Conduction
1. In which case the thermal conductivity increases from left to right[NCERT 1974, 76; AFMC 2000]
(a) Al, Cu, Ag (b) Ag, Cu, Al
(c) Cu, Ag, Al (d) Al, Ag, Cu
2. Which of the following cylindrical rods will conduct most heat, when
their ends are maintained at the same steady temperature [CPMT 1981; NCERT 1973, 81;
MP PMT 1987; CBSE PMT 1995]
(a) Length 1 m; radius 1 cm
(b) Length 2 m; radius 1 cm
(c) Length 2 m; radius 2 cm
(d) Length 1 m; radius 2 cm
3. The heat is flowing through two cylindrical rods of same material.
The diameters of the rods are in the ratio 1 : 2 and their lengths are
in the ratio 2 : 1. If the temperature difference between their ends is
the same, the ratio of rate of flow of heat through them will be
[NCERT 1982; CBSE PMT 1995; EAMCET 1997]
(a) 1 : 1 (b) 2 : 1
(c) 1 : 4 (d) 1 : 8
4. Two identical square rods of metal are welded end to end as shown
in figure (i), 20 calories of heat flows through it in 4 minutes. If the
rods are welded as shown in figure (ii), the same amount of heat
will flow through the rods in
[NCERT 1982]
0oC 100oC
o
0C 100oC
(i) (ii)
(a) 1 minute (b) 2 minutes
(c) 4 minutes (d) 16 minutes
5. For cooking the food, which of the following type of utensil is most
suitable
[MNR 1986; MP PET 1990; CPMT 1991;
SCRA 1998; MP PMT/PET 1998, 2000; RPET 2001]
(a) High specific heat and low conductivity
(b) High specific heat and high conductivity
(c) Low specific heat and low conductivity
(d) Low specific heat and high conductivity
, Transmission of Heat 707
6. Under steady state, the temperature of a body [CPMT 1978] 14. Two rods A and B are of equal lengths. Their ends are kept between
(a) Increases with time the same temperature and their area of cross-sections are A1 and
(b) Decreases with time A2 and thermal conductivities K1 and K 2 . The rate of heat
(c) Does not change with time and is same at all the points of the
transmission in the two rods will be equal, if [MP PMT 1991; CBSE PMT 2002]
body
(d) Does not change with time but is different at different points (a) K1 A2 K2 A1 (b) K1 A1 K2 A2
of the body
7. The coefficient of thermal conductivity depends upon (c) K1 K2 (d) K1 A12 K2 A22
[MP PET/PMT 1984; AFMC 1996; Orissa JEE 2005] 15. In variable state, the rate of flow of heat is controlled by
(a) Temperature difference of two surfaces
(a) Density of material (b) Specific heat
(b) Area of the plate
(c) Thermal conductivity (d) All the above factors
(c) Thickness of the plate
16. If the ratio of coefficient of thermal conductivity of silver and copper
(d) Material of the plate is 10 : 9, then the ratio of the lengths upto which wax will melt in
8. When two ends of a rod wrapped with cotton are maintained at Ingen Hausz experiment will be
different temperatures and after some time every point of the rod [DPMT 2001]
attains a constant temperature, then
[MP PET/PMT 1988] (a) 6 : 10 (b) 10 : 3
(a) Conduction of heat at different points of the rod stops because (c) 100 : 81 (d) 81 : 100
the temperature is not increasing
17. The thickness of a metallic plate is 0.4 cm. The temperature
(b) Rod is bad conductor of heat
between its two surfaces is 20 o C . The quantity of heat flowing
(c) Heat is being radiated from each point of the rod
per second is 50 calories from 5cm 2 area. In CGS system, the
(d) Each point of the rod is giving heat to its neighbour at the coefficient of thermal conductivity will be
same rate at which it is receiving heat
(a) 0.4 (b) 0.6
9. The length of the two rods made up of the same metal and having
the same area of cross-section are 0.6 m and 0.8 m respectively. The (c) 0.2 (d) 0.5
o o
temperature between the ends of first rod is 90 C and 60 C and 18. In Searle's method for finding conductivity of metals, the
