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Idea of energy - class-IX

Description: idea of energy
Number of Questions: 54
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Tags: energy and its forms physics work and energy
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The force acting on a 4gm mass in the energy region ${ U=8x }^{ 2 }$ at x= -2 cm is :

  1. 8 dyne

  2. 4 dyne

  3. 16 dyne

  4. 32 dyne


Correct Option: D
Explanation:

Given,

$m=4gm$
$U=8x^2$

The force acting on mass $m$ in the potential energy $U$  at $x=-2cm$ will be

$F=-\dfrac{dU}{dx}$
$F=-\dfrac{d(8x^2)}{dx}$
$F=-16x$
$|F| _{x=-2cm}=-16\times(-2)=32dyne$
The correct option is D.



Which of the following pairs represent units of the same physical quantity?

  1. kelvin and joule

  2. kelvin and calorie

  3. newton and calorie

  4. joule and calorie


Correct Option: D
Explanation:

joule and calorie are both units of energy .
$1\  joule = 0.24 \ calories$

What is the equivalent of $1$ joule in calorie?

  1. $0.24$

  2. $2.4$

  3. $24$

  4. $240$


Correct Option: A
Explanation:
$1 joule = 1/4.18 \ cal= 0.24 \ calorie$

Name the quantity which is measured in $Wh$.

  1. Force

  2. Power

  3. Energy

  4. All the above


Correct Option: C
Explanation:

We know that $ power = work/ time$.

Power has unit $W$ and time can have either seconds or minutes or hours.Hence work can have either $W\ min$ or $Wh$ or $Ws$. We in general consider work is nothing but energy. Hence energy can have unit $Wh$.

A certain household has consumed $250$ units of energy during a month. How much energy is this in joule ?

  1. $9 \times 10^8:J $

  2. $10 \times 10^8:J $

  3. $9 \times 10^7:J $

  4. $9 \times 10^{-8}:J $


Correct Option: A
Explanation:

Energy consumed in a month
$=250 : units = 250 : kWh$
$= 250 \times 3.6 \times 10^6 = 9 \times 10^8:J $

Kilowatt-hour ($kWh$) represents the unit of :

  1. power

  2. impulse

  3. momentum

  4. none of these


Correct Option: D
Explanation:

The kilowatt-hour ($kWh$) is a unit of energy equivalent to one kilowatt ($1\  kW$) of power expended for one hour ($1\  h$) of time.


State whether the given statement is True or False :
Kilowatt hour is the unit of power.
  1. True

  2. False


Correct Option: B
Explanation:

False
because,The kilowatt-hour (kWh) is a unit of energy equivalent to one kilowatt (1 kW) of power expended for one hour (1 h) of time.

The commercial unit of measurement energy is 

  1. Watt -Hour

  2. Kilowatt

  3. Kilowatt hour

  4. Megawatt


Correct Option: C
Explanation:

The commercial unit of measurement energy is Kilowatt hour.

The power supply in our home is measured in Kilowatt hour units.

$1\ kW$ is equal to :

  1. $1000\ watt$

  2. $10^{-3}\ mega\  watt$

  3. $A$ and $B$ both

  4. neither $A $ nor $B$


Correct Option: C
Explanation:

$1000\ W= 1\ kW$
$1000 \ kW= 1\ MW$

The unit kWh represents

  1. power

  2. momentum

  3. energy

  4. force


Correct Option: C
Explanation:

This is unit of energy.If the energy is being transmitted or used at a constant rate over a period of time, the total energy in kilowatt hours is the power in kilowatts multiplied by the time in hours.

Which of the following is different from the others?

  1. Joule

  2. Kilowatt hour

  3. Erg

  4. Watt


Correct Option: D
Explanation:

Joule, kilowatt Hour and Ergs are different units of the physical quantity Energy.


Watt is a unit of Power

Hence, Option D is correct

Number of kilowatt-hours = $\displaystyle \frac{volt \times ampere \times  \,...........}{1000}$

  1. time in seconds

  2. time in minutes

  3. time in hours

  4. time in days


Correct Option: C
Explanation:

Power, $P=VI$

1 kilowatt-hour means $P=1 kW=1000 W$ and time $t=1 hour$
As kWh is the unit of electrical energy so $1 kWh=VIt/1000$. 
Thus, time will be in hour. 

Which of the following is the biggest unit of energy?

  1. Joule

  2. Kilowatt hour

  3. Erg

  4. Electron volt


Correct Option: B
Explanation:

Since, we know that SI unit of energy is represented by Joule. Hence, let us convert all the different units in the form of Joules. 


