RRB JE Mechanical-Heat Transfer Questionbank 1

Mechanical-Engineering

Which of the following materials is likely to have the highest thermal conductivity? RRB JE Stage 2 (22.04.2025 2:30-4:30 PM)

Aluminium
Rubber
Air
Wood

Explanation:

Aluminium is a metal known for its excellent thermal conductivity. It transfers heat very efficiently, making it a common choice for applications like heat sinks and cookware.

The unit of overall coefficient of heat transfer is —-? (Konkan Railway TA 2017)

W/m²K
W/m²
W/mK
W/m

Explanation:

Overall heat transfer coefficient is used when heat transfer takes place via conduction and convection both modes. 1/U = 1/h₁ + L/k + 1/h₂. Unit of U is W/m²K.

The thermal diffusivity is given by the expression —-? (k-thermal conductivity, ρ-density, Cp-specific heat capacity, μ-dynamic viscosity) (Konkan Railway SSE 2015)

μCp/k
k/μCp
k/ρCp
εT

Explanation:

Thermal diffusivity α = k/(ρC_p) m²/s. It measures the rate of transfer of heat of a material from the hot end to the cold end.

Heat is conducted through a 10 cm thick wall at rate of 30 W/m². When the temperature difference across wall is 10°C, what is the thermal conductivity of wall? (RRB Kolkata Diesel JE 25.10.2009)

0.03 W/mK
0.3 W/mK
3 W/mK
30 W/mK

Explanation:

Q/A = k × ΔT/Δx, 30 = k × 10/0.1, k = 0.3 W/mK.

In water cooling, the water in the jackets obtains heat from the cylinders due to —-? (RRB Jammu JE 25.10.2009)

Combined convection and radiation
Radiation
Conduction
Convection

Explanation:

The cooling system circulates water through water jackets. As water passes through the cooling jacket, it absorbs heat from hot metal surfaces. The heat transfer mode in the cooling jacket is pure convection under normal operating conditions.

Steady state heat flow implies —-? (RRB SSE Bilaspur Yellow paper, 21.12.2014)

negligible flow implies
no difference of temperature between the bodies
constant heat flow rate i.e. heat flow rate independent of time
uniform rate in temperature rise of a body

Explanation:

Steady state heat flow implies constant heat flow rate i.e. heat flow rate independent of time. In steady state heat transfer, the temperature of the body does not vary with time.

What is the ratio of thermal conductivity to electrical conductivity equal to? (RRB JE Bhopal Paper-I (Shift-II), 28.08.2015)

Prandtl number
Schmidt number
Lorenz number
Lewis number

Explanation:

The ratio of thermal conductivity to electrical conductivity is commonly known as the Lorenz number. Prandtl number Pr = μC_p/k. Nusselt number Nu = hL/k.

Which one of the following have a highest thermal conductivity? (RRB JE (Shift-1), 28.08.2015) (RRB JE Jharkhand 2014)

Boiling water
Steam
Solid ice
Rain water

Explanation:

Highest thermal conductivity ⇒ Solid ice. Temperature ↑ thermal conductivity of solid ↓, Temperature ↑ thermal conductivity of air ↑.

The substance for which Prandtl number (Pr) << 1 is characterized by —-? (RRB SSE (Shift-III), 01.09.2015)

High rate of heat diffusion
Low rate of heat diffusion
High rate of mass diffusion
Low rate of mass diffusion

Explanation:

The substance for which Prandtl number (Pr) << 1 is characterized by high rate of heat diffusion. Heat diffuses very quick in liquid metals (Pr << 1).

Heat is transferred by all three modes of transfer, viz, conduction, convection and radiation in —-? (RRB JE Ajmer 2014)

Electric heater
Steam condenser
Melting of ice
Boiler

Explanation:

In boiler, heat transfer takes place by all three modes. The heating surface in furnace area receives heat primarily by radiation. Remaining heat surface receives heat by conduction through pipes and convection from hot flue gases.

Which of the following fluid flow conditions has high heat transfer coefficient? (RRB SSE Secundrabad (Shift-I), 01.09.2015)

Free Convection in air
Forced Convection in air
Free Convection in water
Condensation of steam

Explanation:

Condensation of steam has high heat transfer coefficient (2500-100000 W/m²K). Free convection gases: 2-20, liquids: 50-1000. Forced convection gases: 25-300, liquids: 50-20,000.

The MKS unit of diffusion coefficient is —-? (RRB Mumbai C&G SSE 25.10.2009)

m²s⁻¹
m²s⁻¹
m²s⁻²
m²s⁻²

Explanation:

The M.K.S. unit of diffusion coefficient is m²s⁻¹. Thermal diffusivity α = k/(ρC_p) has unit m²/s. Thermal diffusivity and kinematic viscosity have the same unit.

Thermal diffusivity is —-? (RRB Bhopal TM SSE 25.10.2009)

a physical property of the material
function of temperature
a dimensionless parameter
useful in case of radiative heat transfer

Explanation:

Thermal diffusivity is defined as the ratio of heat conducted to the heat energy stored per unit volume. Its SI unit is m²/s. It describes how fast heat can be transformed across the medium.

Unit of thermal diffusivity is —-? (RRB Kolkata SSE 09.09.2012)

m²/hr
m²/hr°C
kcal/mhr
kcal/mhr

Explanation:

Thermal diffusivity α = k/(ρC_p) has unit m²/sec or m²/hr. It tells about the ability of material to allow heat energy to get diffused or pass through the medium more rapidly.

The critical radius of insulation of cylinder is given by —-? (RRB Bhopal SSE 09.09.2012)

k/h
2k/h
k/4h
k/4h

Explanation:

Critical thickness is defined as radius for which heat transfer rate is maximum. For cylinder r_cr = k/h. For sphere r_cr = 2k/h. k = thermal conductivity of insulating material, h = heat transfer coefficient at outer surface.

If two metallic plates of equal thickness and thermal conductivities k₁ and k₂ are put together face to face and a common plate is constructed, then the equivalent thermal conductivity of this plate will be —-? (RRB Jammu SSE 09.09.2012)

k₁k₂/(k₁+k₂)
2k₁k₂/(k₁+k₂)
√(k₁²+k₂²)/(k₁k₂)
√(k₁²+k₂²)/(2k₁k₂)

Explanation:

If two plates of equal thickness are joined together, thermal resistance of new plate equals sum of individual resistances. 1/k_eq = 1/k₁ + 1/k₂, k_eq = 2k₁k₂/(k₁+k₂).