temperature gradient along the bar [MP PMT 1984]
that for the other rod is 150 and 110 o C . For which rod the rate of
conduction will be greater (a) Is greater nearer the hot end
(a) First (b) Second (b) Is greater nearer to the cold end
(c) Same for both (d) None of the above (c) Is the same at all points along the bar
10. The ratio of thermal conductivity of two rods of different material is (d) Increases as we go from hot end to cold end
5 : 4. The two rods of same area of cross-section and same thermal 19. The dimensions of thermal resistance are
resistance will have the lengths in the ratio [MP PET 1984; BVP 2003]
(a) 4 : 5 (b) 9 : 1 (a) M 1 L2 T 3 K (b) ML2 T 2 K 1
(c) 1 : 9 (d) 5 : 4 (c) ML2 T 3 K (d) ML2 T 2 K 2
11. The thermal conductivity of a material in CGS system is 0.4. In 20. A piece of glass is heated to a high temperature and then allowed to
steady state, the rate of flow of heat 10 cal/sec-cm , then the thermal
2
cool. If it cracks, a probable reason for this is the following property
gradient will be [MP PMT 1989] of glass [CPMT 1985]
(a) 10C / cm (b) 12C / cm (a) Low thermal conductivity
(c) 25C / cm (d) 20C / cm (b) High thermal conductivity
(c) High specific heat
12. Two rectangular blocks A and B of different metals have same
length and same area of cross-section. They are kept in such a way (d) High melting point
that their cross-sectional area touch each other. The temperature at
one end of A is 100°C and that of B at the other end is 0°C. If the 21. Two walls of thicknesses d and d and thermal conductivities k and
1 2 1
ratio of their thermal conductivity is 1 : 3, then under steady state, the k are in contact. In the steady state, if the temperatures at the outer
2
temperature of the junction in contact will be [MPsurfaces
PMT 1985]are T1 and T2 , the temperature at the common wall is
(a) 25°C (b) 50°C [MP PMT 1990; CBSE PMT 1999]
(c) 75°C (d) 100°C
k 1 T1 d 2 k 2 T2 d 1 k 1 T1 k 2 d 2
13. Two vessels of different materials are similar in size in every respect. (a) (b)
k1 d 2 k 2 d1 d1 d 2
The same quantity of ice filled in them gets melted in 20 minutes
and 30 minutes. The ratio of their thermal conductivities will be [MP PMT 1989; CMEET Bihar
k d1995] k 2d2 k 1 d 1 T1 k 2 d 2 T2
(c) 1 1 T1 T2
(d)
(a) 1.5 (b) 1 T 1 T 2 k1 d1 k 2 d 2
(c) 2/3 (d) 4
22. A slab consists of two parallel layers of copper and brass of the
same thickness and having thermal conductivities in the ratio 1 : 4. If
, 708 Transmission of Heat
the free face of brass is at 100 o C and that of copper at 0 o C , (a) 5 (b) 2.4
the temperature of interface is (c) 1.5 (d) 1.2
[IIT 1981; MP PMT 1987, 2001] 31. If the radius and length of a copper rod are both doubled, the rate
of flow of heat along the rod increases
(a) 80°C (b) 20°C
(a) 4 times (b) 2 times
(c) 60°C (d) 40°C
(c) 8 times (d) 16 times
23. The temperature gradient in a rod of 0.5 m long is 80 o C / m . If 32. The coefficients of thermal conductivity of copper, mercury and glass are
o respectively K, K and K such that K > K > K . If the same quantity of heat
the temperature of hotter end of the rod is 30 C , then the c m g c m g
is to flow per second per unit area of each and corresponding
temperature of the cooler end is temperature gradients are X , X and X , then
c m g
(a) 40 o C (b) 10 o C (a) X c X m X g (b) X c X m X g
(c) 10 o C (d) 0 o C (c) Xc Xm X g (d) Xm Xc X g
24. On heating one end of a rod, the temperature of whole rod will be 33. If two metallic plates of equal thicknesses and thermal conductivities
uniform when K 1 and K 2 are put together face to face and a common plate is
(a) K = 1 (b) K = 0 constructed, then the equivalent thermal conductivity of this plate
will be [MP PMT 1991]
(c) K = 100 (d) K
25. Snow is more heat insulating than ice, because K1 K2
(a) Air is filled in porous of snow
K1 K 2 2 K1 K 2
(b) Ice is more bad conductor than snow (a) (b)
K1 K 2 K1 K 2
(c) Air is filled in porous of ice
(d) Density of ice is more (K12 K 22 ) (K12 K 22 )
(c) (d)
26. Two thin blankets keep more hotness than one blanket of thickness K1 K 2 2 K1 K 2
equal to these two. The reason is 34. The quantity of heat which crosses unit area of a metal plate during
(a) Their surface area increases conduction depends upon
(b) A layer of air is formed between these two blankets, which is [MP PMT 1992; JIPMER 1997]
bad conductor (a) The density of the metal
(c) These have more wool (b) The temperature gradient perpendicular to the area
(d) They absorb more heat from outside (c) The temperature to which the metal is heated
27. Ice formed over lakes has (d) The area of the metal plate
(a) Very high thermal conductivity and helps in further ice 35. The ends of two rods of different materials with their thermal
formation conductivities, radii of cross-sections and lengths all are in the ratio 1
: 2 are maintained at the same temperature difference. If the rate of
(b) Very low conductivity and retards further formation of ice flow of heat in the larger rod is 4 4 cal / sec , that in the shorter
(c) It permits quick convection and retards further formation of rod in cal / sec will be
ice
[EAMCET 1986]
(d) It is very good radiator
(a) 1 (b) 2
28. Two rods of same length and material transfer a given amount of
heat in 12 seconds, when they are joined end to end. But when they (c) 8 (d) 16
are joined lengthwise, then they will transfer same heat in same 36. Two spheres of different materials one with double the radius and
conditions in one-fourth wall thickness of the other, are filled with ice. If the time
[BHU 1998; UPSEAT 2002] taken for complete melting ice in the large radius one is 25 minutes
(a) 24 s (b) 3 s and that for smaller one is 16 minutes, the ratio of thermal
(c) 1.5 s (d) 48 s conductivities of the materials of larger sphere to the smaller sphere
is [EAMCET 1991]
29. Wires A and B have identical lengths and have circular cross-
sections. The radius of A is twice the radius of B i.e . rA 2rB . For (a) 4 : 5 (b) 5 : 4
a given temperature difference between the two ends, both wires (c) 25 : 1 (d) 1 : 25
conduct heat at the same rate. The relation between the thermal
37. The ratio of the diameters of two metallic rods of the same material is
conductivities is given by
2 : 1 and their lengths are in the ratio 1 : 4. If the temperature
(a) K A 4 KB (b) K A 2K B difference between their ends are equal, the rate of flow of heat in
them will be in the ratio [MP PET 1994]
(c) K A KB / 2 (d) K A KB / 4
(a) 2 : 1 (b) 4 : 1
30. Two identical plates of different metals are joined to form a single
plate whose thickness is double the thickness of each plate. If the (c) 8 :1 (d) 16 : 1
coefficients of conductivity of each plate are 2 and 3 respectively,
then the conductivity of composite plate will be
, Transmission of Heat 709
38. Two cylinders P and Q have the same length and diameter and are (d) Metal has low thermal conductivity
made of different materials having thermal conductivities in the ratio 45. In order that the heat flows from one part of a solid to another part,
2 : 3. These two cylinders are combined to make a cylinder. One end what is required
of P is kept at 100°C and another end of Q at 0°C. The temperature
at the interface of P and Q is [MP PMT 1994; EAMCET 2000] [Pb. PMT 1999; EAMCET 1998]
(a) Uniform density (b) Density gradient
(a) 30 o C (b) 40 o C
(c) Temperature gradient (d) Uniform temperature
(c) 50 o C (d) 60 o C 46. At a common temperature, a block of wood and a block of metal
39. Two identical rods of copper and iron are coated with wax feel equally cold or hot. The temperatures of block of wood and