Option A- 
1 Joule= $1\quad Joule\quad =\quad [kg]{ [\frac { m }{ s } ] }^{ 2 }$

Option B-
$kWh\quad =\quad 1000W\times 3600s\quad =\quad 3600000\quad W\times s$

As we know that 1 Ws = 1 Joule

So 1 kWh= 3600000 Joules

Option C- 
$1\quad Joule\quad =\quad [kg]{ [\frac { m }{ s } ] }^{ 2 }=\quad \left[ 1000\quad gm \right] \left[ \frac { 100\quad cm }{ 1\quad s }  \right] \left[ \frac { 100\quad cm }{ 1\quad s }  \right] \ S0,\quad 1\quad Joule\quad =\quad { 10 }^{ 7 }gm\frac { { cm }^{ 2 } }{ { s }^{ 2 } } \quad =\quad { 10 }^{ 7 }\quad erg\ So,\quad 1\quad erg\quad =\quad { 10 }^{ -7 }Joule$

Option D- 
Electron Volt is the unit of energy in terms of change in energy for one electron in the potential difference of one volt. So, mathematically it is-

$1\quad ev\quad =\quad Charge\quad of\quad one\quad electron\quad \times \quad 1\quad Volt\quad =\quad 1.6\quad \times { 10 }^{ -19 }Coloumb\quad \times \quad 1\frac { Joule }{ Coloumb } \ So,\quad 1\quad ev=\quad =\quad 1.6\quad \times { 10 }^{ -19 }Joule$

Hence, Option B is the largest unit of energy and hence it is practically used in everyday life of electric consumption. 

How many joules are there in one kilowatt hour?

  1. ${35 \times 10 ^6 }$ J

  2. ${3.5 \times 10 ^6 }$ J

  3. ${36 \times 10 ^6 }$ J

  4. ${3.6\times 10 ^6 }$ J


Correct Option: D
Explanation:

As, $1kW=1000W ; 1hr=3600s$
$1kWh=1000Wh=3600000=3.6\times10^6Ws=3.6\times10^6J$

Electron volt is the unit of

  1. energy

  2. potential difference

  3. charge

  4. charge to mass


Correct Option: A
Explanation:

We know that the electrical potential energy of electron $U=eV$ where e is the charge of electron and V be the voltage. So electron volt will be the unit of energy.  

The ratio of SI units to CGS units of energy is

  1. 10$^5$

  2. 10$^6$

  3. 10$^7$

  4. 10$^{-7}$


Correct Option: C
Explanation:

Unit of energy is $ML^2T^{-2}$ 

So in  $CGS$ unit, mass = gram, length = centimeter, time = second.
And in $SI$ unit, mass = kilo gram, length = meter ,time = second.
So ratio of both SI to CGS

=$\dfrac{kg.m^2.s^{-2}  }{g.cm^2.s^{-2}}$

=$\dfrac {1000g.100^2cm^2.s^{-2}}{g.cm^2s^{-2}}$

=$10^7$.

1 horse power is equal to

  1. 740 watts

  2. 746 watts

  3. 648 watts

  4. 748 watts


Correct Option: B
Explanation:

1 horse power is equal to 746 watts.

i.e. $1 \ H.P.=746W$.

1 kWh equals

  1. $ 3.6 \times 10^6 J$

  2. $ 3.6 \times 10^7 J$

  3. $ 3.6 \times 10^8 J$

  4. $ 3.6 \times 10^{-6} J$


Correct Option: A
Explanation:

1 watt is defined as 1 joule/second.

So $1kW = 1000W$, and $1 hour =60 \times 60 seconds $

So $1kWh = 3600kWs = 3,600,000Ws = 3,600,000J$

$1kWh=3.6*{{10}^{6}}J$

One kWh is equal to

  1. $3.6 \times 10^6 J$

  2. $3.6 \times 10^5 J$

  3. $3.6 \times 10^4 J$

  4. $3.6 \times 10^3 J$


Correct Option: A
Explanation:

Watt is defined as joule per second. Kilowatt is $10^3$joule per second and kwh is kilowatt hour $10^3\times60\times60$ 

So 1kwh=$3.6\times10^7$J. 

The number of joules that is equal in value to 1 kWh is.