A high value of thermal diffusivity represents —-? (RRB Bangalore SSE 09.09.2012)

High storage, less conduction of heat
Less storage, more conduction of heat
There is always equal amount of conduction and storage since it is a property
It has no relevance

Explanation:

A high value of thermal diffusivity represents less storage, more conduction of heat. Thermal diffusivity α = k/(ρC_p). Large value of α means faster heat will diffuse through the material.

The following has least value of thermal conductivity —-? (RRB Jammu JE 25.10.2009)

Rubber
Air
Water
Plastic

Explanation:

Air has the least thermal conductivity among the given options. Thermal conductivity: Diamond 2300 W/m-K, Glass 1.2, Air 0.024, Water 0.6, Asbestos 0.2 W/m-K.

Lumped parameter analysis for transient heat conduction is essentially valid of —-? (RRB Kolkata Diesel JE 25.10.2009)

Bi < 0.1
0.1 < Bi < 0.5
1 < Bi < 10
Bi → ∞

Explanation:

Lumped parameter analysis for transient heat conduction is essentially valid for Biot number less than 0.1 (Bi < 0.1).

Cork is good insulator because it has —-? (DMRC JE 20.04.2019)

Low density
Atoms colliding frequency
Free electron
Porous body

Explanation:

Cork is a good insulator because it has a porous body and air is trapped in the pores. Air is a bad conductor of heat, making cork a very good thermal insulator.

Unsteady state of heat flow occurs in —-? (RRB Mumbai JE 19.12.2010)

Flow of heat through furnace walls
Flow of heat through insulated pipe with constant surface temperature
Annealing of castings
Flow of heat through refrigerator walls

Explanation:

In unsteady state heat transfer, temperature varies with time as well as location: T = f(x, y, z, t) and ∂T/∂t ≠ 0. In annealing, temperature changes with time.

In a cylinder under steady state conduction with uniform heat generation, the temperature gradient at half the radius location will be —-? (RRB Allahabad SSE 19.12.2010)

One half of that at surface
One fourth of that at surface
Twice that at surface
Four times that at surface

Explanation:

In a cylinder under steady state conduction with uniform heat generation, dT/dr = qr/(2k). At r = R/2, temperature gradient is one half of that at r = R.

If Nusselt number is 390, Reynolds number is 39 and Prandtl number is 20, then Stanton number will be —-? (BMRCL JE 24 Feb. 2019)

0.2
0.3
0.4
0.5

Explanation:

Stanton number = Nusselt number/(Prandtl number × Reynolds number) = 390/(20 × 39) = 0.5.

Unit of thermal conductivity is —-? (RRB JE (Shift-III), 26.08.2015) (RRB SSE (Shift-III), 03.09.2015) or Unit of thermal conductivity in SI unit is —-? (RRB Mumbai SSE 05.10.2008)

J/m/s
W/m²K
W/mK
J/°C

Explanation:

According to Fourier’s Law, Q = -kA(dT/dx). k = Q/[A(dT/dx)] = (J/s)/(m² × K/m) = W/mK.

The amount of heat transferred under steady state across a slab of cross section area 0.1 m² and thickness of 0.02 m with following assumptions: Material conductivity 150 W/mK, Temperature difference 20°C is —-? (DMRC JE 20.04.2018)

2 kW
6 kW
3 kW
15 kW

Explanation:

Q = kA(dT/dx) = 150 × 0.1 × 20/0.02 = 15000 W = 15 kW.

As per Fourier law of heat conduction in one dimension, which is correct (k: Thermal conductivity, A: Area of cross section, T: Temperature, x: co-ordinate along the direction of heat flow)? (DMRC JE 22.09.2017) (Konkan Railway TA 2017)

-kA dT/dx
kA dT/dx
-kA dx/dT
kA dx/dT

Explanation:

Fourier’s Law: Q = -kA dT/dx. The negative sign shows the temperature in the direction of heat transfer is decreasing.

Drive by wire is technology in which —-? (JMRC JE 10.06.2017)

wheels are steered with the help of strong wires
it is introduced to reduce CO emission
vehicle control system is achieved by electromechanical actuator
none of the above

Explanation:

Drive by wire is technology in which vehicle control system is achieved by electromechanical actuator.

The conduction in gases occurs due to —-? (BMRCL JE 24 Feb. 2019)

conduction is not possible in gases
kinetic theory
free electrons
lattice vibration

Explanation:

Heat conduction occurs in gases by molecular momentum transfer when high velocity and high temperature molecules collide with low velocity, low temperature molecules. RMS velocity C ∝ √T (from kinetic theory of gases).

Thermal conductivity of air with rise in temperature —-? (RRB Bhubneshwar JE-II 19.12.2010)

Increases
Decreases
Remains constant
May increase or decrease depending on temperature

Explanation:

Temperature ↑ thermal conductivity of air ↑. For gases, k ∝ √T/√M, so if T ↑ then k ↑.

Thermal conductivity of non-metallic amorphous solids with decrease in temperature —-? (RRB Allahabad SSE 19.12.2010)

increases
decreases
remains constant
may increase or decrease depending on temperature

Explanation:

Thermal conductivity of non-metallic amorphous solids decreases with decrease in temperature. Thermal conductivity of gases increases with temperature, metals decreases with temperature.

Temperature of steam at around 540°C can be measured by —-? (RRB Mumbai JE 05.10.2008)

thermometer
radiation-pyrometer
thermistor
thermocouple

Explanation:

Temperature of steam at around 540°C can be measured by thermocouple. Thermocouple works on Seebeck effect, made of two dissimilar metals joined together producing voltage with temperature change.

The amount of heat flow through a body by conduction is —-? (RRB Mumbai SSE 05.10.2008)

directly proportional to the surface area of the body
directly proportional to the temperature difference on the two faces of the body
dependent upon the material of the body
All options are correct

Explanation:

According to Fourier’s law, heat flow is directly proportional to surface area, directly proportional to temperature difference, and dependent upon material of the body.

