Tag: heat energy transfers

Questions Related to heat energy transfers

In the Ingen Hausz's experiment, the wax melts up to lengths $10cm$ and $25cm$ on two identical rods of different materials. The ratio of thermal conductivities of the two materials is

physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

  1. $1:6.25$

  2. $6.25:1$

  3. $1: $$\sqrt{2.5}$

  4. $1:2.5$

Show answer
Correct Option: D
Explanation:

In the Ingen Hauszs experiment, as rate of heat transfer is constant
$\dfrac { { K } _{ 1 } }{ { K } _{ 2 } } =\dfrac { { l } _{ 1 } }{ { { l } _{ 2 } } } $
So, putting the value in above formula's we find
$\dfrac{{K} _{1}}{{K} _{2}}=\dfrac{10}{25}=\dfrac{1}{2.5}$

Two blocks of steel A and B, A being two times heavier than B, are at 40$^o$C. The ratio of heat content of A to B is:

physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

  1. 1

  2. 4

  3. 2

  4. $\displaystyle \frac{1}{2}$

Show answer
Correct Option: C
Explanation:
Let the mass of block B be $m$.
So, mass of block A is $2m$
Both are made of steel, so, both has same specific heat capacity (let 'S')

So, $\cfrac{Heat\;Capacity\;(A)}{Heat\;Capacity\;(B)}=\cfrac{2ms}{1ms}=2$

Electric heaters used for cooking have the filament or the heating coil on a plate made up of clay because:

physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

  1. clay is a bad conductor of heat

  2. clay is a bad conductor of electricity

  3. clay is necessary for good taste

  4. both A & B

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Correct Option: D
Explanation:
Electric heaters used for cooking have the filament or the heating coil on a plate made up of clay because clay is bad conductor of heat and electricity.

Heat capacity of clay is $0.33cal/g^oc$ so bad conductor of heat.
And clay is an insulator, so bad conductor of electricity.

Thermocole lined box is used as a cold storage because.

physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

  1. Thermocole is a bad conductor

  2. It is cheaper

  3. It is easier to construct

  4. None of these

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Correct Option: A
Explanation:

Thermocole lined box is used as cold storage because these material is a poor conductor of heat . And it has millions of tiny air bubbles that show the progress of heat through material.

There are two hollow spheres made of different materials, one with double the radius and one fourth wall thickness of the other are filled with ice. If the time taken for melting the ice completely in larger sphere is $25$ minutes and that for smaller sphere is $16$ minutes, the ratio of the thermal conductivity of the larger sphere to the smaller sphere is :

physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

  1. $4:5$

  2. $5:4$

  3. $25:8$

  4. $8:25$

Show answer
Correct Option: D
Explanation:

Smaller sphere

Radius $\to \dfrac{R}{2}$. wall thickness $\to 4d$

Larger sphere
Radius $\to R$, wall thickness $\to d$

Vol of ice in smaller sphere $= \dfrac{4}{3} \pi \left(\dfrac{R}{2} - 4d\right)^3$

$m _S = $ Mass of ice in smaller sphere $=\rho _{ice} \times \dfrac{4}{3}\pi \left(\dfrac{R}{2} - 4d\right)^3$

$m _L = $ Mass of ice in larger sphere $= \rho _{ice} \times \dfrac{4}{3} \pi (r-d)^3$

Heat required to melt $m _S = m _sL = \dfrac{4}{3}\pi \left(\dfrac{R}{2} 4d\right)^3\times \rho _{ice} \times L = Q _s$        ...(i)

Heat required to melt $m _L = m _l L = \dfrac{4}{3} \pi (R-d)^3 \times \rho _{ice} \times L = Q _L$          ...(ii)

$\dfrac{Q}{t} = \dfrac{KA}{x} (\Delta T)$

$\dfrac{Q _s}{16} = \dfrac{K _s4\pi\left(\dfrac{R}{2}\right)^2}{4d}\Delta T$      ...(for small sphere)      ...(1)

$\dfrac{A _L}{25} = \dfrac{K _24\pi R^2}{d}\Delta T$        ...(for large sphere)   ....(2)

Dividing eq(1)  by eq(2)
$\therefore \dfrac{Q _L}{25} \times \dfrac{16}{Q _s} = \left(\dfrac{K _L}{K _S}\right)\dfrac{4\pi R^2\Delta T\times 4d}{4\pi\left(\dfrac{R}{2}\right)^2\Delta T \times d}$

$\therefore \dfrac{Q _L}{Q _S} \times \dfrac{16}{25} = \left(\dfrac{K _L}{K _S}\right)\times 16$

but $\dfrac{Q _L}{Q _S} = \dfrac{(R-d)^3}{\left(\dfrac{R}{2}-4d\right)^3}$

here $R > > d$,

$\therefore \dfrac{Q _L}{Q _S} = \dfrac{R^3}{\left(\dfrac{R}{2}\right)^3} = 8$

$\therefore \dfrac{8}{25} = \dfrac{K _L}{K _S}$

a corked bottle full of water when frozen will break because.

physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

  1. The volume of water increases on freezing

  2. The volume of water decreases on freezing

  3. The bottle contracts on freezing

  4. Glass is a bad conductor of heat

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Correct Option: A
Explanation:
A corked bottle of water on being frozen at $0^{\circ}$ undergoes anomalous expansion of water between $4^{\circ}$ and $0^{\circ}$ wherein density of water increases on cooling , causing expansion due to orderly hydrogen bonding, this increasing volume cracks the bottle.
$Density=\cfrac{Mass}{Volume}$
$\Rightarrow D\propto \cfrac{1}{V}$
$\Rightarrow D\uparrow\Rightarrow V\downarrow$

Read the given statements and select the correct option.
Statement 1: An iron ball at ${35}^{o}C$ is dropped in a mug containing water at $305.15K$. Then the heat will flow from the water to the iron ball.
Statement 2: In convection, heated molecules of a liquid themselves move to carry heat from the hot region to the cold region.

physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

  1. Both statement 1 and 2 are true and statement 2 is the correct explanation of statement 1

  2. Both statement 1 and 2 are true and statement 2 is not the correct explanation of statement 1

  3. Statement 1 is true but statement 2 is false

  4. Statement 1 is false and statement 2 is true

Show answer
Correct Option: D
Explanation:

${35}^{o}C$ is equivalent to $(273.15+35)308.15K$. So iron ball is at higher temperature and heat will flow from iron ball to the water.

In a room contain air , heat can go from one place to another

physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

  1. By condition only

  2. by convection only

  3. By radiation only

  4. By all the three modes

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Correct Option: D

Which of the following is a good conductor of heat?

physics transfer of heat heat and heat transfer heat energy transfer heat energy transfers fundamentals of heat transfer conduction

  1. Steel

  2. Glass

  3. Wood

  4. Rubber

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Correct Option: A
Explanation:

Steel being an alloy of metals, it has free electrons in its atoms. They start moving along with the heat flow and help in the conduction of heat. All metals are good conductors. The other three being non-metals, do not have free electrons. Hence they are bad conductors

Heat travels much faster in insulators than in conductors.

physics transfer of heat heat and heat transfer heat energy transfer heat energy transfers fundamentals of heat transfer conduction

  1. True

  2. False

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Correct Option: B
Explanation:
The statement if false, heat travels much faster in insulators than in conductors.
Heat travels faster in conductor. Insulators donot even conduct heat, When heat is consumed by the conductor, it has 3 modes of transfer of heat conduction, convection and radiation.