  1. $3.6\times 10^8$

  2. $0.36\times 10^4$

  3. $3.6\times 10^5$

  4. $3.6\times 10^6$


Correct Option: D
Explanation:

1 kWh is equal to 3.6 $\times$ 10$^{6}$

1 Watt-sec  is a Joule a watt expended for a second.

Because there’s 3600 seconds in an hour,

then 3600 Ws = 1 Wh = 3600 Joules = 3.6 kJ

so 1000 Wh = 1 kWh = 1000 $\times$ 3.6 kJ = 3.6 MJ = 3.6 $\times$ 10$^{6}$

Kilowatt hour (kWh) represents the unit of____

  1. power

  2. impulse

  3. mementum

  4. none of these


Correct Option: D
Explanation:

The kilowatt hour is a unit of energy equal to $3.6\times 10^6J$. If the energy is being transmitted or used at a constant rate (power) over a period of time, the total energy in kilowatt hours is the power in kilowatts multiplied by the time in hours. so if kWh is unit of energy so none of the option is correct hence option D is the answer.

Watt sec represents the unit for:

  1. energy

  2. power

  3. force

  4. none of these


Correct Option: A
Explanation:

A watt second (also watt-second, symbol W s or W. · s) is a derived unit of energy equivalent to the joule. The watt-second is the energy equivalent to the power of one watt sustained for one second.

So this is unit of energy.

One joule is approximately equal to:

  1. 0.28 cal

  2. 0.32 cal

  3. 0.24 cal

  4. 4.2 cal


Correct Option: C
Explanation:

calorie (cal) is the energy needed to increase 1 gram of water by 1°C at a pressure of 1 atmosphere.

1 cal = 4.184 J. 
So 1J =0.24cal.

1 MeV is equal to:

  1. $1.6 \times 10^{-19} J$

  2. $1.6 \times 10^{-14} J$

  3. $1.6 \times 10^{-13} J$

  4. $1.6 \times 10^{13} J$


Correct Option: C
Explanation:

 Electron volt (symbol eV) is a unit of energy equal to approximately 1.6×1019  joules (symbol J). By definition, it is the amount of energy gained (or lost) by the charge of a single electron moving across an electric potential difference of one volt.

And mega electron volt (MeV) is $10^6\times eV$ so $1MeV=1.6 \times 10^{-13}$.
so best possible option is optionC.

The form of energy present in a wound spring is :

  1. Chemical

  2. Heat energy

  3. Magnetic

  4. Mechanical


Correct Option: D
Explanation:

no chemical reaction takes place while wounding a spring so no chemical energy.

no energy is stored in the form of temperature change so no heat energy is stored.
no magnetic field is around the spring so no magnetic energy.
but due to change in shape of the metal potential energy is stored, so answer is option D.

1erg is equal to:

  1. $ 1 g cm s^{-2}$

  2. 1 Nm

  3. $ 10^7 J$

  4. $10^{-7} J$


Correct Option: D
Explanation:

An erg is the amount of work done by a force of one dyne exerted for a distance of one centimeter. In the CGS base units, it is equal to one gram centimeter-squared per second-squared

And joule is SI unit of work done or energy equals work done by force of 1N for a displacement of 1m.
So converting cgs to SI unit.
$1 erg= 10^{-3}N\times(10^{-2}m)^2\times1s$ $=10^{-7}$J.

A helicopter flying in the air has:

  1. only kinetic energy but not potential energy

  2. Only potential energy but not kinetic energy

  3. both potential and kinetic energy

  4. neither kinetic nor potential energy


Correct Option: C
Explanation:

A flying helicopter has potential energy,mgh(mass by g by weight) and this potential energy does not depend in any way on how fast it is flying in a horizontal direction.

It also possesses kinetic energy at a constant velocity which is same at any height it flies.
The total energy is sum of both potential energy and kinetic energy.

One kilowatt-hour is equivalent to ______  joules.

  1. $\displaystyle 3.6\times 10^{6}$

  2. $\displaystyle 6\times 10^{6}$

  3. $\displaystyle 3\times 10^{6}$

  4. $\displaystyle 1.8\times 10^{6}$


Correct Option: A

The form of energy in presence of which other objects are seen is known as:

  1. wind energy

  2. heat energy

  3. light energy

  4. chemical energy


Correct Option: C
Explanation:

In presence of light energy, other objects are seen.

Which of the following is not a form of energy.

  1. Heat

  2. Light

  3. Sound

  4. Charge


Correct Option: D
Explanation:

Charge is a fundamental property of matter. It is not a form of energy.

What type of energy conversion takes place when a compressed spring is released?