In the figure given below, curve A will be applicable when thermal conductivity of the material —-? (RRB Mumbai JE 19.12.2010)

increases with increase in temperature
decreases with increase in temperature
is very large
is constant at all the temperatures

Explanation:

Q = K_mA[(t₁ – t₂)/t], K_m = K₀(1 + βt). When β ↑, K_m ↑. Curve A is applicable when thermal conductivity increases with increase in temperature.

Which of the following is a case of steady state heat transfer? (RRB Mumbai SSE 19.12.2010)

I.C. engine
Air preheaters
Heating of building in winter
None of these

Explanation:

In steady state heat transfer, temperature at each point is constant with time. Heat transfer in IC engine, air preheater, and heating of building in winter is time dependent (unsteady heat transfer).

Which of the following is expected to have highest thermal conductivity? (DMRC JE 2013)

steam
solid ice
melting ice
water

Explanation:

Solid ice has high thermal conductivity compared to melting ice as solids have systematic arrangement of atoms with stronger intermolecular forces than liquids.

Provision of fins on a given heat transfer surface will be more if there are —-? (DMRC JE 20.04.2018)

Fewer number of thin fins
Fewer number of thick fins
Large number of thick fins
Large number of thin fins

Explanation:

Provision of fins will be more effective with large number of thin fins because more surface area provides higher heat transfer rate. Q = hA(t₁ – t₂).

Inspite of large heat transfer coefficients in boiling liquids, fins are used advantageously when the entire surface is exposed to —-? (Konkan Railway TA 2017)

Nucleate boiling
Film boiling
Transition boiling
All modes of boiling

Explanation:

Fins are used advantageously when the entire surface is exposed to film boiling – a stage where heater surface is totally covered by a film of vapour and liquid does not contact the solid.

The parameter(s) responsible for loss of heat from a hot pipe surface in a room without fans would include —-? (Konkan Railway SSE 2015)

Temperature of the surface and air in the room
Emissivity of the surface
Length and diameter of the pipe
All options are correct

Explanation:

All parameters affect heat loss: temperature difference, emissivity (polished vs rough surface), and length/diameter of pipe.

The figure given below shows the variation of temperature across the thickness of materials with different thermal conductivities under steady states. Curve C will be applicable when thermal conductivity of the material —-? (RRB Kolkata Diesel JE 25.10.2009)

increases with increase in temperature
decreases with increase in temperature
is very large
is constant at all temperatures

Explanation:

Curve C is applicable when thermal conductivity of material is very large. Curve A: k ↑ with temp, Curve B: k ↓ with temp, Curve D: k is constant.

The highest thermal diffusivity is of —-? (RRB Chandigarh SSE 25.10.2009)

Iron
Wood
Concrete
Lead

Explanation:

Thermal diffusivity α = k/(ρC_p). Lead has the highest thermal diffusivity among the given options.

Highest thermal conductivity is of —-? (RRB Mumbai C&G JE 25.10.2009)

Solid ice
Melting ice
Water
Steam

Explanation:

Highest thermal conductivity is of solid ice. Thermal conductivity: Solid ice 0.54-0.65 W/mK, Melting ice 0.025-0.56, Water 0.598, Saturated steam 0.0184 W/mK.

Metals are good heat conductors because —-? (RRB Gorakhpur RDSO SSE 25.10.2009)

of free electrons present
their atoms are relatively far apart
their atoms collide frequently
All options are correct

Explanation:

Metals are good heat conductors because of free electrons contribution in heat conduction. In non-metals, lattice vibration contributes to heat transfer.

Heat is transferred by conduction, convection and radiation in —-? (RRB Jammu JE 25.10.2009)

Boiler furnaces
Melting of ice
Condensation of steam in condenser
None of these

Explanation:

In boiler furnaces, heating surface receives heat by radiation, then travels through metal by conduction, and then transferred from metal to water by convection.

In regarding nuclear boiling —-? (RRB Kolkata SSE 09.09.2012)

The temperature of the surface is greater than the saturation temperature of the liquid
Bubbles are created by expansion of entrapped gas or vapour at small cavities in the surface
The temperature is greater than that of film boiling
All options are correct

Explanation:

In nucleate boiling, temperature is greater than that of film boiling. Nucleate boiling occurs when surface temperature is hotter than saturated fluid temperature but heat flux is below critical heat flux.

Two insulating materials of thermal conductivity k and 2k are available for lagging a pipe carrying a hot fluid. If the radial thickness of each material is the same —-? (RRB Gorakhpur Design SSE 09.09.2012)

Material with higher thermal conductivity should be used for the inner layer and one with lower thermal conductivity for the outer
Material with lower thermal conductivity should be used for the inner layer and one with higher thermal conductivity for the outer
It is immaterial in which sequence the insulating materials are used
None of these

Explanation:

If radial thickness is the same, material with lower thermal conductivity should be used for the inner layer and higher thermal conductivity for the outer, resulting in lower overall heat transfer rate.

The critical radius of insulation for spheres is given by —-? (where k is the coefficient of thermal conductivity and h is the convective heat transfer coefficient) (RRB Bhopal SSE 09.09.2012)

k/h
k/4πh
h/2k
2k/h

Explanation:

Critical radius of insulation: For cylinder r_c = k/h. For sphere r_c = 2k/h.

The process of heat transfer from one particle of the body to another without the actual motion of the particle, is known as —-? (RRB Chandigarh SSE 09.09.2012)

Conduction
Convection
Radiation
All options are correct

Explanation:

Conduction is the process of heat transfer from one particle to another without actual motion of the particle. Direct contact is required for heat transfer.

With increase in temperature, thermal conductivity of air —-? (RRB Bhubneshwar JE II 29.11.2008)

Increases
Decreases
Remains the same
None of these

Explanation:

With increase in temperature, thermal conductivity of air increases. For gases, k ∝ √T/√M.

Minimum thermal diffusivity is of —-? (RRB Mumbai C&G SSE 25.10.2009)

Aluminium
Rubber
Iron
Lead

Explanation:

Minimum thermal diffusivity is of rubber (0.089-0.13 mm²/s). Aluminium: 98, Iron: 23, Lead: 2.5 mm²/s.

Critical radius of a hollow cylinder is defined as —-? (DMRC JE 22.09.2017)

Outer radius which gives maximum heat flow
Outer radius which gives minimum heat flow
Inner radius which gives minimum heat flow
Inner radius which gives maximum heat flow

Explanation:

Critical radius of a hollow cylinder is defined as outer radius which gives maximum heat flow, thus heat resistance is minimum at this point. For hollow cylinder, r_c = k/h.

