Tag: energy conversions

Questions Related to energy conversions

In a hydroelectric power plant more electrical power can be generated if water falls from a greater height because

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

  1. its temperature increases

  2. larger amount of potential energy is converted into kinetic energy

  3. the electricity content of water increases with height

  4. more water molecules dissociate into ions

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Correct Option: B
Explanation:

Hydroelectric power plant uses the potential energy stored in water to generate electricity.
Therefore, if we increase the height of it's fall, energy stored will also increase and large amount of potential energy will be converted into kinetic energy, which means more electric energy will be generated.

Choose the incorrect statement regarding wind power.

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

  1. Energy  from moving wind is wind energy.

  2. The potential energy content of wind blowing at high altitudes is the source of wind power.

  3. Wind hitting at the blades of a windmill causes them to rotate. The rotation thus achieved can be utilized further.

  4. One possible method of utilising the energy of rotational motion of the blades of a windmill is to run the turbine of an electric generator.

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Correct Option: B
Explanation:

The energy that we harness from a windmill is the wind energy due to moving air.
Here, option B is wrong as the wind energy does not depend on the altitude when we harness energy from the windmill.

Water falls from a height 500 m. What is the rise in  temperature of water at bottom if whole energy remains in the water?

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

  1. $0.96^0C$

  2. $1.02^oC$

  3. $1.17^oC$

  4. $0.23^oC$

Show answer
Correct Option: C
Explanation:
Water is at height $(h) = 500 \,m$.
Potential energy $= mgh$
$= 1 \times 9.81 \times 500$
$= 4905 \,J/litre$

Specific heat of water $= 4180 \,J/kg \,k$.
So, $E = mc \Delta T$.
$4905 = 1 \times 4180 \times \Delta T$
$\Delta T = \dfrac{4905}{4180}$
$\Delta T = 1.16^o C$.

When we rub our hands, which of the following is true?

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

  1. Mechanical energy converted to heat energy.

  2. kinetic energy converted to heat energy.

  3. Heat energy converted to mechanical energy.

  4. Heat energy converted to kinetic energy.

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Correct Option: A

A body off mass $5kg$ falls from a height of $30metre$. If its all mechanical energy is changed into heat, then heat produced will be 

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

  1. $ \text{350 cal} $

  2. $ \text{150 cal} $

  3. $ \text{60 cal} $

  4. $ \text{6 cal} $

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Correct Option: A

a piece of lead falls from a height of $100m$ on a fixed non-conducting slab which brings it to rest. If the specific heat of lead is $30.6{\rm{ }}cal/kg{\,^ \circ }C,$, the increase in temperature of the slab immediately after collision is 

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

  1. ${6.72^ \circ }C$

  2. ${7.62^ \circ }C$

  3. ${5.62^ \circ }C$

  4. ${8.72^ \circ }C$

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Correct Option: D

A metal ring of mass  2.1 kg and of 10 cm radius is revolving about its axis, Making 100/$\pi $ revoluation/s in the liquid is equal to 

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

  1. 100 cal

  2. 1000 cal

  3. 2100 cal

  4. 10 cal

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Correct Option: A

The temperature inside and outside a refrigerator are $273\ K$ and $300\ K$ respectively Assuming that the refrigerator cycle is reversible, for every joule of work done, the heat delivered to the surrounding will be nearly

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

  1. $11\ J$

  2. $22\ J$

  3. $33\ J$

  4. $50\ J$

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Correct Option: D

The height of the Niagara falls is $50$ metres, $(1 cal= 4.2 \mathrm\ { J }).$  Assume its mechanical energy can be completely converted into heat energy.

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

  1. Heat energy gained by each gram of water is $49 \times 10 ^ { 5 }\ \mathrm { cal }$

  2. Rise in temperature of water is $0.166^{ o }\ C/g$

  3. Rise in temperature of water is $0.12^{ o }\ C$

  4. Heal energy gained is $500\ joule/g$

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Correct Option: C,D

The units of fore and length are made three times of their earlier values. Earlier the energy of a system was $81\ J$. What will be the energy of the same system in new units?

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

  1. $243$

  2. $729$

  3. $9$

  4. $None\ of\ the\ above$

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Correct Option: B