  1. Molecular energy to potential energy

  2. Kinetic energy to potential energy

  3. Potential energy to kinetic energy

  4. Potential energy to molecular energy


Correct Option: C

The negative of the work done  by the conservative internal forces on a system equals to the change in 

  1. total energy

  2. Kinetic energy

  3. Potential energy

  4. None of these


Correct Option: C
Explanation:

The negative of the work done  by the conservative internal forces on a system equals to the change in potential energy


Hence, correct answer is option $C$

$1\  hp$ is equal to:

  1. $0.746\  kW$

  2. $7.46\  kW$

  3. $74.6\  kW$

  4. $746 \ kW$


Correct Option: A
Explanation:

Horsepower (hp) is a unit of measurement of power (the rate at which work is done).A power level of 1 hp is approximately equivalent to $746$ watt (W) or $0.746$ kilowatt (kW).

The unit of work is joule $(J)$ in the $SI$ system and erg in the $CGS$ system. Find number of ergs in $1\ joule$.

  1. $10^{17} ergs$.

  2. $10^{9} ergs$.

  3. $10^{8} ergs$.

  4. $10^{7} ergs$.


Correct Option: D

Which one is not a unit of energy.

  1. Electron-volt

  2. Joule

  3. Calorie

  4. Watt


Correct Option: C

How many joules are equivalent to $1$ unit of electricity.

  1. $3.6\times{10}^{3}$

  2. $3.6\times{10}^{4}$

  3. $3.6\times{10}^{5}$

  4. $3.6\times{10}^{6}$


Correct Option: D

A body of $1000 kg$ moving with $0.3m/s$ in +x direction. A hole is present at the bottom of body such that, mass is dropping at rate of $20 gm/sec$, inn -y direction. Find out force exerted by ejecting mass on body.

  1. 0.4 N

  2. 0 N

  3. 0.6 N

  4. None of these


Correct Option: A

Work done charge of mass 2 Kg due to external force against electrostatics force is - 10 J if charge is displaced from A to B. Velocity of charge at point A is 4m/s and at B is 2m/s then find difference is electropotential energy $ (U _s - U _A) $

    • 10J
    • 10J
    • 21J
    • 2J

Correct Option: A

Taking the unit of work as Joule and the unit of amount of heat as K cal, the magnitude in Joule for mechanical equivalence of heat is:

  1. $1$

  2. $4.2 \times 10^7$

  3. $4.2$

  4. $4.2 \times 10^3$


Correct Option: D
Explanation:

$1\ kcal=4.2\times1000=4.2\times10^3\ J$

Define the term calorie. How is it related to joule ?

  1. The amount of energy requires to raise the temperature of 1 gram of water by 1° Celsius, 1 Cal = 4.18 J

  2. The amount of energy requires to raise the temperature of 1 gram of water by 1 Kelvin, 4.18 Cal = 1 J

  3. The amount of energy requires to raise the temperature of 1 gram of water by 1 Fahrenheit, 4.18 Cal = 1 J

  4. The amount of energy requires to raise the temperature of 1 gram of ether by 1° Celsius, 1 Cal = 4.18 J


Correct Option: A
Explanation:
The term calorie can be defined as the energy needed to raise the temperature of 1 gram of water by $ 1$ °C. 
$1 \ cal = 4.18 \ J$

The energy required to accelerate a car from $10:m/s$ to $20:m/s$ is how many times the energy required to accelerate the car from the rest to $10:m/s$:

  1. Equal

  2. 4 times

  3. 2 times

  4. 3 times


Correct Option: D
Explanation:

Energy required to accelerate from $\displaystyle 10:m/s$ to $20:m/s$ $E _{1}=\frac{1}{2}m(400-100)$


Energy  required to accelerate from rest to $\displaystyle 10:m/s$  $E _{2}=\frac{1}{2}m(100-0)$

$\displaystyle \frac{E _{1}}{E _{2}}=\frac{\frac{1}{2}m(300)}{\frac{1}{2}m(100)}=3:times$

Cost of electricity for home use is Rs. 1.50 per unit. This unit is:

  1. 1 ampere

  2. 1 volt

  3. 1 joule

  4. 1 kilowatt hour


Correct Option: D
Explanation:

The electrical unit for the electricity of home is used as kilowatt hour (kWh). Here the cost of 1 kWh electricity is Rs 150.  

The unit $kg m^2s^{-2}$ is associated with:

  1. work

  2. kinetic energy

  3. potential energy

  4. all the above


Correct Option: D
Explanation:

This is unit of energy $ML^2T^{-2}$ (in dimensional form).