uniformly. When one end of each is kept at temperature of boiling block of metal are [AIIMS 1999]
water, the length upto which wax melts are 8.4 cm and 4.2cm (a) Equal to temperature of the body
respectively. If thermal conductivity of copper is 0.92, then thermal
(b) Less than the temperature of the body
conductivity of iron is [MP PET 1995]
(c) Greater than temperature of the body
(a) 0.23 (b) 0.46
(d) Either (b) or (c)
(c) 0.115 (d) 0.69
47. According to the experiment of Ingen Hausz the relation between
40. Mud houses are cooler in summer and warmer in winter because[BVP 2003] the thermal conductivity of a metal rod is K and the length of the
(a) Mud is superconductor of heat rod whenever the wax melts is
(b) Mud is good conductor of heat [UPSEAT 1999]
(c) Mud is bad conductor of heat (a) K/l = constant (b) K 2 / l = constant
(d) None of these
(c) K / l 2 = constant (d) Kl = constant
41. The temperature of hot and cold end of a 20cm long rod in
thermal steady state are at 100 o C and 20 o C respectively. 48. Temperature of water at the surface of lake is 20 o C. Then
Temperature at the centre of the rod is [MP PMT 1996] temperature of water just below the lower surface of ice layer is
(a) 50 o C (b) 60 o C (a) 4o C (b) 0 o C
(c) 40 o C (d) 30 o C (c) 4o C (d) 20 o C
42. Two bars of thermal conductivities K and 3K and lengths 1cm and 49. One end of a metal rod of length 1.0 m and area of cross section
2cm respectively have equal cross-sectional area, they are joined 100cm 2 is maintained at 100 o C. If the other end of the rod is
lengths wise as shown in the figure. If the temperature at the ends maintained at 0 o C , the quantity of heat transmitted through the
o o
of this composite bar is 0 C and 100 C respectively (see rod per minute is (Coefficient of thermal conductivity of material of
figure), then the temperature of the interface is rod =100W/m-K)
[EAMCET (Engg.) 2000]
o
0Co
K 3K 100 C (a) 3 10 J
3
(b) 6 10 J 3
(c) 9 10 3 J (d) 12 10 3 J
1 cm 2 cm
50. The coefficient of thermal conductivity of copper is nine times that
o 100 o of steel. In the composite cylindrical bar shown in the figure. What
(a) 50 C (b) C
3 will be the temperature at the junction of copper and steel [
o
200 o (a) 75 C
(c) 60 o C (d) C 100oC 0oC
3 o
(b) 67 C Copper Steel
43. A heat flux of 4000 J / s is to be passed through a copper rod of o
(c) 33 C
length 10 cm and area of cross-section 100 cm 2 . The thermal 18 cm 6 cm
(d) 25 o C
conductivity of copper is 400 W / m o C . The two ends of this rod 51. The lengths and radii of two rods made of same material are in the
must be kept at a temperature difference of ratios 1[MP
: 2PMT
and1999]
2 : 3 respectively. If the temperature difference
between the ends for the two rods be the same, then in the steady
(a) 1 o C (b) 10 o C state, the amount of heat flowing per second through them will be
in the ratio [MP PET 2000]
(c) 100 o C (d) 1000 o C
(a) 1: 3 (b) 4 : 3
44. On a cold morning, a metal surface will feel colder to touch than a
wooden surface because [AIIMS 1998] (c) 8 : 9 (d) 3 : 2
(a) Metal has high specific heat 52. A slab consists of two parallel layers of two different materials of
same thickness having thermal conductivities K and K . The
(b) Metal has high thermal conductivity 1 2
equivalent conductivity of the combination is
(c) Metal has low specific heat [BHU 2001]
1. In which case the thermal conductivity increases from left to right[NCERT 1974, 76; AFMC 2000]
(a) Al, Cu, Ag (b) Ag, Cu, Al
(c) Cu, Ag, Al (d) Al, Ag, Cu
2. Which of the following cylindrical rods will conduct most heat, when
their ends are maintained at the same steady temperature [CPMT 1981; NCERT 1973, 81;
MP PMT 1987; CBSE PMT 1995]
