Tag: physics

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Multiple choice energy efficiency energy transformations and energy transfers 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

  1. $1.36W$

  2. $13.6W$

  3. $0.136W$

  4. $136W$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Pressure P = h * rho * g = 0.12m * 13600 * 10 = 16320 Pa. Volume V = 5 * 10^3 cc = 5 * 10^-3 m^3. Work = P * V = 16320 * 5 * 10^-3 = 81.6 J. Power = Work / time = 81.6 J / 60s = 1.36 W.

Multiple choice energy efficiency energy transformations and energy transfers physics

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

  1. zero

  2. +ve

  3. -ve

  4. either of these
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Free expansion into a vacuum means there is no external pressure (P_ext = 0). Since work done W = integral(P_ext * dV), the work done is zero.

Multiple choice energy efficiency energy transformations and energy transfers physics

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:

  1. 10%

  2. 20%

  3. 30%

  4. 40%
Reveal answer Fill a bubble to check yourself
B Correct answer
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
$\eta = \dfrac{P _f}{P _i}$

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

$\eta = \dfrac{1}{5} \times 100$ 

$\eta = 20$%

Multiple choice energy efficiency energy transformations and energy transfers physics

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 ? ($g = 10 \ ms^{-1}$)

  1. $0.75 %$

  2. $83.3 %$

  3. $75 %$

  4. $17.5 %$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Efficiency is calculated as (useful work output / total energy input) * 100. The useful work is potential energy mgh = 100 * 10 * 25 = 25,000 J or 25 kJ. Efficiency = (25 kJ / 30 kJ) * 100 = 83.33%.

Multiple choice energy efficiency energy transformations and energy transfers physics

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:

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

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

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

  4. $\sqrt{32} $ m/s
Reveal answer Fill a bubble to check yourself
B Correct answer
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.
$\therefore \displaystyle \frac{1}{2} mv^2 = 9$
$\Rightarrow \displaystyle \frac{1}{2} \times 1 \times v^2 = 9$
$\Rightarrow v^2 = 18$
$\Rightarrow v = \sqrt{18} m/s$

Multiple choice energy efficiency energy transformations and energy transfers physics

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

  1. $39$%

  2. $69$%

  3. $93$%

  4. $96$%
Reveal answer Fill a bubble to check yourself
B Correct answer
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 %

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

Filament of torch bulb is ?

  1. A metal case

  2. Metal tip at the centre of the base

  3. 2 thick wires

  4. Thin wires
Reveal answer Fill a bubble to check yourself
D Correct answer
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.

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

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:


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

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

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

  4. $\frac { 2 P } { 6U }$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Energy stored U = (1/2)LI^2. Power dissipated P = I^2R. Time constant tau = L/R. From equations, L = 2U/I^2 and R = P/I^2. Thus, tau = (2U/I^2) / (P/I^2) = 2U/P.

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

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:

  1. open electric circuit

  2. closed electric circuit

  3. complete electric circuit

  4. both b and c
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

A circuit that provides a continuous path for current from one terminal to the other is called a closed or complete circuit.