INSULATING BODY 4. GUARD HEATER SELECT THE CORRECT ANSWER USING THE...

3.

Insulating

body

4.

Guard

heater

Select the correct answer using the codes given below:

Codes:

A B C D A B C D

(a)

1

2

3

4

(b)

2

1

3

4

(c)

1

2

4

3

(d)

2

1

4

3

Thermal Conductivity of Materials

IES-12. Match

the

following:

[IES-1992]

List-I

List-II

A. Normal boiling point of oxygen

1. 1063°C

B. Normal boiling point of sulphur

2. 630.5°C

C. Normal melting point of Antimony

3.

444°C

D. Normal melting point of Gold

4. –182.97°C

(a)

4 2 3 1 (b)

4 3 1 2

(c) 4 2 3 1 (d)

4 3 2 1

IES-13. Assertion (A): The leakage heat transfer from the outside surface of a

steel pipe carrying hot gases is reduced to a greater extent on

providing refractory brick lining on the inside of the pipe as compared

to that with brick lining on the outside.

[IES-2000]

Reason (R): The refractory brick lining on the inside of the pipe offers

a higher thermal resistance.

(a) Both A and R are individually true and R is the correct explanation of A

(b) Both A and R are individually true but R is not the correct explanation of A

(c) A is true but R is false

(d) A is false but R is true

IES-14. Assertion (A): Hydrogen cooling is used for high capacity electrical

generators. [IES-1992]

Reason (R): Hydrogen is light and has high thermal conductivity as

compared to air.

Page 6 of 97

IES-15. In

MLT

θ

system (T being time and

θ

temperature), what is the

dimension of thermal conductivity?

[IES-2009]

(a)

ML T

⁻

¹

⁻

¹

θ

⁻

³

(b)

MLT

⁻

¹

θ

⁻

¹

(c)

ML

θ

⁻

¹

T

⁻

³

(d)

ML

θ

⁻

¹

T

⁻

²

IES-16. Assertion (A): Cork is a good insulator.

[IES-2009]

Reason (R): Good insulators are highly porous.

(b) Both A and R individually true but R in not the correct explanation of A

(d) A is false but R is true

IES-17. In which one of the following materials, is the heat energy propagation

minimum due to conduction heat transfer?

[IES-2008]

(a) Lead

(b) Copper

(c) Water

(d) Air

IES-18. Assertion (A): Non-metals are having higher thermal conductivity than

metals. [IES-2008]

Reason (R): Free electrons In the metals are higher than those of non

metals.

(a) Both A and R are true and R is the correct explanation of A

(b) Both A and R are true but R is NOT the correct explanation of A

Page 7 of 97

Answers with Explanation (Objective) Previous 20-Years GATE Answers

GATE-1. Ans. (c)

Q

kA

^dT

= −

dx

^Qdx

kdT

kdT

cons tan t

or dT

¹

A

= −

∴

=

∞

k

Which one has minimum thermal conductivity that will give maximum

temperature drop.

GATE-2. Ans. (d) Heat entry = Heat exit

(

²

^B

)

^dT

(

¹

^B

)

^dT

×

= ×

dx

dy

GATE-3. Ans. (a) Heat loss by hot body = Heat gain by cold body

( ) ( )

−

=

−

m c

t

t

m c

t

t

h ph

h

f

c pc

f

c

×

×

−

= ×

×

−

=

°

or 1 0.4

60

1 4.2

20

or

13.5 C

t

t

t

f

f

f

GATE-4. Ans. (c)

Previous 20-Years IES Answers

IES-1. Ans. (b)

IES-2. Ans. (d) Thermal conductivity may constant or variable.

IES-3. Ans. (a) Q = kA

^dT

0.5 1

⁶⁰

W

120 W

dx

=

× ×

0.25

=

∂

= −

+

+

⇒

∂

=

+

IES-4. Ans. (b)

T

10 40

x

30

x

²

²

T

₂

40 60

x

x

x

∂

∂

1

40 60 1

1

∂

∂

=

∂

∂

⇒

+

= ⎜

⎛

⎝

×

⎞

⎟

⎠

∂

∂

T

T

T

2

( )

α τ

τ

−

2

3

2 10

x

1

=

⇒

∂

=

×

=

°

T

( )

⁽

⁾

3

2 10

100

0.2 C/hour

τ

∂

k

k

IES-5. Ans. (b) Thermal diffusivity (α) =

;

ρ

^{∴ ∞}

c

α

p

k

IES-6. Ans. (c) α =

ρ

c

IES-7. Ans. (d)

IES-8. Ans. (a)

IES-9. Ans. (d)

IES-10. Ans. (b) For thick place homogeneous wall, heat loss =

dt

kA

dx

Page 8 of 97

⎛

× ×

⎞

=

⎛

×

⎞

Δ =

=

dt

dt

⎜

⎟

⎜

⎟

0.7

0.14

0.1

[

constant]

or

A

A

or x

m

dt

⎝

⎠

⎝

⎠

∵

0.5

dx

red brick

diatomic

IES-11. Ans. (a) Temperature slope is higher for low conducting and lower for high

conducting fluid. Thus A is for 1, B for 2. Temperature profile in C is for

insulator. Temperature rise is possible only for heater and as such D is for

guard heater.

IES-12. Ans. (d)

IES-13. Ans. (a)

IES-14. Ans. (a) It reduces the cooling systems size.

IES-15. Ans. (c)

^Q

^{= −}

^KA

^dT

_dx

^;

(

^{ML T}

²

⁻

³

)

⁼

^{K L}

( )

²

_{( )}

^{( )}

_L

^θ

−

−

⎡

−

−

⎤

( )( )

²

³

2

3

ML T

3

1

ML T

K L

K

MLT

⇒

=

⇒

=

= ⎣

⎦

θ

θ

L

θ

IES-16. Ans. (a)

IES-17. Ans. (d) Heat energy propagation minimum due to conduction heat transfer in

case of Air as its thermal conductivity is high.

IES-18. Ans. (d) Non-metals have lower thermal conductivity and free electrons in metal

higher then non metal therefore (d) is the answer.

Page 9 of 97