Tag: power

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Multiple choice physics work and energy commercial unit of energy power work and power

When we pay for our electricity bill, we are paying for the ____________.

  1. charge used

  2. current used

  3. power used

  4. energy used
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

A unit is defined as $kWh$, means a $1000 joule/ sec$ is used for $1 hour$ ,

$1KwH=1000\times 3600=3.6\times10^6joules$
we pay for unit and that is energy 

Multiple choice physics work and energy commercial unit of energy power work and power

$1kWh= $ _________?

  1. $3600000\ J$
  2. $10000\ J$
  3. $4.2\ J$
  4. $25000\ J$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
Kilowatt hour is the energy consumed by a body of power $1\ kW$ in $1\ hr$. 
Hence, 
$1\ kWh = 1 kW \times 1\ hr$
              $=  10^3 W \times 3600\ s$
              $= 3600000\ J$
Multiple choice physics work and energy commercial unit of energy power work and power

Kilowatt-hour is the unit of :

  1. potential difference.

  2. electric power.

  3. electrical energy.

  4. charge.
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The kilowatt-hour (symbolized kWh) is a unit of energy equivalent to one kilowatt (1 kW) of power expended for one hour. The kilowatt-hour is commercially used as a billing unit for energy delivered to consumers by electric utilities.

Multiple choice physics work and power commercial unit of energy power work and energy

A body is moved along a straight line by a machine delivering a constant power. The distance moved by the body in time $t$ is proportional to  :

  1. ${t^{3/4}}$
  2. ${t^{3/2}}$
  3. ${t^{1/4}}$
  4. ${t^{1/2}}$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Let's consider a body is moved along a straight line by a machine delivering a constant power $P$. The distance moved by the body is $S$. 


Power, $P=F.v$.. . . . . . (1)


Force, $F=ma$ . . . . . . . .(2)

where, $v=\dfrac{S}{t}$ 

acceleration, $a=\dfrac{S}{t^2}$

$m=$ mass

from  equation (1) and equation (2), we get

$P=\dfrac{mS}{t^2}\times \dfrac{S}{t}$

$S^2=\dfrac{Pt^3}{m}$

From the above equation, we get

$S^2\propto t^3$

$S\propto t^{3/2}$

The correct option is B.

Multiple choice physics work and power commercial unit of energy power work and energy

A body is moving along a straight line delivering power given as P = at, then work (W) done is given for time 0 to t is

  1. W = a
  2. W = $\frac{1}{2}a t^2$
  3. W = $2at^2$
  4. W = $a^2t^2$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Given that ,

Power , $P= at$
We know that 
$P=\dfrac{dW}{dt}$

$\implies \dfrac{dW}{dt}= at$

$\implies dW= at dt$
$\implies W= \int _0^t atdt$
$\implies W=\dfrac12 at^2$

$\therefore $ Work done for time 0 to t ,   $W=\dfrac12 at^2$  

Multiple choice physics work and power commercial unit of energy power work and energy

A force of $ 2\hat { i } +3\hat { j } +2kN $ acts on a body for 4 s and produces a displacement of $3\hat {i} +4\hat {j} +5 \hat {k} m $ calculate the power ?

  1. 5 w

  2. 6 w

  3. 7 W

  4. 9 w
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Given that,

Force  ,$F= 2 \hat i + 3\hat j + 2\hat k $  N
Displacement , $S= 3\hat i + 4\hat j +5 \hat k $ m 
Time Taken , $t= 4\ s$

Power, $P=\dfrac Pt = \dfrac{F\cdot S}{t}= \dfrac{6+12+10}{4}= \dfrac{28}4 = 7\ W$

Multiple choice physics work and power commercial unit of energy power work and energy

A pump of $200W$ power is lifting $2kg$ water from an average depth of $10m$ in one second. Velocity of water delivered by the pump is :

$(g=10m/s^2)$

  1. $10m/s$
  2. $2m/s$
  3. $4 m/s$
  4. $1 m/s$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

acceleration due to gravity $g = 10m/sec^2$

height $H = 10 meters$
mass $= 2kg$
Potential energy $= mgh = 2\times 10 \times 10 = 200J$
as power $= \dfrac{work \ done}{time}$
when power of motor $= 200w$
$200 = 200/t$
$\Rightarrow t = 1sec$
here displacement of water= height $= 10m$
time = 1sec
Hence,
Velocity $V= \dfrac{Displacement}{Time}$
              $V= \dfrac{10}{1}$
              $V= 10m/sec$

Multiple choice physics work and power commercial unit of energy power work and energy

A small diesel engine uses a volume of $1.5 \times 10^4\, cm^3$  of fuel per hour to produce a useful power
output of 40 kW. It may be assumed that 34 kJ of energy is transferred to the engine when it uses $1.0\, cm^3$  of fuel.
What is the rate of transfer from the engine of energy that is wasted?

  1. 850 kW

  2. 920 kW

  3. 840 kW

  4. 810 kW
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Energy produced by $1.0\ cm^3$ of fuel = $34\ kJ$

So, energy produced by  $1.5 \times 10^4\ cm^3$ of fuel in one hour = $1.5 \times 10^4 \times 34\ kJ$
                                                                                                   = $5.1 \times 10^5\ kJ$
Energy produced in one second = $\dfrac{5.1 \times 10^5}{60}\ kJ/s$
                                                         = $850\ kW$
So, rate of energy wasting = $(850-40)\ kW$
                                             = $810\ kW$

Multiple choice physics work and power commercial unit of energy power work and energy

A body projected vertically from the earth reaches a height equal to earth's radius before returning to the earth. The power exerted by the gravitational force is greatest :

  1. at the highest position of the body

  2. at the instant just before the body hits the earth

  3. it remains constant throughout

  4. at the instant just after the body is projected
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$Power,$ $P$ $=$$\overrightarrow{F}$.$\overrightarrow{v}$ $=$ $Fv$ $cos$$\theta$
$Just$ $before$ $hitting$ $the$ $earth$ $θ$ $=$ $0°.$ $Hence,$ $the$ $power$ $exerted$ $by$ $the$ $gravitational$ $force$ $is$ $greatest$ $at$ $the$ $instant$ $just$ $before$ $the$ $body$ $hits$ $the$ $earth.$