(a) Length 1 m; radius 1 cm
(b) Length 2 m; radius 1 cm
(c) Length 2 m; radius 2 cm
(d) Length 1 m; radius 2 cm
3. The heat is flowing through two cylindrical rods of same material.
The diameters of the rods are in the ratio 1 : 2 and their lengths are
in the ratio 2 : 1. If the temperature difference between their ends is
the same, the ratio of rate of flow of heat through them will be
[NCERT 1982; CBSE PMT 1995; EAMCET 1997]
(a) 1 : 1 (b) 2 : 1
(c) 1 : 4 (d) 1 : 8
4. Two identical square rods of metal are welded end to end as shown
in figure (i), 20 calories of heat flows through it in 4 minutes. If the
rods are welded as shown in figure (ii), the same amount of heat
will flow through the rods in
[NCERT 1982]
0oC 100oC
o
0C 100oC
(i) (ii)
(a) 1 minute (b) 2 minutes
(c) 4 minutes (d) 16 minutes
5. For cooking the food, which of the following type of utensil is most
suitable
[MNR 1986; MP PET 1990; CPMT 1991;
SCRA 1998; MP PMT/PET 1998, 2000; RPET 2001]
(a) High specific heat and low conductivity
(b) High specific heat and high conductivity
(c) Low specific heat and low conductivity
(d) Low specific heat and high conductivity
, Transmission of Heat 707
6. Under steady state, the temperature of a body [CPMT 1978] 14. Two rods A and B are of equal lengths. Their ends are kept between
(a) Increases with time the same temperature and their area of cross-sections are A1 and
(b) Decreases with time A2 and thermal conductivities K1 and K 2 . The rate of heat
(c) Does not change with time and is same at all the points of the
transmission in the two rods will be equal, if [MP PMT 1991; CBSE PMT 2002]
body
(d) Does not change with time but is different at different points (a) K1 A2 K2 A1 (b) K1 A1 K2 A2
of the body
7. The coefficient of thermal conductivity depends upon (c) K1 K2 (d) K1 A12 K2 A22
[MP PET/PMT 1984; AFMC 1996; Orissa JEE 2005] 15. In variable state, the rate of flow of heat is controlled by
(a) Temperature difference of two surfaces
(a) Density of material (b) Specific heat
(b) Area of the plate
(c) Thermal conductivity (d) All the above factors
(c) Thickness of the plate
16. If the ratio of coefficient of thermal conductivity of silver and copper
(d) Material of the plate is 10 : 9, then the ratio of the lengths upto which wax will melt in
8. When two ends of a rod wrapped with cotton are maintained at Ingen Hausz experiment will be
different temperatures and after some time every point of the rod [DPMT 2001]
attains a constant temperature, then
[MP PET/PMT 1988] (a) 6 : 10 (b) 10 : 3
(a) Conduction of heat at different points of the rod stops because (c) 100 : 81 (d) 81 : 100
the temperature is not increasing
17. The thickness of a metallic plate is 0.4 cm. The temperature
(b) Rod is bad conductor of heat
between its two surfaces is 20 o C . The quantity of heat flowing
(c) Heat is being radiated from each point of the rod
per second is 50 calories from 5cm 2 area. In CGS system, the
(d) Each point of the rod is giving heat to its neighbour at the coefficient of thermal conductivity will be
same rate at which it is receiving heat
(a) 0.4 (b) 0.6
9. The length of the two rods made up of the same metal and having
the same area of cross-section are 0.6 m and 0.8 m respectively. The (c) 0.2 (d) 0.5
o o
temperature between the ends of first rod is 90 C and 60 C and 18. In Searle's method for finding conductivity of metals, the
temperature gradient along the bar [MP PMT 1984]
that for the other rod is 150 and 110 o C . For which rod the rate of
conduction will be greater (a) Is greater nearer the hot end
