Tag: energy transformations and energy transfers

Questions Related to energy transformations and energy transfers

The power of a heart which pumps $5\times{10}^{3}cc$ of blood per minute at a pressure of $120mm$ of mercury ($g=10m{s}^{-2}$ and density of $Hg=13.6\times{10}^{3}kg/m$) is

energy efficiency energy transformations and energy transfers physics

  1. $1.36W$

  2. $13.6W$

  3. $0.136W$

  4. $136W$

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Correct Option: A

The work done during the process when 1 mole of gas is allowed to expand freely into vacuum is:

energy efficiency energy transformations and energy transfers physics

  1. zero

  2. +ve

  3. -ve

  4. either of these

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Correct Option: A

A man weighing 60 kg lifts a body of 15 kg to the top of a building 10 m high in 30 minutes. His efficiency is:

energy efficiency energy transformations and energy transfers physics

  1. 10%

  2. 20%

  3. 30%

  4. 40%

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Correct Option: B
Explanation:

Here, a man weighing 60 kg lifts a body of 15 kg to the top of a building 10 m high in 30 minutes. initially, while lifting the body(doing work) the power is 
$P _i = \dfrac{mgh}{t}$

$P _i = \dfrac{(60+15)(10)(10)}{(30)(60)}$

$P _i = \dfrac{7500}{1800}$

After reaching to the top the power is

$P _f = \dfrac{mgh}{t}$

$P _f = \dfrac{(15)(10)(10)}{(30)(60)}$

$P _f = \dfrac{1500}{1800}$

The efficiency of the man is the ratio of final power to the initial one
$\eta = \dfrac{P _f}{P _i}$

$\eta = \dfrac{\dfrac{1500}{1800}}{\dfrac{7500}{1800}} \times 100$ 

$\eta = \dfrac{1}{5} \times 100$ 

$\eta = 20$%

A motor has an electrical input of $30 kJ$ and is used to raise $100 kg$ load to a height of $25 m$ when fired to a crane winch. What is the efficient of winch ? ($g = 10 \ ms^{-1}$)

energy efficiency energy transformations and energy transfers physics

  1. $0.75 %$

  2. $83.3 %$

  3. $75 %$

  4. $17.5 %$

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Correct Option: B

A machine which is 75% efficient, uses 12 J of energy in lifting 1 kg mass through a certain distance. The mass is then allowed to fall through the same distance. The velocity at the end of its fall is:

energy efficiency energy transformations and energy transfers physics

  1. $\sqrt{12} $ m/s

  2. $\sqrt{18} $ m/s

  3. $\sqrt{24} $ m/s

  4. $\sqrt{32} $ m/s

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Correct Option: B
Explanation:

Efficiency = 75%, Input energy = 12 J
$\therefore \displaystyle \frac{75}{100} = \frac{\text{Output energy}}{\text{Input energy}}$
$\Rightarrow $ Output energy $= \displaystyle \frac{75}{100} \times 12 = 9 J$
$\therefore$ P.E. of the mass = 9 J
At the end of the fall it will be converted to K.E.
$\therefore \displaystyle \frac{1}{2} mv^2 = 9$
$\Rightarrow \displaystyle \frac{1}{2} \times 1 \times v^2 = 9$
$\Rightarrow v^2 = 18$
$\Rightarrow v = \sqrt{18} m/s$

An installation consisting of an electric motor driving a water pump left $75 L$ of water per second to a height of $4.7 m$. If the motor consumes a power of $5 kW$, then the efficiency of the installation is

energy efficiency energy transformations and energy transfers physics

  1. $39$%

  2. $69$%

  3. $93$%

  4. $96$%

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Correct Option: B
Explanation:

Power consumed by motor$5kW= 5 \times 10^3 W= 5000W$

power used i lifting water = $\dfrac{mgh}{t}=7.5 \times 9.8 \times 4.7= 3454.5 W$
Efficiency = $\dfrac{\text{Power used}}{\text{Power consumed}} \times 100$% = $\dfrac{3454.5}{5000} \times 100$% = 69 %

A man of 60 kg gains 1000 cal of heat by eating 5 mangoes. His efficiency is 56%. To what height he can jump by using this energy?

energy efficiency energy transformations and energy transfers physics

  1. 4m

  2. 20 m

  3. 078 m

  4. 0.2 m

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Correct Option: C

A domestic gas cylinder contains about 14.2 kg of LPG. A strong smelling substance called ethyl mercaptan is added to the LPG to detect the leakage of gas from the cylinder. On being lighted, it burs with a blue flame. One gram of LPG produces about 50 kJ of heat. Weight of the domestic cylinder is -

physics energy transformations and energy transfers law of conservation of energy the law of conservation of energy types of energy

  1. 142 N

  2. 152 N

  3. 132 N

  4. 122 N

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Correct Option: A

According to the law of conservation of energy:

physics energy transformations and energy transfers law of conservation of energy the law of conservation of energy types of energy

  1. energy exists in only one form

  2. energy can be created but not destroyed and it can be transformed from one form to another

  3. energy exists in many forms but it cannot be transformed

  4. energy can neither be produced nor be destroyed and it can be transformed from one form to another

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Correct Option: D
Explanation:

The law of conservation of energy states that the total amount of energy in a system remains constant ("is conserved"), although energy within the system can be changed from one form to another or transferred from one object to another.

Energy cannot be created or destroyed, but it can be transformed.

According to law of conservation of energy :

physics energy transformations and energy transfers law of conservation of energy the law of conservation of energy types of energy

  1. energy can be created but not destroyed and it can be transformed from one form into another

  2. energy exists in only one form

  3. energy exists in many forms but it cannot be transformed

  4. energy can neither be created nor be destroyed but can be changed into different forms

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Correct Option: D
Explanation:

$Law\ of conservation\ of\ energy$ states that the total energy of an isolated system remains constant and it is said to be conserved over time. Energy can neither be created nor be destroyed, but it can be transformed from one form to another, for instance, chemical energy can be converted to kinetic energy in the explosion of a stick of dynamite.