Tag: physics

Questions Related to physics

Three rods of identical cross-sectional area and made from the same metal from the sides of an isosceles triangles ABC, right-angled at B. The point A and B are maintained at temperatures T and $(\sqrt{2})$T respectively. In the steady state, the temperature of the point C is $T _C$. Assuming that only heat conduction takes place, $T _C/T$ is?

  1. $\dfrac{1}{2\left(\sqrt{2}-1\right)}$

  2. $\dfrac{3}{\sqrt{2}+1}$

  3. $\dfrac{1}{\sqrt{3}\left(\sqrt{2}-1\right)}$

  4. $\dfrac{1}{\sqrt{2}+1}$


Correct Option: B

If two bodies A and B of $50^{0} C$ comes in contact with each other. The heat will flow 

  1. Body A $\rightarrow$ Body B

  2. Body B $\rightarrow$ Body A

  3. nowhere

  4. from both the bodies


Correct Option: C
Explanation:

Both the bodies are at same temperature, so there will be no transfer of heat from one body to other body.

If the pressure of a gas is doubled then its thermal conductivity will :

  1. remain constant

  2. decrease

  3. decrease exponentially

  4. increase


Correct Option: A
Explanation:

Thermal conductivity of a gas is independent of the pressure as compared to solid and liquid. So the thermal conductivity of gas will remain constant.

If $K$ denotes coefficient of thermal conductivity, $d$ the density and $C$ the specific heat, the unit of $X$, where $X = K/dc$, will be

  1. $cm\space sec$

  2. $cm^2\space sec^{-2}$

  3. $cm \space sec^{2}$

  4. $cm^2 \space sec^{-1}$


Correct Option: D
Explanation:

The Units of the respective quantities in SI are:

  • $[K] = J(mKs)^{-1}$
  • $[\rho] = kg(m)^{-3}$
  • $[c] = J(kgK)^{-1}$
$[X] = \dfrac{[K]}{[\rho] [c]} = \dfrac{J(mKs)^{-1}}{(kg(m)^{-3})(J(kgK)^{-1})}$
$[X] = m^{2}s^{-1}$ or in CGS $cm^{2}s^{-1}$

A body of length 1 m have an area of cross-section as 0.75 $m^{2}$. If rate of heat conduction of the body is 6000 J/s and coefficient of thermal conductivity is 200 $Jm^{-1}$ $K^{-1}$, then the temperature difference between the two ends of the body is

  1. $30^{\circ}C$

  2. $20^{\circ}C$

  3. $40^{\circ}C$

  4. $80^{\circ}C$


Correct Option: C

Two rods one hallow and the other solid made of the same material have the same length of $20 cm $ and radius of $2 cm$. When their temperature is increased through the same amount of $50^oC$, their expansion ratio $E _h:E _s$ will be:

  1. $1:8$

  2. $1:4$

  3. $1:1$

  4. $1:2$


Correct Option: A

Equal temperature difference exists between the ends of two metallic rods $1 $ and $2$ of length. Their thermal conductivities are $K _1$ and $K _2$ and cross sectional areas represents $A _1$ and $A _2$. The condition for equal rate of heat transfer is:

  1. $K _1A _2=K _2A _1$

  2. $K _1A _1=K _2A _2$

  3. $K _1A _1^2=K _2A _2^2$

  4. $K _1^2A _2=K _2^2A _1$


Correct Option: B
Explanation:
Equal temperature difference exists between the two metallic rods 1 and 2.
The heat transfer$=\dfrac{KA \Delta T}{l}$
For equal rate of heat transfer,
$\dfrac{K _1 A _1 \Delta T}{l}=\dfrac{K _2A _2 \Delta T}{l}$
$K _1A _1=K _2A _2$
The correct option is B.

Conduction of heat is possible:

  1. when the bodies are apart from each other

  2. when the bodies have same temperature and in thermal contact

  3. when they have different temperatures maintaining distance between them

  4. bodies should be in contact and should have different temperatures


Correct Option: D
Explanation:

Conduction is only possible when the bodies in contact have a temperature difference.

More energetic molecules of a body transfer some of their energy to other molecules, without any change in their position in:

  1. conduction

  2. convection

  3. radiation

  4. none of these


Correct Option: A
Explanation:

Conduction is defined as the process of transferring heat from molecule to molecule, without the actual movement of molecules.

Here, more energetic molecules of a body transfer some of their energy to other molecules, without any change in their position.

The thermal conductivity of the plate depends upon

  1. the temperature difference between the two sides

  2. the thickness of the plate

  3. the area of the plate

  4. nature of the material


Correct Option: D
Explanation:

Thermal conductivity of any object depends solely on the material of the object.
It is an intrinsic property and is independent on dimensions/mass/etc of the body.
For example, all iron rods will have same thermal conductivity.
All copper rods will have same thermal conductivity.
But, thermal conductivity of iron rod will be different from that of copper rod.