(a) First (b) Second (b) Is greater nearer to the cold end
(c) Same for both (d) None of the above (c) Is the same at all points along the bar
10. The ratio of thermal conductivity of two rods of different material is (d) Increases as we go from hot end to cold end
5 : 4. The two rods of same area of cross-section and same thermal 19. The dimensions of thermal resistance are
resistance will have the lengths in the ratio [MP PET 1984; BVP 2003]
(a) 4 : 5 (b) 9 : 1 (a) M 1 L2 T 3 K (b) ML2 T 2 K 1
(c) 1 : 9 (d) 5 : 4 (c) ML2 T 3 K (d) ML2 T 2 K 2
11. The thermal conductivity of a material in CGS system is 0.4. In 20. A piece of glass is heated to a high temperature and then allowed to
steady state, the rate of flow of heat 10 cal/sec-cm , then the thermal
2
cool. If it cracks, a probable reason for this is the following property
gradient will be [MP PMT 1989] of glass [CPMT 1985]
(a) 10C / cm (b) 12C / cm (a) Low thermal conductivity
(c) 25C / cm (d) 20C / cm (b) High thermal conductivity
(c) High specific heat
12. Two rectangular blocks A and B of different metals have same
length and same area of cross-section. They are kept in such a way (d) High melting point
that their cross-sectional area touch each other. The temperature at
one end of A is 100°C and that of B at the other end is 0°C. If the 21. Two walls of thicknesses d and d and thermal conductivities k and
1 2 1
ratio of their thermal conductivity is 1 : 3, then under steady state, the k are in contact. In the steady state, if the temperatures at the outer
2
temperature of the junction in contact will be [MPsurfaces
PMT 1985]are T1 and T2 , the temperature at the common wall is
(a) 25°C (b) 50°C [MP PMT 1990; CBSE PMT 1999]
(c) 75°C (d) 100°C
k 1 T1 d 2 k 2 T2 d 1 k 1 T1 k 2 d 2
13. Two vessels of different materials are similar in size in every respect. (a) (b)
k1 d 2 k 2 d1 d1 d 2
The same quantity of ice filled in them gets melted in 20 minutes
and 30 minutes. The ratio of their thermal conductivities will be [MP PMT 1989; CMEET Bihar
k d1995] k 2d2 k 1 d 1 T1 k 2 d 2 T2
(c) 1 1 T1 T2
(d)
(a) 1.5 (b) 1 T 1 T 2 k1 d1 k 2 d 2
(c) 2/3 (d) 4
22. A slab consists of two parallel layers of copper and brass of the
same thickness and having thermal conductivities in the ratio 1 : 4. If
, 708 Transmission of Heat
the free face of brass is at 100 o C and that of copper at 0 o C , (a) 5 (b) 2.4
the temperature of interface is (c) 1.5 (d) 1.2
[IIT 1981; MP PMT 1987, 2001] 31. If the radius and length of a copper rod are both doubled, the rate
of flow of heat along the rod increases
(a) 80°C (b) 20°C
(a) 4 times (b) 2 times
(c) 60°C (d) 40°C
(c) 8 times (d) 16 times
23. The temperature gradient in a rod of 0.5 m long is 80 o C / m . If 32. The coefficients of thermal conductivity of copper, mercury and glass are
o respectively K, K and K such that K > K > K . If the same quantity of heat
the temperature of hotter end of the rod is 30 C , then the c m g c m g
is to flow per second per unit area of each and corresponding
temperature of the cooler end is temperature gradients are X , X and X , then
c m g
(a) 40 o C (b) 10 o C (a) X c X m X g (b) X c X m X g
(c) 10 o C (d) 0 o C (c) Xc Xm X g (d) Xm Xc X g
24. On heating one end of a rod, the temperature of whole rod will be 33. If two metallic plates of equal thicknesses and thermal conductivities
uniform when K 1 and K 2 are put together face to face and a common plate is
(a) K = 1 (b) K = 0 constructed, then the equivalent thermal conductivity of this plate
will be [MP PMT 1991]
(c) K = 100 (d) K
25. Snow is more heat insulating than ice, because K1 K2