The heat transfer by conduction through a pipe is given by the relation —-? (RRB Allahabad JE 19.12.2010)

Q = πlk(T₁ – T₂)/log(A₂/A₁)
Q = 2πkL(T₁ – T₂)/log(r₂/r₁)
Q = 2πkL/(T₁ – T₂)
Q = 4πkr₁r₂(T₁ – T₂)/(r₂ – r₁)

Explanation:

Heat transfer by conduction through pipe: Q = 2πkL(T₁ – T₂)/ln(r₂/r₁). For sphere: Q = 4πkr₁r₂(T₁ – T₂)/(r₂ – r₁).

Which dimensionless number has the significant role in forced convection? (RRB Mumbai SSE 05.10.2008)

Mach number
Reynold number
Prandtl number
Peclet number

Explanation:

In forced convection, Prandtl number has significant role. Pr = μC_p/K. Nu = hL/K, St = h/(ρC_pV), Gr = βgρ²D³θ/μ².

A body cools from 90°C to 70°C in 5 minutes. The time required by body for further cooling to 50°C will be —-? (RRB Mumbai SSE 19.12.2010)

5 minutes
Less than 5 minutes
More than 5 minutes
10 minutes

Explanation:

As temperature decreases, rate of cooling decreases (Newton’s law of cooling). So time required for further cooling will be more than 5 minutes.

A composite wall is made of two layers of thickness σ₁ and σ₂ having thermal conductivities ‘K’ and ‘2K’ and equal surface areas normal to the direction of heat flow. The outer surfaces are 100°C and 200°C respectively. If junction temperature is 150°C, what will be ratio σ₁:σ₂? (DMRC JE 20.04.2018)

1:1
2:1
1:2
2:3

Explanation:

For steady state: q = K(100-150)/σ₁ = 2K(150-200)/σ₂. 50K/σ₁ = 100K/σ₂. σ₁/σ₂ = 1/2 ⇒ σ₁:σ₂ = 1:2.

A fin will be effective only when Biot number is —-? (DMRC JE 2013)

Less than one
Equal to one
More than one
Infinite

Explanation:

A fin will be more effective if Biot number is less than one. Bi = hL/k. Small Bi represents small resistance to heat conduction and small temperature gradients within the body.

On heat transfer surface, fins are provided in order to —-? (RRB Kolkata Diesel JE 25.10.2009)

increase temperature gradient so as to enhance heat transfer
increase turbulence in flow for enhancing heat transfer
decrease the pressure drop of the fluid
increase surface area to promote the rate of heat transfer

Explanation:

Fins are provided on heat transfer surface to increase surface area and promote the rate of heat transfer.

Which one of the following is electrically most conductive? (RRB Mumbai C&G JE 25.10.2009)

Copper
Silver
Aluminium
Gold

Explanation:

Conductivity order: Silver > Copper > Gold > Aluminium > Tungsten > Zinc > Nickel > Iron > Platinum > Tin.

In the heat flow equation Q = kA(t₁ – t₂)/x, the term (t₁ – t₂)/x is known as —-? (RRB Jammu JE 25.10.2009)

Thermal conductivity
Thermal coefficient
Thermal resistance
Temperature gradient

Explanation:

From Fourier’s equation Q = kAΔT/x = kA(t₁ – t₂)/x, where (t₁ – t₂)/x is the temperature gradient.

With rise in temperature thermal conductivity of solid materials —-? (RRB Allahabad JE 25.10.2009)

Decreases
Increases
Remains constant
Cannot be predicted

Explanation:

For solids (metals): k ∝ 1/T, so k decreases as T increases. For non-metals and semiconductors: k ∝ T.

6.0 kJ of conduction heat transfer has to take place in 10 minutes from one end to other end of a metallic cylinder of 10 cm³ cross sectional area, length 1 metre and thermal conductivity as 100 W/mK. What is the temperature difference between the two ends? (RRB Patna JE 25.10.2009)

80°C
100°C
120°C
160°C

Explanation:

Q/t = kA(dT/dx). (6×10³)/600 = 100 × 10×10⁻⁴ × dT/1. dT = 100°C.

Which of the following phenomenon is responsible for the heat transfer through the walls of steel radiator? (RRB Allahabad SSE 09.09.2012)

Conduction only
Convection only
Radiation only
Conduction and convection

Explanation:

Conduction is responsible for heat transfer through the walls of steel radiator. In solids, heat transfer takes only by conduction.

Which pair, out of the following alternatives, is correctly matched? (RRB Kolkata SSE 09.09.2012)

A-ii, B-iv, C-iii, D-i
A-i, B-ii, C-iv, D-iii
A-iv, B-iii, C-ii, D-i
A-i, B-ii, C-iii, D-iv

Explanation:

Fourier’s law → Conduction (Q = kAΔT/x). Newton’s law of cooling → Convection (Q = hAΔT). Stefan-Boltzmann law → Radiation (Q = σAT⁴). Kirchhoff’s law → Radiation.

The rate of heat transfer through a hollow cylinder of inner and outer radii r₁ and r₂ respectively, depends on —-? (RRB Bhopal SSE 09.09.2012)

difference of radii, (r₂ – r₁)
sum of radii, (r₂ + r₁)
product of radii, (r₁r₂)
ratio of radii, (r₂/r₁)

Explanation:

Heat transfer through hollow cylinder: Q = 2πkL(T₁ – T₂)/ln(r₂/r₁). It depends on ratio of radii (r₂/r₁).

The time constant of a thermocouple is the time taken to —-? (RRB Jammu SSE 09.09.2012)

attain the final value
attain 63.2% of the value of initial temperature difference
attain 50% of initial temperature difference
minimum time taken to record a temperature reading

Explanation:

The time constant of a thermocouple is the time taken to attain 63.2% of the value of initial temperature difference. Large time constant corresponds to slow system response.

As the value of diffusivity of material increases, the propagation of heat into the medium —-? (RRB Bhubneshwar JE II 29.11.2008)

increases
decreases
remains same
may increase or decrease

Explanation:

As the value of diffusivity of material increases, the propagation of heat into the medium increases. Thermal diffusivity α = k/(ρC_p) m²/sec.

