Tag: power

Questions Related to power

A two kilowatt motor pumps out water from a well 10 meter deep. The quantity of water pumped out per hour  is ( g = 10 $m/s^2$)

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

  1. $72 \times 10^2$ kg

  2. $72 \times 10^3$ kg

  3. $72 \times 10^4$ kg

  4. $72 \times 10^5$ kg

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Correct Option: B
Explanation:

Two kilowatt motor  pump means that 2kJ wotk is done in $1$ sec 

Let $m$ be the mass of water that is pumped out 
Work done , $W=2\ kJ=2000\ J$
Time taken , $t=1 sec$

Displacement of water , $h= 10\ m$

As in this case  $W=mgh $

$\implies 2000= m\times 10\times 10$
$\implies m=20\ kg$

Quantity of water pumped out per sec , $m=20\ kg$

$\therefore \text{ Quantity of water that is pumped out per hour }= m\times 3600=20\times 3600= 72\times 10^3 \ kg $

If the heart pushes 1 cc of blood in one second under pressure 20000 $N/m^2$ the power of heart is:

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

  1. 0.02 W

  2. 400 W

  3. 20 mW

  4. 0.2 W

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Correct Option: A
Explanation:

Given that , 
Pressure ,$P=2000 N/m^2$
Change in Volume = $1 cc= 10^{-6 } L $

$\text{Power}=\dfrac{\text{workdone}}{\text{time}}=\dfrac{\text{pressure }\times \text{ change in volume}}{\text{time}}=\dfrac {2000\times 10^{-6}}{1}= 0.02 \ W$

The pump of a water pumping system uses $2.0\ kW$ of electrical power when raising water. The
pumping system lifts $16kg$ of water per second through a vertical height of $7.0\ m$.
What is the efficiency of the pumping system?

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

  1. $1.8\%$

  2. $5.6\%$

  3. $22\%$

  4. $55\%$

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Correct Option: B
Explanation:

The power of pump = $\dfrac{d(mgh)}{dt}$

                                 = $(16 \times 10 \times 7 ) W$
                                  = $112 W$
So, efficiency = $\dfrac{P _{out}}{P _{in}}\times 100$
                       = $\dfrac{112}{2000} \times 100$
                       =  $5.6 %$%

Choose the correct option.

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

  1. kW$=$ unit of power and kWh $=$ unit of energy.

  2. kW$=$ unit of energy and kWh $=$ unit of power.

  3. kW and kWh are both of unit of energy.

  4. kW and kWh are both of unit of power.

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Correct Option: A
Explanation:

Electrical Power is measured in watts (W), kilo-watts (kW), mega-watts (MW), and giga-watts (GW). The power of a load can change quickly, especially if the load is turning on and off.

Electrical Energy is measured in kilowatt-hours (kWh), megawatt-hours, and sometimes (less commonly) in watt-hours or watt-seconds. Energy is the primary measurement used by the utility company to determine your bill each month (although demand and power factor are also sometimes used). Unlike power, energy does not change quickly, but instead accumulates gradually.

In our household applications, commercial unit of electricity is used. One unit is equal to:

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

  1. $1\ kW$

  2. $1\ kWh$

  3. $1\ kW/h$

  4. $1\ Wh$

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Correct Option: B
Explanation:

Electrical Energy is measured in kilowatt-hours (kWh). A unit is the primary measurement used by the utility company to determine your bill each month (although demand and power factor are also sometimes used). Unlike power, energy does not change quickly, but instead accumulates gradually.

$250 kg$ of water per minute is to be drawn from a well $150 m$ deep. An electric pump of _______ can be used. $(g = 10 m/s^2) $ 

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

  1. 6 horse power

  2. 7 horse power

  3. 8 horse power

  4. 9 horse power

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Correct Option: D
Explanation:

Power, $P = \dfrac{w}{t} = \dfrac{mgh}{t} =\dfrac{mgh}{t\times 746} HP = 8.35HP$
$\therefore Power$  $= 9HP$ 

Which of the following can be used to express energy? Symbols have their usual meanings for the units of physical quantities.
(i) Wh   (ii) VC    (iii) VAs$^2$  (iv)  A$^2\Omega$s

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

  1. (i) , (ii)

  2. (ii), (iii), (iv)

  3. (i), (ii), (iv)

  4. (i), (ii), (iii), (iv)

Show answer
Correct Option: C
Explanation:

Using   $E = Pt$       $\implies$ Energy can be expressed in $Wh$

Using  $W = qV$     $\implies$ Energy can be expressed in $VC$
Using  $H = i^2Rt$  $\implies$ Energy can be expressed in $A^2\Omega s$
Whereas  Energy cannot be expressed by  $VAs^2$
Hence option C is correct.

Electric energy supplied to homes is measured by

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

  1. voltmeter

  2. ammeter

  3. galvanometer

  4. kilowatt-hour meter

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Correct Option: D
Explanation:

The kilowatt hour (symbolized kW⋅h as per SI) is a composite unit of energy equivalent to one kilowatt (1 kW) of power sustained for one hour. One watt is equal to 1 J/s. One kilowatt hour is 3.6 MJoules, which is the amount of energy converted if work is done at an average rate of one thousand watts for one hour.

1 kilowatt-hour is the amount of .... by 1000 watt electric appliance when it operates for one hour.

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

  1. Power

  2. Voltage

  3. Electric energy

  4. None

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Correct Option: C
Explanation:

1 kilowatt-hour is the amount of energy energy consumed by a 1000 watt electric appliance when it is operated for one hour.

Convert 1 kWh to SI unit of energy.

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

  1. $3.6\times 10^6J$

  2. $3.6\times 10^8J$

  3. $1.8\times 10^6J$

  4. $1.8\times 10^8J$

Show answer
Correct Option: A
Explanation:

$1\ kWh$ $ = 1000 \times 3600$ $ = 3.6\times 10^6$ $J$