Concept of energy - class-X

Energy is produced in a star's center, where pressures are enormous and temperatures reach 27 million°F (15 million°C). This causes nuclear fusion—atoms of hydrogen are ripped apart and fuse (join) to form helium. These reactions release vast amounts of energy, which makes the star shine.

Answer is C.

Potential energy is energy stored in an object. This energy has the potential to do work. Gravity gives potential energy to an object. This potential energy is a result of gravity pulling downwards. The gravitational constant, g, is the acceleration of an object due to gravity. This acceleration is about 10 meters per second on earth. The formula for potential energy due to gravity is PE = mgh. As the object gets closer to the ground, its potential energy decreases while its kinetic energy increases. 
In this case, a body of mass 5 kg kept at a height of 10 m. So the potential energy is given as 5 * 10 *10 = 500 J.
Hence, the potential energy of a body of mass 5 kg kept at a height of 10 m is 500 J.

One kilo-calorie is defined as the amount of heat required to raise the temperature of one kilogram of water by 1 °C. 

Electron volt or $eV$ is the unit of energy (electrical energy). $1\ eV = 1.6 \times 10⁻¹⁹ $ coulombs (charge of an electron) $\times $ 1 volt.

As energy is defined via work so,the SI unit for energy is the same as the unit of work that is joule (J)

Energy is defined as the capacity to do work.

The kilowatt hour is a unit of energy equal to 1,000 watt-hours, or 3.6 megajoule. 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 product of the power in kilowatts and the time in hours.
1 kilowatt hour $= 1000 \times 3600 joule = 3.6 \times 10^{6}\ J$

Energy is the capacity of a physical system to perform work. Energy exists in several forms such as heat energy, kinetic or mechanical energy, light energy, potential energy, electrical energy, or other forms. According to the law of conservation of energy the total energy of a system remains constant, though energy may transform into another form.
The SI unit of energy is the joule (J) or newton-meter (N-m). Joule is also the SI unit of work.

A body at rest will have thermal energy, and may even have Potential energy.

Work done is same as energy which is measured in joules. 1 joule is same as 1 newton expanded over 1 meter which is force multiplied by distance.

If a body is at rest then its position does not changes if position is not changing then $\frac{dx}{dt}=0$ so velocity is zero.

Work is defined as force multiplied by displacement in the direction of the force. When carrying a load horizontally, the force (gravity) acts vertically, while the displacement is horizontal. Since the angle between force and displacement is 90 degrees, the work done is zero.

Energy and work Energy is the capacity to do work, so option A is best possible answer. 

It is a fact that

S.I. unit of energy is $N-m$ which is also known as $joule$ and 

$1eV$ is the energy gained by an electron when it is accelerated through a potential difference of $1volt$.
So, $1eV=$ charge of an electron $\times$ $1volt$ $=1.6\times{10}^{-19}coulomb$ ${\times}1volt=1.6\times{10}^{-19}J$

Heat energy is usually measured in $calorie$.

Heat energy is usually measured in $calorie$. $1calorie$ is the heat energy required in raising the temperature of $1g$ of water from ${14.5}^{0}C$ to ${15.5}^{0}C$. It is related to $J$ as $1J=0.24calorie$.

Erg is a unit of Energy.

Energy is defined as the capacity to do work.

$1calorie=4.18J$

$1kWh=1kilowatt{\times}1hour=1000{J}{s}^{-1}\times{3600}s=3.6\times{10}^{6}J$
So, none of option A, B and C are correct.

$1kWh$ is the amount of work done (or, energy spent) by a source of power $1kW$ in $1h$ time.
So, ${1kW}\times{1h}=1kWh$

Gravitational Potential energy, $W = mgh$

$h=$ vertical displacement $=5\ m$

$m =$ Mass of object $= 40\ Kg$

$g =$ Acceleration due to gravity $= 9.8\ ms^{-2}$

$W = 40 \times 5 \times 9.8= 1960\ J$

For half way down$=\dfrac{1960}{2}= 980\ J$

At this point i.e. half way object has an equal amount of potential and kinetic energy.

$1$ unit of energy $=1\ kWh$

$1\ kwh=3.6 \times 10^{6}\ J$

Thus $250$ unit$=250 \times 3.6 \times 10^{6}\ J$$=9 \times 10^{8}\ J$

Energy is defined as the ability to do work and its SI unit is Joule.

Using E = mc^2. m = 4.2 mg = 4.2 * 10^-6 kg. c = 3 * 10^8 m/s. E = 4.2 * 10^-6 * (9 * 10^16) = 37.8 * 10^10 J. Converting to calories (1 cal = 4.184 J), the result is approximately 9 * 10^10 cal.

Lenzs law is a consequence of the law of conservation of energy. Emf induced is produced to oppose the motion.

kilowatt is the unit of power.

The electric motor works on principle of electromagnetic induction, which is when a voltage is induced by a changing magnetic field.

The power consumption

For a central force F = -k/r^2, the potential energy U = -k/r. The kinetic energy K = 1/2 * |U| = k/2r. Total energy E = K + U = k/2r - k/r = -k/2r.

Watt-sec, Kilowatt-hour, and eV are all units of energy. J-sec (Joule-second) is the unit of action or angular momentum, not energy.

Kilo-watt-hour (kWh) is a standard unit of energy (power * time). The other expressions do not simplify to energy units.

$W=QV$
$=(10\times 10^{-3})(12V)$
$=12\times 10^{-2}J$
$=0.12J$

The erg is a unit of energy and work equal to 10⁻⁷ joules. It originated in the centimetre–gram–second(CGS) system of units. It has the symbol erg. The erg is not an SI unit.

When a body does some work it uses its energy to provide external force due to which its energy decreases.

Definition of Energy “Capacity to do work”. If a ball falls due to free fall, its weight will do work, which is transferred to Kinetic Energy when dropped from a certain height. But then it says this is as 'energy is defined as the capacity to do work'

Joule.

 Force is represented as $\left[ ML{ T }^{ -2 } \right] \quad and\quad length\left[ L \right] $

When the man moves on a horizontal surface, then work done by gravity is zero since gravitational force acts perpendicular to the displacement. 

Energy is the capacity of a physical system to perform work. Energy exists in several forms such as heat, kinetic or mechanical energy, light, potential energy, electrical, or other forms.

flowing water possess kinetic energy which is used to do work