Consider that two solid bodies A and B are touching each other and transmitting heat through conduction. In the graph below, OX represents the first body and XY represents the second body. State True (T) of False (F): 1. Temperature gradient is more 2. The heat flow is determined by Fourier’s law 3. Area under the curve represents heat dissipation rate (RRB SSE Secunderabad Green paper, 21.12.2014)

T.T.T
T.T.F
T.F.T
F.F.T

Explanation:

All above options are true. T₄ > T₁ so temperature gradient is more. Heat transfer follows Fourier’s law. Area under the curve represents heat dissipation rate.

Thermal diffusivity of substance is inversely proportional to —-? (RRB Mumbai C&G JE 25.10.2009)

Specific heat
Density of substance
Both a and b
None

Explanation:

Thermal diffusivity α = k/(ρC_p). So α is inversely proportional to both density (ρ) and specific heat (C_p).

Heat transfer takes places as per —-? (RRB Allahabad JE 25.10.2009)

Zeroth law of thermodynamics
First law of thermodynamics
Second law of thermodynamics
Third law of thermodynamics

Explanation:

Heat transfer takes place as per second law of thermodynamics. Concept of temperature measurement = Zeroth law. Internal energy = First law.

Which one of the following is NOT a temperature measuring instrument? (RRB Mumbai SSE 19.12.2010)

Thermocouple
Rotameter
Thermistor
Pyrometer

Explanation:

Rotameter is not a temperature measurement device. It is used to measure discharge of fluid.

A flat plate has thickness 6 cm, thermal conductivity 1 W/m-K, convective heat transfer coefficients on its two flat faces are 50 W/m²K and 20 W/m²K. Determine its overall heat transfer coefficient. (RRB Mumbai JE 05.10.2008)

6.33 W/m²K
7.69 W/m²K
20 W/m²K
None of these

Explanation:

1/U = 1/h₁ + L/k + 1/h₂ = 1/50 + 0.06/1 + 1/20 = 0.02 + 0.06 + 0.05 = 0.13. U = 7.69 W/m²K.

Determine the shape factor of a hemispherical body placed on a flat surface with respect to itself. (DMRC JE 2013)

0
0.25
0.5
1

Explanation:

The shape factor of a hemispherical body placed on a flat surface with respect to itself is 0.5.

A copper wire of radius 0.65 mm is insulated with a sheathing of thickness 1.25 mm having thermal conductivity 0.7 W/mK. The outside surface convective heat transfer coefficient is 10 W/m²K. If thickness of insulation is raised by 15 mm, what will be the result on electrical current-carrying capacity of the wire? (Konkan Railway STA 2017)

It will increase
It will decrease
It will remain same
will vary depending upon the electrical conductivity of the wire

Explanation:

Critical radius r = k/h = 0.7/10 = 0.07 m = 70 mm. Since wire radius (0.65 + 1.25 = 1.9 mm) is below critical radius, heat transfer increases with added insulation up to critical radius.

A large cylindrical vessel was sealed in summer. What is likely to happen to it in winter? (RRB Kolkata Diesel JE 25.10.2009)

Nothing
Explode
Buckle & Collapse
Become lighter

Explanation:

A large cylindrical vessel sealed in summer will buckle and collapse in winter due to pressure difference caused by temperature reduction.

For which of the given conditions heat transfer from the insulated tip can be considered the case of fin of finite length? (where m is slope of differential equation and L is length of fin) (RRB Mumbai C&G JE 25.10.2009)

m = 0.75, L = 3
m = 1, L = 3
m = 3, L = 0.72
m = 2, L = 1.2

Explanation:

Q ∝ tanh(mL). For m=1, L=3: tanh(3)=0.995 which gives highest heat transfer.

The transfer of heat by molecular collision is known as —-? (RRB JE (Shift-1), 28.08.2015)

Conduction
Convection
Radiation
None of the above

Explanation:

Conduction is heat transfer from one particle to another without movement of the object due to molecular collision, lattice vibration, and transportation of free electrons.

In which one of the following materials is the heat energy propagation minimum due to conduction heat transfer? (RRB SSE (Shift-III), 03.09.2015) (RRB JE (Shift-2), 29.8.2015)

Lead
Copper
Water
Air

Explanation:

Heat energy propagation is minimum due to conduction heat transfer in case of air.

The heat flow through solids only by —-? (RRB JE (Shift-III), 27.08.2015)

Conduction
Convection
Radiation
Does not flow

Explanation:

Heat flows in solids only by conduction, which occurs when molecules in hot object transfer energy to cooler object through molecular collisions.

Fourier’s law of heat conduction gives the heat flow for —-? (RRB JE (Shift-I), 27.08.2015)

Irregular surface
Non-uniform temperature surface
One dimensional cases only
Two dimensional cases only

Explanation:

Fourier’s law assumes heat flow is unidirectional (one dimensional), steady state condition, constant temperature gradient, linear temperature profile, no internal heat generation.

Which of the following is not a method of heat transfer? (RRB SSE (shift-III), 02.09.2015)

Conduction
Convection
Condensation
Radiation

Explanation:

Condensation is not a method of heat transfer. The three modes are conduction (occurs in all media), convection (liquids and gases only), and radiation (all media and vacuum).

Forced convection is primarily characterised by —-? RRB JE (Re-Exam) 04.06.2025

the absence of any temperature gradients in the fluid
heat transfer occurring solely through radiation
the natural buoyancy-driven flow of the fluid
the use of external devices such as fans or pumps to move the fluid

Explanation:

Forced convection is primarily characterized by the use of external devices such as fans or pumps to move the fluid. This external force enhances heat transfer compared to natural convection.

Which of the following is an example of forced convection? RRB JE Stage 2 (22.04.2025 9:00 AM-11:00 AM)

Air blown over a car radiator by a fan
Thermal energy transmitted by electromagnetic waves
Heat transfer through a stationary fluid layer
Warm air naturally rising from a hot surface

Explanation:

Forced convection is a mode of heat transfer in which fluid motion is generated by an external source like a pump, fan, or mixer. Air blown over a car radiator by a fan is an example.

The by-pass factor of cooling coil is 0.2. If the surface temperature is 5°C and air enters at 40°C, the exit temperature of air will be —-? (RRB SSE (shift-III), 02.09.2015)

5°C
12°C
20°C
40°C

Explanation:

By-pass factor = (Tₑ – Tₛ)/(Tᵢ – Tₛ). 0.2 = (Tₑ – 5)/(40 – 5). Tₑ = 5 + 0.2×35 = 5 + 7 = 12°C.

