Tag: energy conversions

Questions Related to energy conversions

Work done by 100 calorie of heat is __________.

physics the essence of change different forms of energy energy conversions energy transformations and energy transfers

  1. 418.4 J

  2. 4.184 J

  3. 41.84 J

  4. None of these

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Correct Option: A
Explanation:
For an isothermal expansion of an ideal gas, the change in internal energy is zero.

According to the first law of thermodynamics, 

Change in internal energy U = Q-W = 0

So, all the heat energy is utilized to do work. 

Q = W

We know that, one calorie is equal to 4.184 J

Therefore, Work done by 100 calorie of heat in an isothermal expansion of any ideal gas will be 4.184 * 100 =  418.4 Joule


A person of weight 60 kg wants to loose 5 kg by going up and down 10m high stairs. Assume he burns twice as much fat while going up than going down. If 1 kg of fat is burnt on expending 7000 kcal. How many times must he go up and down to reduce his 7 kg weight? (Take $  g=10 \mathrm{ms}^{-2} )  $

physics the essence of change different forms of energy energy conversions energy transformations and energy transfers

  1. $ 1.8 \times 10^{3} $ times

  2. $ 2.4 \times 10^{3} $ times

  3. $ 1.7 \times 10^{3} $ times

  4. $ 2.1 \times 10^{3} $ times

Show answer
Correct Option: C
Explanation:
Energy used to go up $=mgh=60\times 10\times 10=6000\,J$

Energy used to come down $\dfrac{6000}{2}=3000\,J$

Energy used in one round trip $=9000\,J$

$1\,cal=4.5\,J$

$1\,J=\dfrac{1}{4.2\,cal}$

$9000\,J=\dfrac{9000}{4.2}=2142.85\,cal$

$7000\,kilo\,cal$ is required to burn $1\,kg$ mass

To reduce $5\,kg$ mass, energy required $=7000\times 5=35000\,kilo\,val$

Number of trip $=\dfrac{35000\times 1000}{2142.85}=1.7\times 10^{3}$

What is $\Delta E$  for a system that does 5 kcal of work by the surroundings when 3 kcal heat is absorbed by the system.  

physics the essence of change different forms of energy energy conversions energy transformations and energy transfers

  1. -2 kcal

  2. +2 kcal

  3. +8kcal

    • 7 kcal
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Correct Option: C

A bullet of mass 10 gm moving with a speed of 20 m/s hits an ice block of mass 990 gm kept on a frictionless floor and gets stuck in it. The amount of ice that melts, if 50% of the lost kinetic energy goes to ice, will be 

physics the essence of change different forms of energy energy conversions energy transformations and energy transfers

  1. $0.003 g$

  2. $0.30 g$

  3. $0.0003 g$

  4. $3.0 g$

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Correct Option: A

A steel ball of mass $5$ ${ g }$ is thrown downward with velocity $10$ ${ ms } ^ { - 1 }$ from height $19.5$ ${ m }$ . It penetrates sand by $50$ ${ cm }$ . The change in mechanical energy will be ( ${ g } = 10$ ${ ms } ^ { - 2 }$ )

physics the essence of change different forms of energy energy conversions energy transformations and energy transfers

  1. $1$ ${J}$

  2. $1.25$ ${J}$

  3. $1.5$ ${J}$

  4. $1.75$ ${J}$

Show answer
Correct Option: B
Explanation:

$\begin{array}{l} The\, \, change\, \, in\, \, mechanic\, \, energy\, \, \Delta U=mg\left( { h+x } \right) +\frac { 1 }{ 2 } m{ v^{ 2 } } \ here\, \, m=5g=0.00\, 5kg\cdot h=19.5\, mx=50cm=0.5m,v=10\, m/s \ So,\, \Delta U=0.005\times 10919.5+0.5+\frac { 1 }{ 2 } \times 0.005\times { \left( { 10 } \right) ^{ 2 } }=0.005\times 10\times 20+\frac { 1 }{ 2 } \times 0.005\times 100=1.25J \end{array}$

Hence,
option $(B)$ is correct answer.

A block of ice falls from certain height and completely melts. If only 3/4th of the energy is absorbed by the block the height of fall should be

physics the essence of change different forms of energy energy conversions energy transformations and energy transfers

  1. 48.4 m

  2. 84.4m

  3. 88.4m

  4. 44.8m

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Correct Option: D

A block of ice at 0 C whose mass is initially 50.0 kg slides along a horizontal surface starting at a speed of 5.38 m/s and finally coming of ice melted as a result of the friction between the block and the surface will be

physics the essence of change different forms of energy energy conversions energy transformations and energy transfers

  1. 2.16 g

  2. 4.0 g

  3. 1 g

  4. 50 g

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Correct Option: A

An ice block is falling from a height. If $75\%$ of the block melts then $h = (g = 10\ ms^{-2})$.

physics the essence of change different forms of energy energy conversions energy transformations and energy transfers

  1. $10.2\ Km$

  2. $12.8\ Km$

  3. $25.2\ Km$

  4. $26.8\ Km$

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Correct Option: A

A block of mass 500Kg has dimension $10 \times 5 \times 2m$. It is placed on a table with the largest surface in contract. The work that must be done to turn it an place it so that the smallest surface is in contact with the table  

physics the essence of change different forms of energy energy conversions energy transformations and energy transfers

  1. Zero

  2. 19600J

  3. 14700J

  4. 9800J

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Correct Option: C

A lead ball moving with velocity v strikes a wall and stops. I 50% of its energy is converted into heat, then what will b the increase in temperature? (Specific heat of lead is s)

physics the essence of change different forms of energy energy conversions energy transformations and energy transfers

  1. $\dfrac { 2{ v }^{ 2 } }{ Js } $

  2. $\dfrac { { v }^{ 2 } }{ 4Js } $

  3. $\dfrac { { v }^{ 2 }s }{ J } $

  4. $\dfrac { { v }^{ 2 }s }{ 2J } $

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Correct Option: B