Tag: transfer of heat

Questions Related to transfer of heat

A sphere, a cube and a thin circular plate, all of same material and same mass, are initially heated to same high temperature. Choose the correct statement.

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

  1. The plate will cool fastest and cube the slowest

  2. The sphere will cool faster and cube and the slowest

  3. The plate will cool faster and sphere the slowest

  4. The cube will cool fastest and plate the slowest.

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Correct Option: C

A metal cylinder of mass 0.5 kg is heated electrically by a 12 W heater in a room and cylinder temperature rises uniformly to $ 25^oC in 5 min $ excess temperature surroundings,

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

  1. The rate of loss of heat of the cylinder to surrounding at $ 20^oC $ is 2 W

  2. The rate of loss heat of the cylinder to surrounding at $ 45^oC $ is 12 W

  3. Specific heat capacity of metal is $ \frac {240}{Ib(3/2)} $

  4. None of these

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Correct Option: B

Heat current is maximum in which of the following (rods are of identical dimension)

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

  1. Copper

  2. Copper steel

  3. Steel Copper

  4. Steel

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Correct Option: A

Assertion (A): When hot water is poured in athick glass tumbler then the tumbler breaks.
Reason (R): Glass is a bad conductor of heat

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

  1. A and R are correct and R is correct explanation for A

  2. A and R are correct and R is not correct explanation for A

  3. A is true and R is false

  4. A is wrong and R is true.

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Correct Option: A
Explanation:

Since glass is bad conductor of heat the heat will try to escape but the glass would not allow it and in the process ultimately glass breaks down.

Metals are good conductors of heat because

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

  1. They contain large number of free electrons

  2. Their atoms are relatively apart

  3. Their atoms collide frequently

  4. They have reflecting surfaces

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Correct Option: A
Explanation:

Free electrons are not associated to any atom in particular. They are usually the valence electrons of the metal atoms and are loosely bound. These electrons form a "charge fluid" that travels from one place to other in the conductor and thus convey electric current.

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. $1:2.5$

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

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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. $8:25$

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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 $R > > d$,

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

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