

Free convection

Forced convection

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# The critical thickness of insulation for a sphere is

k/h₀

2k/h₀

h₀/k

h₀/2k

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4

# Two long parallel surfaces each of emissivity 0.7 are maintained at different temperatures and accordingly have radiation heat exchange between them. It is desired to reduce 75% of the radiant heat transfer by inserting thin parallel shields of emissivity 1 on both sides. The number of shields should be

One

Two

Three

Four

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4

# The ratio of surface convection resistance to the internal conduction resistance is known as

Grashoff number

Biot number

Stanton number

Prandtl number

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4

# The thermal diffusivities for solids are generally

Less than those for gases

Less than those for liquids

More than those for liquids and gases

More or less same as for liquids and gases

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4

# Thermal conductivity of a material may be defined as the

Quantity of heat flowing in one second through one cm cube of material when opposite faces ^re maintained at a temperature difference of 1°C

Quantity of heat flowing in one second through a slab of the material of area one cm square, thickness 1 cm when its faces differ in temperature by 1°C

Heat conducted in unit time across unit area through unit thickness when a temperature difference of unity is maintained between opposite faces

All of the above

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4

# The heat transfer from a hot body to a cold body is directly proportional to the surface area and difference of temperatures between the two bodies. This statement is called

First law of thermodynamics

Newton's law of cooling

Newton's law of heating

Stefan's law

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4

# A steam pipe is to be lined with two layers of insulating materials of different thermal conductivities. For the minimum heat transfer,

The better insulation must be put inside

The better insulation must be put outside

One could place either insulation on either side

One should take into account the steam temperature before deciding as to which insulation is put where

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4

# Metals are good conductors of heat because

Their atoms collide frequently

Their atoms are relatively far apart

They contain free electrons

They have high density

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# Thermal diffusivity of a substance is

Directly proportional to the thermal conductivity

Inversely proportional to density of substance

Inversely proportional to specific heat

All of the above

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4

# The amount of radiation mainly depends upon the

Nature of the body

Temperature of the body

Type of surface of the body

All of these

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4

# Total heat is the heat required to

Change vapour into liquid

Change liquid into vapour

Increase the temperature of a liquid or vapour

Convert water into steam and superheat it

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4

# In counter current flow heat exchanger, the logarithmic temperature difference between the fluids is ________ as compared to parallel flow heat exchanger.

Same

Less

Greater

None of these

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4

# The amount of radiation mainly depends on

Nature of body

Temperature of body

Type of surface of body

All of the above

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4

# In heat transfer, conductance equals conductivity (kcal/hr/sq.m/ °C/cm) divided by

Hr (time)

Sq. m (area)

°C (temperature)

K.cal (heat)

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# Thermal diffusivity is

A dimensionless parameter

Function of temperature

Used as mathematical model

A physical property of the material

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4

# Unit of thermal diffusivity is

m²/hr

m²/hr °C

kcal/m² hr

kcal/m. hr °C

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4

# Thermal conductivity of water at 20°C is of the order of

0.1

0.23

0.42

0.51

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4

# Film coefficient is defined as Inside diameter of tube

Equivalent thickness of film

Thermal conductivity Equivalent thickness of film Specific heat × Viscosity

Thermal conductivity Molecular diffusivity of momentum Thermal diffusivity

Film coefficient × Inside diameter Thermal conductivity

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4

# In a heat exchanger with one fluid evaporating or condensing, the surface area required is least in

Parallel flow

Counter flow

Cross flow

All of these

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4

# Sensible heat factor is given by (where S.H. = Sensible heat, and L.H. = Latent heat)

S.H/(S.H + L.H)

(S.H + L.H) /S.H

(L.H - S.H)/S.H

S.H/(L.H - S.H)

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4

# According of Kirchhoff's law

Radiant heat is proportional to fourth power of absolute temperature

Emissive power depends on temperature

Emissive power and absorptivity are constant for all bodies

Ratio of emissive power to absorptive power for all bodies is same and is equal to the emissive power of a perfectly black body.