Tag: energy conversions

Questions Related to energy conversions

A Wind-mill converts mechanical energy to ________ energy

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

  1. chemical

  2. electrical

  3. solar

  4. potential

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Correct Option: B
Explanation:

Wind power is generated by the force wind exerts on the blades of a turbine, causing the turbine's shaft to rotate at a speed of 10 to 20 revolutions per minute (rpm). The rotor shaft is connected to a generator that converts mechanical energy into electrical energy.

A cyclist free-wheels from the top of a hill, gathers speed going down the hill, apply his brakes and eventually came to rest at the bottom of the hill. Which one of the following energy changes take place?

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

  1. Potential to kinetic to heat energy

  2. Kinetic to potential to heat energy

  3. Chemical to heat to potential energy

  4. Kinetic to heat to chemical energy

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Correct Option: A
Explanation:

Considering ground as zero potential
Initially $E = U + K = U + 0 = U$
Finally $E = U + K = 0 + 0 = 0$
So, initially it had some potential energy, it was converted to kinetic energy by gaining speed, it finally became zero so energy must have been lost as heat while applying brakes.

The temperature of water at the bottom of a large waterfall is generally higher than that of the water at the top, because

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

  1. the falling water absorbs heat from the sun

  2. the KE of the falling water is converted into heat

  3. the water at the bottom has greater PE

  4. rocks on the bed of the river give out heat

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Correct Option: B
Explanation:

Water is slightly warmer at the bottom of a waterfall than at the top because the gravitational potential energy is converted first to kinetic energy of the falling water, and finally to heat energy as it comes to rest at the bottom.

The temperature of water at the bottom of a waterfall is higher than that ofthe water at the top, because

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

  1. falling water absorbs heat from the sun

  2. kinetic energy of the falling water is converted into heat

  3. water at the bottom has greater potential energy

  4. the rocks on the river bed give out heat

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Correct Option: B
Explanation:

When water falls from top of a waterfall, it has a force of friction being exerted on it due to which some of its kinetic energy gets converted into heat energy.

Thus the temperature of water at the bottom of waterfall is higher than that at the top.

A pendulum rocks back and forth. In an ideal setting, this motion would go on perpetually without loss of mechanical energy. Realistically, if you held a pendulum up and made it go in motion, it would slow to a stop.
What is responsible for the pendulum gradually losing mechanical energy? Ignore the mass of the string holding the pendulum bob in motion.

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

  1. The work done by gravity

  2. The work done by tension

  3. The work done by air resistance

  4. The pendulum does not lose mechanical energy

  5. There is not enough information to determine the cause

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Correct Option: C
Explanation:

The vibrations of the pendulum die after some time , it is because of the frictional force of air which opposes the motion of bob .So air resistance is responsible for the loss in mechanical energy .

The correct sequence of energy changes in hydroelectric power station is:

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

  1. Kinetic to potential to mechanical to electrical

  2. Potential to kinetic to mechanical to electrical

  3. Potential to electrical to mechanical

  4. Potential to electrical to kinetic to mechanical

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Correct Option: A
Explanation:

First of all moving water is stored against a dam and then it is allowed to fall on turbine and then as turbine rotates it will produce electricity.

So sequence of energy conversion is 
Kinetic to potential to mechanical to electrical.
Hence option A is best possible.

Electric heater converts :

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

  1. electric energy into chemical energy

  2. chemical energy into electric energy

  3. heat energy into electric energy

  4. electric energy into heat energy

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Correct Option: D
Explanation:

An electric heater is an electrical device that converts electric current to heat. The heating element inside every electric heater is an electrical resistor, and works on the principle of Joule heating: an electric current passing through a resistor will convert that electrical energy into heat energy.

The height of water in a dam reduces by $20  m$, which is used to generate electricity. The water further fell by $10  m$ into the tunnel to strike the turbine plates. If the volume of water is ${10}^{4}  {m}^{3}$, find the hydel energy generated. Assume all the potential energy of the water being converted into electricity. Take $g = 10  m{s}^{-2}$.

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

  1. 2000 MJ

  2. 200 MJ

  3. 1000 MJ

  4. 500 MJ

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Correct Option: A
Explanation:

The potential energy of the stored water is converted into kinetic energy which is then converted into electrical energy.
The potential energy of water $= mgh$        ...... (1)
The volume of water $= 10000  {m}^{3}$.
The mass of water $=$ volume $\times$ density $= 10000 \times 1000 = {10}^{7}  kg$.
Also, the depth of water $= 20  m$.
$\therefore$ The height of CG of stored water $= 10  m$.
The total height through which water falls $= 20  m$.
Substituting it in (1), we get potential energy of water $= {10}^{7} \times 20 \times 10 = 2000  MJ$.