Tag: physics

Questions Related to physics

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

Filament of torch bulb is ?

physics chemical effects of electric current study about cells electric cell types of cells

  1. A metal case

  2. Metal tip at the centre of the base

  3. 2 thick wires

  4. Thin wires

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Correct Option: D
Explanation:

A filament of a bulb is spirally wounded wire inside the bulb supported by two thick wires at its ends. The filament is wire, which is mostly made up of tungsten. 

As electric current passes through the filament, it glows to produce light.

An inductor coil stores $U$ when a current is passed through it and dissipates heat energy at the rate of $P .$ The time constant of the circuit when this connected across a battery of zero internal resistance is:


physics chemical effects of electric current study about cells electric cell types of cells

  1. $\frac { 4 U } { P }$

  2. $\frac { U } { P }$

  3. $\frac { 2 U } { P }$

  4. $\frac { 2 P } { 6U }$

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Correct Option: C

An electric circuit which starts from one terminal of a cell and ends at the other terminal of the cell without any break is called:

physics chemical effects of electric current study about cells electric cell types of cells

  1. open electric circuit

  2. closed electric circuit

  3. complete electric circuit

  4. both b and c

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Correct Option: D