In a flat plate convection heat transfer, local convection heat transfer coefficient at the leading edge is —-? (RRB Bangalore SSE 09.09.2012)

Greater than the average heat transfer coefficient
Equal to the average heat transfer coefficient
No local heat transfer coefficient at leading edge
Smaller than the average heat transfer coefficient

Explanation:

hₓ ∝ x⁻¹⁄² shows that at the leading edge, heat transfer coefficient is maximum and decreases as distance increases from leading edge.

In an automobile radiator is used to cool cooling water of the engine. The mode of heat transfer between radiator body and surrounding is —-? (RRB Bangalore SSE 09.09.2012)

Conduction
Convection
Radiation
All the above

Explanation:

The mode of heat transfer between the walls of radiator and surrounding fluid is through convection, which is dominant when there is bulk fluid motion.

The Prandtl number for liquid, which has velocity boundary layer much thicker than thermal boundary layer, is —-? (RRB SSE (Shift-II), 01.09.2015)

Pr > > 1
Pr < < 1
Pr = 1
Pr = 0

Explanation:

δ/δₜ = (Pr)¹⁄³. If Pr >> 1, then δ >> δₜ, so velocity boundary layer is much thicker than thermal boundary layer.

For flow of fluid over a heated plate, viscosity = 0.001 Pa.s, Cp = 1 kJ/kg-K, k = 1 W/mK. Hydrodynamic boundary layer thickness is 1 mm. The thermal boundary layer thickness at same location is —-? (RRB Gorakhpur Design SSE 09.09.2012)

0.001 mm
0.01 mm
1 mm
10 mm

Explanation:

Pr = μCₚ/k = 0.001 × 1000/1 = 1. δₜ = δ(Pr)¹⁄³ = 1 × 1 = 1 mm.

What happens when the thickness of insulation on a pipe exceeds the critical value? (Konkan Railway SSE 2015)

Heat transfer rate increases
Heat transfer rate decreases
Heat transfer rate remains constant
None of these

Explanation:

When thickness of insulation exceeds critical value, heat transfer rate decreases. Critical thickness is the thickness at which heat transfer is maximum.

In free convection heat transfer, Nusselt number is function of —-? (RRB Jammu SSE 09.09.2012)

Grashoff number and Reynold number
Prandtl number and Reynold number
Grashoff number, Prandtl number and Reynold number
Grashoff number and Prandtl number

Explanation:

In free convection, Nusselt number is a function of Grashoff number and Prandtl number: Nu = f(Gr, Pr).

Heat transfer in liquid and gases take place by —-? (RRB Mumbai JE 19.12.2010)

conduction
convection
radiation
conduction and convection both

Explanation:

Heat transfer in liquid and gases takes place by convection. Convection equation: Q = hAΔT.

When heat is transferred from one particle of hot body to another by actual motion of the heated particles, it is referred to as heat transfer by —-? (RRB Allahabad JE 19.12.2010)

conduction
convection
radiation
conduction and convection

Explanation:

When heat is transferred by actual motion of heated particles, it is convection. Convection moves in a circular pattern and is classified as forced or natural convection.

On a summer day, a scooter rider feels more comfortable while on the move than while at a stop light because —-? (JMRC JE 10.06.2017)

An object in motion captures less solar radiation
Air is transparent to radiation and hence it is cooler than the body
More heat is lost by convection and radiation while in motion
Air has a low specific heat and hence it is cooler

Explanation:

Heat transfer takes place from body to surrounding by convection and radiation. When speed is more, contact speed of air increases, so more heat is lost.

Film coefficient is the ratio of —-? (DMRC JE 22.09.2017)

Thickness of film of fluid to thermal conductivity
Thickness of film of fluid to temperature drop through film of fluid
Thermal conductivity to temperature drop through film of fluid
Thermal conductivity to equivalent thickness of film of fluid

Explanation:

Film coefficient h = k/dx, where k = thermal conductivity and dx = thickness of film.

According to Newton’s law of cooling, the rate of heat transfer from a solid surface of area A, at temperature t₁ to a fluid at temperature t₂ is given by —-? (RRB JE Ahmedabad 2014)

Q = hA(t₁ + t₂)
Q = hA(t₁ – t₂)
Q = h/A(t₁ – t₂)
Q = A/h(t₁ – t₂)

Explanation:

Newton’s law of cooling: Q = hA(t₁ – t₂), where h = convective heat transfer coefficient.

The law governing the rate of heat transfer from solid surface to a fluid is —-? (RRB Mumbai SSE 05.10.2008) or The rate of heat transfer from a solid surface to a fluid is obtained from —-? (DMRC JE 2013)

Newton’s law of cooling
Fourier’s law
Kirchhoff’s law
Stefan’s law

Explanation:

Rate of heat transfer from solid surface to a fluid is obtained from Newton’s law of cooling: Q = hA(T₁ – T₂).

A thin flat plate 2m × 2m is hanging freely in air. Surrounding temperature is 25°C. Solar radiation falling on one side at 500 W/m². What should be convective heat transfer coefficient in W/m²°C, if plate temperature is to remain constant at 30°C? (Konkan Railway SSE 2015)

Explanation:

Area = 4 m². Q = hA(Tₛ – T∞). 500 × 4 = h × 8 × 5. h = 50 W/m²°C.

Addition of fin to the surface increases the heat transfer if —-? (RRB Mumbai C&G JE 25.10.2009)

Equal to one
Greater than one
Less than one
Greater than one but less than two

Explanation:

Addition of fin increases heat transfer if hA/(VkP) is less than one. Fin effectiveness ε ∝ 1/√h.

The unit of overall coefficient of heat transfer is —-? (RRB Malda SSE 25.10.2009)

kcal/m²
kcal/hr°C
kcal/m²hr°C
kcal/m hr°C

Explanation:

The unit of overall coefficient of heat transfer is kcal/m²hr°C or kW/m²-K.

The non-dimensional number that gives the relationship between the thermal boundary layer and hydrodynamic boundary layer is —-? (RRB Patna JE 25.10.2009)

Rayleigh number
Peclet number
Grashof number
Prandtl number

Explanation:

Prandtl number = molecular diffusivity of momentum/molecular diffusivity of heat = μCₚ/k.

