Tag: transfer of heat

Quizzes (8) Questions (172)

Questions Related to transfer of heat

Multiple choice physics transfer of heat applications of convection convection thermal energy transfers

If there were no atmosphere around the earth, the temperature of the earth will

  1. Increase

  2. Decrease

  3. Increase during day and decrease during night

  4. Remain unaffected
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
If there were no atmosphere around the earth, the temperature will decrease.
Earth's atmosphere traps heat from sun and keeps it near to the surface of earth, popularly known as green house effect. It keeps warming the planet and sustaining life on earth. If there were no atmosphere, average temperature drops below zero.
So, we can say that it will decrease.
Multiple choice physics transfer of heat applications of convection convection thermal energy transfers

Why it is more hotter for same distance over the top of a candle than it in the side of its flame?

  1. Conduction of heat in air is upward

  2. Heat is maximum radiated in upward direction

  3. Radiation and conduction both contribute in transferring heat upwards

  4. Convection takes more heat in upward direction
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The hot air is less dense (lighter) than the cooler air around it, so it naturally goes upward, making the area above the flame hotter than the area to the sides of it.

On the side of the fire you’re only getting the radiant heat from the burn. On the top you are getting the convective hot air and actual burn chemicals (smoke, etc.) rising with the less dense air.
Hence the D option is correct .

Multiple choice physics transfer of heat applications of convection convection thermal energy transfers

Heat is flowing through two cylindrical rods made of same materials whose ends are maintained at similar temperatures. If diameters of the rods are in ratio 1 : 2 and lengths in the ratio 2 : 1, then the ratio of thermal current through in steady state is:

  1. 1 : 8

  2. 1 : 4

  3. 1 : 6

  4. 4 : 1
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The thermal current is nothing but the rate of flow heat,


$I=\dfrac{Q}{t}=K\Delta T\dfrac{A}{l}$. . . . . .(1)

Where,

$\Delta T= $ change in temperature

$A=\pi r^2$, cross-sectional area of the cylindrical rod.

$K=$ Thermal conductivity.

$l=$ length of the rod

From equation (1)

Thermal current is $I=4\pi K\Delta T \dfrac{d^2}{l}$

$I\alpha \dfrac{d^2}{l}$ where, $d=$ diameter of the cylindrical rod

Given,

$l _1:l _2=2:1$

$d _1:d _2=1:2$

The ratio of the thermal current  is,

$\dfrac{I _1}{I _2}=(\dfrac{d _1}{d _2})^2\times \dfrac{l _2}{l _1}$

$\dfrac{I _1}{I _2}=(\dfrac{1}{2})^2\times \dfrac{1}{2}$

$\dfrac{I _1}{I _2}=\dfrac{1}{8}$

$I _1:I _2=1:8$

Option A is correct.

Multiple choice physics transfer of heat applications of convection convection thermal energy transfers

A sphere ,a cone and a circular plate, all of same material and same mass are initially heated to same high temperature, then

  1. Plate will cool fastest and cone the slowest

  2. Sphere will cool fastest and the cone slowest

  3. Plate will cool fastest and the cone slowest

  4. Cone will cool fastest and plate slowest
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Cooling rate depends on surface area to volume ratio. A plate has the highest surface area for a given volume, leading to fastest cooling, while a sphere has the lowest, leading to slowest cooling.

Multiple choice physics transfer of heat applications of convection convection thermal energy transfers

Temperature of a room is ${ -15 }^{ 0 }C$ and outside temperature is ${ 10 }^{ 0 }C.$ If room temperature is made ${ 40 }^{ 0 }C$, then find outside temperature, if rate of heat flow is same in both cases. 

  1. ${ 10 }^{ 0 }C$

  2. ${ 5 }^{ 0 }C$

  3. ${ 15 }^{ 0 }C$

  4. ${ 20 }^{ 0 }C$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Rate of heat flow dQ/dt = k(T_hot - T_cold). Case 1: k(10 - (-15)) = k(25). Case 2: k(40 - T_outside) = k(25). Thus, 40 - T_outside = 25, so T_outside = 15 degrees Celsius.

Multiple choice physics transfer of heat applications of convection convection thermal energy transfers

Ram want to drink tea, when it is at $50^0C$. He ordered tea which arrives at a temperature of $80^0C$.tea takes one minute to cool from $80^0Cto60^0C$. If room temperature is $30^0C$, how long he has to wait to drink the tea?

  1. Less than one minute

  2. Two more minutes

  3. Half a minute

  4. Nearly three minute
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Newton's Law of Cooling states dT/dt = -k(T - T_env). For the first interval (80 to 60), average T is 70, difference is 40. For the second interval (60 to 50), average T is 55, difference is 25. Since the temperature difference is smaller, the cooling rate is slower, requiring more time.

Multiple choice physics transfer of heat applications of convection convection thermal energy transfers

A metallic sphere cools from $50^{\circ}C$ to $40^{\circ}C$ in $300\ s$. If the room temperature is $20^{\circ}C$, then its temperature in the next $5$ min will be

  1. $38^{\circ}C$

  2. $33.3^{\circ}C$

  3. $30^{\circ}C$

  4. $36^{\circ}C$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Using Newton's Law of Cooling: (T1 - T2)/t = k(T_avg - T_env). (50 - 40)/300 = k(45 - 20) => 10/300 = k(25) => k = 1/750. For the next 300s: (40 - T)/300 = (1/750)( (40+T)/2 - 20). Solving gives T = 30 degrees Celsius.

Multiple choice physics transfer of heat applications of convection convection thermal energy transfers

In natural convection, a heated portion of a liquid moves because

  1. Its molecular motion becomes aligned

  2. Of molecular collisions within it

  3. Its density is less than that of the
    surrounding fluid

  4. Of currents of the surrounding fluid
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

In natural convection, heating a fluid decreases its density, causing it to rise due to buoyancy forces, while cooler, denser fluid moves in to replace it.