(a) Air is filled in porous of snow
K1 K 2 2 K1 K 2
(b) Ice is more bad conductor than snow (a) (b)
K1 K 2 K1 K 2
(c) Air is filled in porous of ice
(d) Density of ice is more (K12 K 22 ) (K12 K 22 )
(c) (d)
26. Two thin blankets keep more hotness than one blanket of thickness K1 K 2 2 K1 K 2
equal to these two. The reason is 34. The quantity of heat which crosses unit area of a metal plate during
(a) Their surface area increases conduction depends upon
(b) A layer of air is formed between these two blankets, which is [MP PMT 1992; JIPMER 1997]
bad conductor (a) The density of the metal
(c) These have more wool (b) The temperature gradient perpendicular to the area
(d) They absorb more heat from outside (c) The temperature to which the metal is heated
27. Ice formed over lakes has (d) The area of the metal plate
(a) Very high thermal conductivity and helps in further ice 35. The ends of two rods of different materials with their thermal
formation conductivities, radii of cross-sections and lengths all are in the ratio 1
: 2 are maintained at the same temperature difference. If the rate of
(b) Very low conductivity and retards further formation of ice flow of heat in the larger rod is 4 4 cal / sec , that in the shorter
(c) It permits quick convection and retards further formation of rod in cal / sec will be
ice
[EAMCET 1986]
(d) It is very good radiator
(a) 1 (b) 2
28. Two rods of same length and material transfer a given amount of
heat in 12 seconds, when they are joined end to end. But when they (c) 8 (d) 16
are joined lengthwise, then they will transfer same heat in same 36. Two spheres of different materials one with double the radius and
conditions in one-fourth wall thickness of the other, are filled with ice. If the time
[BHU 1998; UPSEAT 2002] taken for complete melting ice in the large radius one is 25 minutes
(a) 24 s (b) 3 s and that for smaller one is 16 minutes, the ratio of thermal
(c) 1.5 s (d) 48 s conductivities of the materials of larger sphere to the smaller sphere
is [EAMCET 1991]
29. Wires A and B have identical lengths and have circular cross-
sections. The radius of A is twice the radius of B i.e . rA 2rB . For (a) 4 : 5 (b) 5 : 4
a given temperature difference between the two ends, both wires (c) 25 : 1 (d) 1 : 25
conduct heat at the same rate. The relation between the thermal
37. The ratio of the diameters of two metallic rods of the same material is
conductivities is given by
2 : 1 and their lengths are in the ratio 1 : 4. If the temperature
(a) K A 4 KB (b) K A 2K B difference between their ends are equal, the rate of flow of heat in
them will be in the ratio [MP PET 1994]
(c) K A KB / 2 (d) K A KB / 4
(a) 2 : 1 (b) 4 : 1
30. Two identical plates of different metals are joined to form a single
plate whose thickness is double the thickness of each plate. If the (c) 8 :1 (d) 16 : 1
coefficients of conductivity of each plate are 2 and 3 respectively,
then the conductivity of composite plate will be
, Transmission of Heat 709
38. Two cylinders P and Q have the same length and diameter and are (d) Metal has low thermal conductivity
made of different materials having thermal conductivities in the ratio 45. In order that the heat flows from one part of a solid to another part,
2 : 3. These two cylinders are combined to make a cylinder. One end what is required
of P is kept at 100°C and another end of Q at 0°C. The temperature
at the interface of P and Q is [MP PMT 1994; EAMCET 2000] [Pb. PMT 1999; EAMCET 1998]
(a) Uniform density (b) Density gradient
(a) 30 o C (b) 40 o C
(c) Temperature gradient (d) Uniform temperature