The effectiveness of a fin will be maximum in an environment with —-? (RRB JE (Shift-3), 29.8.2015) RRB JE Bhopal Paper-1 (Shift-II) 28.08.2015

Free convection
Forced convection
Radiation
Convection and radiation

Explanation:

Fin effectiveness is maximum in an environment with free convection. ε ∝ 1/√h, so lower h (free convection) gives higher effectiveness.

The critical radius of insulation of cylinder is given by —-? (RRB SSE (Shift-III), 01.09.2015)

k/h
2k/h
k/4h
k/2h

Explanation:

Critical radius of insulation for cylinder r_c = k/h. For sphere r_c = 2k/h.

What is the primary mechanism by which thermal radiation transfers energy? RRB JE (Re-Exam) 04.06.2025

Convection due to fluid motion
Direct transfer via physical contact
Conduction through molecular collisions
Electromagnetic waves

Explanation:

Thermal radiation transfers energy through electromagnetic waves. It does not require any medium, is the fastest mode of energy transfer, and is volumetric phenomenon.

The Stefan-Boltzmann law is derived from —-? RRB JE (Re-Exam) 04.06.2025

Wien’s Displacement Law
Kirchhoff’s Law
Fourier’s Law
Planck’s Law

Explanation:

The Stefan-Boltzmann law is derived from Planck’s law. Planck’s constant h = E/v. Stefan-Boltzmann law: Q = σAT⁴, σ = 5.67×10⁻⁸ W/m²K⁴.

What does the Inverse Square Law state in the context of radiation? RRB JE Stage 2 (22.04.2025 9:00 AM-11:00 AM)

Radiation intensity doubles when the distance doubles
Radiation intensity is inversely proportional to the square of the distance
Radiation intensity is directly proportional to the distance
Radiation intensity is unaffected by distance

Explanation:

Inverse Square Law: I ∝ 1/d², where I = intensity of radiation, d = distance from the radiation source.

What defines a black body in the context of thermal radiation? RRB JE Stage 2 (22.04.2025 9:00 AM-11:00 AM)

An object with perfect thermal insulation
A material that only emits visible light
An idealized object that absorbs all incident radiation and emits the maximum possible radiation at a given temperature
A surface that reflects all incident radiation without absorption

Explanation:

A black body is an idealized object that perfectly absorbs all incident radiation of all wavelengths and angles and emits radiation with a spectrum determined by temperature.

A gray body is defined as a body that —-? RRB JE Stage 2 (22.04.2025 2:30-4:30 PM)

reflects all radiation incident upon its surface
has an emissivity that varies with wavelength of radiation
absorbs all radiation incident upon its surface
has emissivity less than 1 but constant for all wavelengths

Explanation:

A grey body is a theoretical object that has an emissivity less than 1 but remains constant over all wavelengths of radiation.

If the temperature of a black body doubles, how many times will its emissive power increase? RRB JE Stage 2 (22.04.2025 2:30-4:30 PM)

2 times
8 times
16 times
4 times

Explanation:

From Stefan-Boltzmann law: E ∝ T⁴. If T doubles, E₂ = σ(2T)⁴ = 16σT⁴ = 16E₁.

Stefan Boltzmann’s constant is expressed in the unit —-? RRB JE 30-08-2019

W/m²K
W/m²K²
W/m²K⁴
W/m²K⁴

Explanation:

Stefan Boltzmann’s constant is expressed in unit W/m²K⁴.

A solar thermal collector —-? (DMRC JE 2013)

Collects the solar energy and reflects it back
Absorbs the solar radiation and dissipates it to the ambient
Collects and converts the solar energy into electrical energy
Collects and converts the solar energy into thermal energy and delivers it to the next stage of the system

Explanation:

Solar thermal collector collects and converts solar energy into thermal energy and delivers it to the next stage of the system. It is a device used to collect heat by absorbing sunlight.

A solar cell is basically —-? (Konkan Railway STA 2017)

A voltage source, controlled by flux of radiation
A current source, controlled by flux of radiation
An uncontrolled current source
An uncontrolled voltage source

Explanation:

Solar cell is basically a current source, controlled by flux of radiation. It is also known as photovoltaic cell that converts light energy into electrical energy through photovoltaic effect.

The rate of energy emission from unit surface area through unit solid angle, along a normal to the surface, is known as —-? (RRB Mumbai C&G JE 25.10.2009)

emissivity
transmissivity
reflectivity
intensity of radiation

Explanation:

Intensity of radiation is the rate of energy emission from unit surface area through unit solid angle along a normal to the surface. Emissivity ε = E/E_b.

In radiative heat transfer, a gray surface is one —-? (RRB Mumbai SSE 25.10.2009)

Which appears gray to the eye
Whose emissivity is independent of wavelength
Which has reflectivity equal to zero
Which appears equally bright from all directions

Explanation:

A gray surface has emissivity independent of wavelength. For gray body, emissivity varies between 0 and 1. For black body ε = 1, for white body ε = 0.

Heat is transferred from an insulated pipe to the surrounding still air by —-? (RRB Patna JE 25.10.2009)

Conduction
Convection
Radiation
All options are correct

Explanation:

Heat is transferred from an insulated pipe to surrounding still air by radiation – emission or transmission of energy in the form of waves or particles through electromagnetic radiation.

Heat transfer by radiation mainly depends upon —-? (RRB Bangalore SSE 09.09.2012)

nature of the body
its temperature
kind and extent of its surface
all of these

Explanation:

Heat transfer through radiation depends on nature of the body, its temperature, and kind and extent of its surface.

Which surface will have the least emissivity? (RRB Chandigarh SSE 09.09.2012)

Smooth glass
Plaster
Aluminium foil
Concrete

Explanation:

Aluminium foil has the least emissivity (0.04). Smooth glass: 0.92-0.95, Plaster: 0.98, Concrete: 0.85.

The process of heat transfer from a hot body to a cold body in a straight line, without affecting the intervening medium, is known as —-? (RRB Allahabad JE 09.09.2012)

Conduction
Convection
Radiation
All options are correct

Explanation:

Radiation is heat transfer from hot body to cold body in a straight line without affecting the intervening medium. No medium is required for heat transfer in radiation.

