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Note on Energy

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Energy is the capacity or ability of a body to do work. It is a scalar quantity. Its SI unit is the same as that of work i.e. joule (J). In CGS system, energy is measured in calorie (4.2 joule= 1 calorie).

There are various forms of energy. Some of them are as follows:

  • Mechanical energy
  • Electrical energy
  • Heat energy
  • Light energy
  • Sound energy
  • Chemical energy
  • Electrical energy
  • Magnetic energy
  • Nuclear energy

Mechanical energy

The energy possessed by a body by the virtue of its motion or its position or its configuration is called mechanical energy. It is two types: Kinetic energy and potential energy.

Kinetic energy

The energy possessed by a body by the virtue of its motion is called kinetic energy. Examples: A flying bird, a moving car, running water etc.

The Kinetic energy of a moving body is determined by the formula:

Kinetic energy (KE)=1/2mv2
Here, m = mass of the body
V =velocity of the body

Derivation of KE = \(\frac{1}{2}\)mv2

Let us consider a body of mass 'm' is at rest on a smooth surface whose initial velocity is 0. If a constant force 'F' is applied on it then it displaces the body by displacement 'd' in its own position. If 'v' be the final velocity then the workdone by the force is given by

W = Fd
Or, W = mas...........(i) (since,F=ma and d=s)
We also have,
a = \(\frac{v - u}{t}\) and s = \(\frac{ u + v}{2} \times t\)

substuting these values on equation (i)
Or, W = m\(\frac{ v - u}{t}\) \(\times\) \(\frac{u +v}{2} \times t\)

Or, W = m \(\frac{v^2 - u^2}{2}\)
Or, W = \(\frac{1}{2}\)mv2 (since, initial velocity u=0)

The workdone becomes the kinetic energy of the body. Thus, the kinetic energy of a body of mass 'm' moving with the velocity equal to 'v' is\(\frac{1}{2}\)mv2.

Potential energy

The energy possessed by a body by the virtue of its position or its configuration is called potential energy.

The potential energy (PE) of a body at a height is determined by the formula:

P.E. = mgh
Here, m = mass
g = acceleration due to gravity
h = Height

Work done is stored in the body as potential energy which is called gravitational potential energy.

The potential energy possessed by a stretched rubber, a compressed spring etc. is called elastic potential energy.

Derivation of PE = mgh

Let us consider a body of mass 'm' . The weight 'mg' of the body acts vertically downwards, where 'g' is the acceleration due to gravity. Let 'h' be the height upto which body is lifted. In order to lift the body, force 'mg' should be applied in upward direction.
Now, the workdone is given by
W = F.d
i.e. W =mgh (Since F= mg and d =h)
This work done is stored in the body as potential energy which is called gravitational potential energy.
\(\therefore\) Gravitational potential energy (P.E.) = mgh.

Electrical energy

Electrical energy is the energy possessed by the continuous flow electrons or by the change in a number of electrons in bodies. Electronic devices like computers, telephones, televisions, radios, calculators etc. use electrical energy.

Heat energy

A substance is made of tiny particles called molecules. The sum total of the kinetic energy of all constituent molecules of the body is heat energy. Sun is the main source of heat energy. Heater is an example of heat energy.

Light energy

Light is a form of energy which produces the sensation of sight. The main source of heat and light energy in the earth is the sun. Some examples of light energy are lightbulb, torch light, tube light, etc.

Sound energy

A sound is a form of energy which is produced by the vibration of sounding bodies. Sound requires material medium to for its propagation. Speakers, radio, television etc are some examples of sound energy

Magnetic energy

The energy possessed by the magnet or a current carrying conductor that effect on magnetic bodies is called magnetic energy. Magnets are used in many electronic devices such as radio, telegraph. Loudspeaker, etc.

Chemical energy

The energy stored in a body, which is released from it, when it undergoes chemical change is called chemical energy. Generally, food, fuel and cells store chemical energy.

Nuclear energy

The energy possessed from a nuclear fission or nuclear fusion is called nuclear energy. Nuclear fission is a process of splitting up of a heavy nucleus into light nuclei with the release of energy. Nuclear fusion is a process in which lighter nuclear fuse to form heavier nuclei.

Transformation of Energy

Conversion of energy from one form to another form is called transformation of energy.

When a candle burns, it changes chemical energy into light and heat energy. Similarly, when a lamp glows, it changes electrical energy into light and heat energy. These examples show that one type of energy can be changed into another type.

Examples:

  • A steam engine changes heat energy into mechanical energy.
  • Electric fan converts electric energy into mechanical energy.
  • A microphone converts electrical energy into sound energy.
  • Bulb converts electrical energy into light energy.
  • Solar panels convert light energy into electrical energy.

Conservation of energy

Energy can neither be created nor destroyed but can only be changed from one form to other. This is called the principle of conservation of energy.