(c) 50 o C (d) 60 o C 46. At a common temperature, a block of wood and a block of metal
39. Two identical rods of copper and iron are coated with wax feel equally cold or hot. The temperatures of block of wood and
uniformly. When one end of each is kept at temperature of boiling block of metal are [AIIMS 1999]
water, the length upto which wax melts are 8.4 cm and 4.2cm (a) Equal to temperature of the body
respectively. If thermal conductivity of copper is 0.92, then thermal
(b) Less than the temperature of the body
conductivity of iron is [MP PET 1995]
(c) Greater than temperature of the body
(a) 0.23 (b) 0.46
(d) Either (b) or (c)
(c) 0.115 (d) 0.69
47. According to the experiment of Ingen Hausz the relation between
40. Mud houses are cooler in summer and warmer in winter because[BVP 2003] the thermal conductivity of a metal rod is K and the length of the
(a) Mud is superconductor of heat rod whenever the wax melts is
(b) Mud is good conductor of heat [UPSEAT 1999]
(c) Mud is bad conductor of heat (a) K/l = constant (b) K 2 / l = constant
(d) None of these
(c) K / l 2 = constant (d) Kl = constant
41. The temperature of hot and cold end of a 20cm long rod in
thermal steady state are at 100 o C and 20 o C respectively. 48. Temperature of water at the surface of lake is 20 o C. Then
Temperature at the centre of the rod is [MP PMT 1996] temperature of water just below the lower surface of ice layer is
(a) 50 o C (b) 60 o C (a) 4o C (b) 0 o C
(c) 40 o C (d) 30 o C (c) 4o C (d) 20 o C
42. Two bars of thermal conductivities K and 3K and lengths 1cm and 49. One end of a metal rod of length 1.0 m and area of cross section
2cm respectively have equal cross-sectional area, they are joined 100cm 2 is maintained at 100 o C. If the other end of the rod is
lengths wise as shown in the figure. If the temperature at the ends maintained at 0 o C , the quantity of heat transmitted through the
o o
of this composite bar is 0 C and 100 C respectively (see rod per minute is (Coefficient of thermal conductivity of material of
figure), then the temperature of the interface is rod =100W/m-K)
[EAMCET (Engg.) 2000]
o
0Co
K 3K 100 C (a) 3 10 J
3
(b) 6 10 J 3
(c) 9 10 3 J (d) 12 10 3 J
1 cm 2 cm
50. The coefficient of thermal conductivity of copper is nine times that
o 100 o of steel. In the composite cylindrical bar shown in the figure. What
(a) 50 C (b) C
3 will be the temperature at the junction of copper and steel [
o
200 o (a) 75 C
(c) 60 o C (d) C 100oC 0oC
3 o
(b) 67 C Copper Steel
43. A heat flux of 4000 J / s is to be passed through a copper rod of o
(c) 33 C
length 10 cm and area of cross-section 100 cm 2 . The thermal 18 cm 6 cm
(d) 25 o C
conductivity of copper is 400 W / m o C . The two ends of this rod 51. The lengths and radii of two rods made of same material are in the
must be kept at a temperature difference of ratios 1[MP
: 2PMT
and1999]
2 : 3 respectively. If the temperature difference
between the ends for the two rods be the same, then in the steady
(a) 1 o C (b) 10 o C state, the amount of heat flowing per second through them will be
in the ratio [MP PET 2000]
(c) 100 o C (d) 1000 o C
(a) 1: 3 (b) 4 : 3
44. On a cold morning, a metal surface will feel colder to touch than a
wooden surface because [AIIMS 1998] (c) 8 : 9 (d) 3 : 2
(a) Metal has high specific heat 52. A slab consists of two parallel layers of two different materials of
same thickness having thermal conductivities K and K . The
(b) Metal has high thermal conductivity 1 2
equivalent conductivity of the combination is
(c) Metal has low specific heat [BHU 2001]