Heat is transferred from an electric bulb by —-? (RRB Bhubneshwar JE II 29.11.2008)

Conduction
Convection
Radiation
All options are correct

Explanation:

Heat is transferred from an electric bulb by radiation, as there is no contact. Radiation is transfer of energy by electromagnetic waves.

The ratio of the energy absorbed by the body to total energy falling on it is called —-? (RRB Jammu JE 25.10.2009)

absorptive power
emissive power
emissivity
None of these

Explanation:

Absorptive power = Energy absorbed by body/Total energy falling on it. Absorptivity measures how much radiation is absorbed by the body.

If a body is at thermal equilibrium, then —-? (Konkan Railway SSE 2015)

Emissivity < absorptivity
Emissivity > absorptivity
Emissivity = absorptivity
None of the above

Explanation:

If a body is at thermal equilibrium, Absorptivity = Emissivity (Kirchhoff’s law).

Assuming the Sun to be a black body emitting radiation with maximum intensity at wavelength 4753 Å, the surface temperature of the sun will be —-? (RRB Mumbai SSE 19.12.2010)

491.4 K
6100 K
49140 K
491.4°C

Explanation:

Wien’s displacement law: T = b/λ_m. b = 2.9×10⁻³ m-K. T = (2.9×10⁻³)/(4753×10⁻¹⁰) = 6100 K.

If the ratio of emission of a body to that of a black body at a given temperature is constant for all wavelengths, the body is called —-? (RRB Allahabad SSE 19.12.2010)

Black body
Grey body
White body
Opaque body

Explanation:

Grey body has constant ratio of emission to black body at given temperature for all wavelengths. It is opaque with τ = 0.

The opaque body is that which —-? (DMRC JE 22.09.2017)

Absorbs all radiations
Reflects all radiations
Transmits all radiation
Partly reflects and partly absorbs the radiation

Explanation:

Opaque body partly reflects and partly absorbs radiation: α + ρ = 1, τ = 0. Black body: α = 1, White body: ρ = 1.

According to Stefan Boltzmann law the relation between the total emission from a black body per unit area and per unit time (E_b) and the absolute temperature (T) is given as —-? (RRB Bhubneshwar JE-II 19.12.2010)

E_b ∝ T⁴
E_b ∝ T³
E_b ∝ T²
E_b ∝ T

Explanation:

Stefan-Boltzmann law: E ∝ T⁴, E = σT⁴, σ = 5.67×10⁻⁸ W/m²K⁴.

The process in which heat energy is transmitted by means of electromagnetic waves is known as —-? (RRB Bhubneshwar JE-II 19.12.2010)

Heat conduction
Heat convection
Heat radiation
None of the above

Explanation:

Heat radiation is the process of transmitting heat energy by electromagnetic waves. This method does not require any medium.

The range of emissivity (ε) in radiation is —-? (RRB Mumbai JE 05.10.2008)

0 ≤ ε ≤ ∞
-∞ ≤ ε ≤ +∞
0 ≤ ε ≤ 1
1 ≤ ε ≤ ∞

Explanation:

The range of emissivity in radiation is 0 ≤ ε ≤ 1.

The material medium between the heat source and receiver is not affected during the process of heat transmission by —-? (RRB JE Allahabad 2014)

Conduction
Convection
Radiation
Conduction as well as convection

Explanation:

During radiation, the material medium between heat source and receiver is not affected. Radiation does not require any medium.

For quick warmth during a cold winter season, a person prefers to sit near a fire. Which of the following modes of heat transfer provides him the maximum heat? (RRB Mumbai SSE 19.12.2010)

conduction from the fire
convection will be better if he is near the fire
combined effect of conduction and convection will be better near the fire
direct unimpeded radiation will provide quick warmth

Explanation:

Direct unimpeded radiation provides quick warmth near a fire during winter.

The heat transfer equation Q = σAT⁴ is called —-? (Konkan Railway STA 2017)

Newton’s law
Stefan-Boltzmann law
Poisson law
Fourier law

Explanation:

Stefan-Boltzmann law: Q = σAT⁴, σ = 5.67×10⁻⁸ W/m²K⁴.

At thermal equilibrium, the absorptivity and emissivity are —-? (Konkan Railway SSE 2015)

unity
zero
different
equal

Explanation:

At thermal equilibrium, absorptivity and emissivity are equal (Kirchhoff’s law).

According to Wien’s displacement law, the wavelength corresponding to maximum energy varies with absolute temperature T as —-? (RRB Mumbai C&G JE 25.10.2009)

T⁻¹
T⁻¹
T⁻¹
T⁻¹

Explanation:

Wien’s displacement law: λ_max × T = constant, so λ_max ∝ T⁻¹.

All radiations in a black-body are —-? (RRB JE (Shift-III), 26.08.2015)

Reflected
Refracted
Transmitted
Absorbed

Explanation:

All radiations in a black-body are absorbed. A black body absorbs all incident electromagnetic radiation regardless of frequency or angle of incidence.

A perfect black body is one which —-? (RRB JE (Shift-III), 27.08.2015)

is black in colour
absorbs heat radiations of all wavelengths falling on it
reflects all the heat radiations
transmits the heat radiations

Explanation:

A perfect black body absorbs heat radiations of all wavelengths falling on it. For black body: α = 1, ρ = 0, τ = 0.

Fraction of radiative energy leaving one surface that strikes the other surface is called —-? (RRB JE (Shift-III), 16.09.2015)

Radiative flux
Emission power of the first surface
View factor
Re-radiation flux

Explanation:

View factor is the fraction of radiant energy leaving one surface which strikes a second surface directly. It depends only on geometry of body.

Intensity of radiation at a surface in perpendicular direction is equal to —-? (RRB JE (Shift-III), 30.08.2015)

Product of emissivity of surface and 1/π
Product of emissivity of surface and π
Product of emissive power of surface and 1/π
Product of emissive power of surface and π

Explanation:

Intensity of radiation at a surface in perpendicular direction = Emissive power/π. E = πI, I = E/π.

If the temperature of a solid surface changes from 27°C to 627°C then its emissive power will increase in the ratio of —-? (RRB JE (Shift-III), 30.08.2015)

Explanation:

T₁ = 27+273 = 300 K, T₂ = 627+273 = 900 K. E₂/E₁ = (T₂/T₁)⁴ = (900/300)⁴ = 3⁴ = 81.