  • Energy is the capacity or ability of a body to do work.
  • The energy possessed by a body by the virtue of its motion or its position or its configuration is called mechanical energy.
  • The energy possessed by a body by the virtue of its motion is called kinetic energy.
  • The energy possessed by a body by the virtue of its position or its configuration is called potential energy.
  • Electrical energy is the energy possessed by the continuous flow electrons or by the change in the number of electrons in bodies.
  • The energy possessed by the magnet or a current carrying conductor that effect on magnetic bodies is called magnetic energy.
  • The energy stored in a body, which is released from it, when it undergoes chemical change is called chemical energy.
  • Conversion of energy from one form to another form is called transformation of energy.
  • Energy can neither be created nor destroyed but can only be changed from one form to other. This is called the principle of conservation of energy.
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Very Short Questions

The energy of a body is the capacity of doing work. Its SI unit is Joule(J).

As the energy means the amount of work that a body can do, they have same SI unit.

A person needs energy to do anything. Even if he is sleeping, the internal activities like respiration, circulation, digestion were doing. To perform these activities, energy is required. So, he will be in energy less condition.

The energy acquired by the bodies subjected to forces is called mechanical energy. For example: A falling stone, the wound up spring a watch, etc.

Energy possessed by a body in virtue of its state or position is called potential energy. Example: water stored in a reservoir, stretched rubber, lifted leg before kicking a football, etc.

Let us suppose a body of mass 'm', at a place 'x' on the ground. The force is applied on the body to displace it up to a point 'y' at height 'h' from the surface. The force to be applied to lift the body is 'mg'.

So, the work done to lift a body from 'x' to 'y' is: W = F x d
or, W = mg x h or, W = mgh
The amount of work is stored in the body as potential energy.
∴ Potential Energy (P.E) = m.g.h

The factors affecting Potential energy are as follows:

  1. Mass of the body (m)
  2. Acceleration due to gravity (g)
  3. Height of the object from the ground (h)

Considering the same capacity of the tank filled with water at position B, potential energy will be more because potential energy is directly proportional to the height and height is more in B than in A.

The factors affecting KE of a body are as follows:

  1. Mass of the body and
  2. Velocity of the body

Kinetic energy of a moving body is directly proportional to the square of velocity of the moving body.
i.e. K.E ∝ v2

Potential energy Kinetic energy
It is the energy possessed by a body in rest. It is the energy possessed by a moving body.
It is calculated by P.E = mgh It is calculated by: K.E = ½ mv2.

The mathematical formula of Kinetic energy (K.E) is ½ mv2.

As we know that potential energy is gained when an object gains height i.e.PE = mgh. Since, PE ∝ h, the stone stores potential energy when it is lifted to a certain height.

The muscular energy, which is potential energy, is used in climbing staircase.

The energy produced from a nuclear fission or fusion is called nuclear energy.

Work and energy are directly related such that work done on a system is equal to the increase in energy.

Solution:
Here,
Mass(m) = 2kg
Height (h) = 2m
Potential energy (P.E) = ?
Acceleration due to gravity (g) = 9.8 m/s2
As we know that,
PE = mgh
Or, PE = 2 x 9.8 x 2
Hence, PE = 39.2 J

Now, when the stone is released, its PE goes on decreasing and KE goes on increasing. At the ground, PE becomes zero such that KE = 39.2 J
For velocity at ground,
Initial velocity (u) = 0
Final velocity (v) = ?
Height (h) = 2m
So, v2 = u2 + 2gh
or, v2 = 2 x 9.8 x 2
or, v2 = 39.2
∴ v = 6.26m/s Thus, the stone has PE 39.2J KE and velocities at ground are 39.2J and 6.26m/s respectively.

Here Mass (m) = 10kg
Velocity (v 0 = 0.4 m / s
Kinetic energy (KE) = ?
we have,

KE = \(\frac{1}{2}\) mv \(^{2}\)
KE = \(\frac{1}{2}\) \(\times\) 10 (0.4)\(^{2}\) = 0.8J

Thus, the kinetic energy of the body is 0.4J .

Here
Mass (m) = 200kg
Potential energy (PE) = 9800J
Height (h) = ?
Acceleration due to gravity (g) = 9.8m / \(s^{2}\)
We have,
PE = mgh
or, 9800 = 200 \(\times\) 9.8 \(\times\) h
or, 9800 = 100 \(\times\) 9.8 \(\times\) h
or, h = \(\frac{9800}{200\times 9.8}\)
= 5m.

Thus, required height is 5m.

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  • Which is not a factor of mechanical energy?

    density
    state
    motion
    position
  • Which is not an example of mechanical potential energy?

    solar rays
    Water in damn
    extended arrow in bow
    footballer lifting his leg to kick ball
  • Which is not source of heat?

    sun
    water
    wood
    Atomic energy
  • How Nuclear power plants produce electricity?

    heat from nuclear fusion drive thermal power plant
    energy of nucleus drive thermal power plant
    heat from nuclear fission drive thermal power plant
    blast from nuclear explosion drive thermal power plant
  • Which contains chemical energy?

    stone
    Uranium
    Coal
    Diamond
  • Which gives nuclear fission energy?

    Uranium Krypton
    Kryptonium
    Barium
    Hydrogen
  • Which gives nuclear fusion energy?

    Barium
    Hydrogen
    Helium
    Kryptonium
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