All the quantities given in the option are different form of energy ao their units will be same. So the given unit is associated with all.

The number of joules contained in 1kWh is_____

  1. $36 \times 10^2$

  2. $36 \times 10^3$

  3. $36 \times 10^4$

  4. $3.6 \times 10^6$


Correct Option: D
Explanation:

The kilowatt hour is a unit of energy equal to 3.6 megajoules. If the energy is being transmitted or used at a constant rate over a period of time, the total energy in kilowatt hours is the power in kilowatts multiplied by the time in hours.

Let energy is used as 1J/s. For an hour or 3600 sec. Then total energy used is 3600J. And kWh means 1000$\times$3600.
Which gives $3.6\times10^6$.

$ kg  m^2 s^{-2}$ represents the unit of_____

  1. kinetic energy

  2. work done

  3. potential energy

  4. all the above


Correct Option: D
Explanation:

the given unit $kgm^2s^{-2}$ is unit of energy and  any of the energy either potential or kinetic will have  same unit 

also work done is given by $F.S$ or $kgms^{-2}.m$ or $kgm^2s^{-2}$
so the unit of work is also same.
so the given unit represent unit for all the three quantities in the options.

Work required to produce $1000   k$ - calorie heat will be

  1. $4.2 Joule$

  2. $0.42 \times {10}^{6} Joule$

  3. $4.2 \times {10}^{6} Joule$

  4. $4.2 \times {10}^{7} Joule$


Correct Option: C
Explanation:

In this problem, we have to convert the heat energy in unit from calorie to Joule. 

According to work-energy theorem, the work is also a energy. 
Thus, $1000k.cal=1000\times 10^3 cal=10^6 cal=4.2\times 10^6 J$  (as $1 cal=4.2 J$)

$1Wh=$

  1. $36kJ$

  2. $3.6kJ$

  3. $3600kJ$

  4. $3.6MJ$


Correct Option: B
Explanation:

$1Wh=1W\times{1h}$

$=1{J}.{s}^{-1}\times{3600}s$

$=3600J=3.6kJ$

Kilowatt-hour is the unit of

  1. potential difference

  2. electric power

  3. electrical energy

  4. charge


Correct Option: C
Explanation:

Electrical energy $V \times l \times t$
Electric energy unit $= kW  hour$

$1kWh=$

  1. $3.6J$

  2. $3.6kJ$

  3. $3.6MJ$

  4. $3.6GJ$


Correct Option: C
Explanation:

$1kWh=1kW\times{1h}$

$=1000{J}.{s}^{-1}\times{3600}s$

$=3.6\times{10}^{6}J=3.6MJ$

A window whose area is $2m^2$ opens on street where the street noise result in an intensity level at the window of $60db$. How much acoustic power enters the window via sound waves ? Now, if an acoustic absorber is fitted at the window? 

  1. $8\mu W$

  2. $10\mu W$

  3. $4\mu W$

  4. $2\mu W$


Correct Option: D
Explanation:

Intensity = 60 dB

60 = 10 log $(\frac{I}{I _{ref}})$

where $I _{ref}$ is the refrence intensity , general value of this is given by 

$I _{ref} = 10^{-12}\frac{watt}{m^2}$

$\frac{I}{10^{-12}}$ = anti log (6)

$I = 10^{-12}  \times{anti log(6)} = 10^{-12} \times{10^6} = 10^{-6}$

Power = I$\times$ area = $2 \times{10^{-6}}$ watt

The internal energy change when a system goes from state A to B is $40$ kJ/mole. If the system goes from A to B by a reversible path and returns to state A by an irreversible path what would be the net change in internal energy?

  1. $40$ kJ

  2. $> 40$ kJ

  3. $< 40$ kJ

  4. Zero


Correct Option: D
Explanation:
For a cyclic process the net change in the internal energy is zero because the change in internal energy doesnot depend on the path.

Which of the following can be negative 

  1. Kinetic energy

  2. Potential energy

  3. Mechanical energy

  4. All of these


Correct Option: B,C
Explanation:

Mechanical energy & potential energy can be negative.

$\therefore$ Option $B$ and $C$.

If the unit of force and length be each increased by four times, then the unit of energy is increased by

  1. 16 times

  2. 8 times

  3. 2 times

  4. 4 times


Correct Option: A
Explanation:

Unit of Energy = Unit of Force $\times$ Unit of length 
So if we increase unit of length and force, each by four times, then unit of energy will increase by sixteen times.  

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