The coefficient of linear expansion of brass and steel are ${\alpha }_1$ and ${\alpha }_2$. If we take a brass rod of length $l_1$ and steel rod of length $l_2$ at 0°C, their difference in length $\left(l_2-l_1\right)$ will remain the same at a temperature if

1. ${\alpha }_1{l}_2={\alpha }_2{l}_1$                                   
2. ${\alpha }_1{l}_2^2={\alpha }_2{l}_1^2$
3. ${\alpha }_1^2{l}_1={\alpha }_2^2{l}_2$                                   
4. ${\alpha }_1{l}_1={\alpha }_2{l}_2$

Under steady state, the temperature of a body 

(1) Increases with time

(2) Decreases with time

(3) Does not change with time and is same at all the points of the body

(4) Does not change with time but is different at different points of the body

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 the free face of brass is at 100°C and that of copper at 0°C, the temperature of interface is
1. 80°C               

2. 20°C

3. 60°C               

4. 40°C

Wires A and B have identical lengths and have circular cross-sections. The radius of A is twice the radius of B i.e. ${\mathrm{r}}_{\mathrm{A}}=2{\mathrm{r}}_{\mathrm{B}}$ . For a given temperature difference between the two ends, both wires conduct heat at the same rate. The relation between the thermal conductivities is given by

(1) ${\mathrm{K}}_{\mathrm{A}}=4{\mathrm{K}}_{\mathrm{B}}$                     
(2) ${\mathrm{K}}_{\mathrm{A}}=2{\mathrm{K}}_{\mathrm{B}}$
(3) ${\mathrm{K}}_{\mathrm{A}}={\mathrm{K}}_{\mathrm{B}}/2$                   
(4) ${\mathrm{K}}_{\mathrm{A}}={\mathrm{K}}_{\mathrm{B}}/4$

Two identical plates of different metals are joined to form a single plate whose thickness is double the thickness of each plate. If the coefficients of conductivity of each plate are 2 and 3 respectively, then the conductivity of the composite plate will be:
1. 5
2. 2.4
3. 1.5                 
4. 1.2

The temperature of the hot and cold ends of a 20 cm long rod in a thermal steady state is at \(100^{\circ}\mathrm{C}\) and \(20^{\circ}\mathrm{C}\) respectively. The temperature at the centre of the rod will be:
1. \(50^{\circ}\mathrm{C}\)
2. \(60^{\circ}\mathrm{C}\)
3. \(40^{\circ}\mathrm{C}\)
4. \(30^{\circ}\mathrm{C}\)

On a cold morning, a metal surface will feel colder to touch than a wooden surface because 

(1) Metal has high specific heat

(2) Metal has high thermal conductivity

(3) Metal has low specific heat

(4) Metal has low thermal conductivity

A cylindrical rod having temperature ${\mathrm{T}}_1$ and ${\mathrm{T}}_2$ at its ends. The rate of flow of heat is ${\mathrm{Q}}_1$ cal/sec. If all the linear dimensions are doubled keeping temperature constant then rate of flow of heat ${\mathrm{Q}}_2$ will be
(a) $4{\mathrm{Q}}_1$           (b) $2{\mathrm{Q}}_1$
(c)$\frac{{\mathrm{Q}}_1}{4}$            (d) $\frac{{\mathrm{Q}}_1}{2}$

A body of length 1m having cross sectional area 0.75 ${\mathrm{m}}^2$ has heat flow through it at the rate of 6000 Joule/sec. Then find the temperature difference if K = 200 ${\mathrm{Jm}}^{-1}{\mathrm{K}}^{-1}$ 

(a) 20°C                     (b) 40°C
(c) 80°C                     (d) 100°C

Air is a bad conductor of heat or partly conducts heat. Still, a vacuum is to be placed between the walls of the thermos flask because: