6th Class Science Work, Force and Energy Energy

Energy

Category : 6th Class

Energy

We require energy to do our daily work. Our body can apply force on an object due to the energy of our body. Therefore, the energy is the capacity to do the work. Energy can neither be created nor destroyed, it is only transformed from one form of energy into another form. Our body obtain energy from the food we eat. A form of stored energy is consumed by the living organism to do the work. The unit for the measurement of energy is Joule is represented by the first letter of its name "J ".

Let us understand energy with a given example:

Take a piece of stone and lift the stone up at a certain height. Put a earthen pot on the surface exactly below the lifted stone. The work will be done by you on lifting the stone and an amount of energy is stored in the stone on lifting the stone at a height. Now, drop the stone on the earthen pot. The pot will break into small pieces. And the energy stored in the stone will be transferred on the earthen pot which will break the pot into small pieces.

Forms of Energy

There are many forms of energy and one form of energy is transferred into another form by the motion, chemical reactions, etc. Some forms of energy are as follows:

(i) Mechanical energy

(ii) Light energy

(iii) Chemical energy

(iv) Heat energy

(v) Electrical energy

(vi) Nuclear energy

(vii) Magnetic energy

(viii) Sound energy

Mechanical Energy

A raised object possess a form of energy, called potential form of mechanical energy. A running object is capable to do the work because, it has kinetic form of mechanical energy. Therefore, the mechanical energy is the sum of both potential and kinetic energy and a body has mechanical energy if it has either potential or kinetic energy or both. There are two forms of mechanical energy, potential energy and kinetic energy, which are discussed below.

(A) Potential Energy: An object possess potential energy even if the object is in rest. An object can be in its static state if placed at a height. Also an object which has elastic property can remain in its static state until the force is applied. The two forms of potential energy, that is, gravitational potential energy and elastic potential energy are discussed below.

(i) Gravitational Potential Energy: The potential energy of an object depends on the mass, raised height from the surface and gravity. If one of all three parameters is increased then the potential energy of the object is also increased. Therefore, the potential energy of an object is the product of its mass, height at which the body is placed and gravity by which the object is pulled. If an object is placed at a height, the force of gravity acts on the object to bring it back on the surface.

A body, A is placed as the given in the picture below:

The body A is placed at a height of h and mass of the body is m. The gravity is represented by the letter g. Therefore, the potential energy (P.E) possess by the object A = Product of the mass, height and gravity = m x $=m\times g=mgh$.

Let us consider an example,

A transmitter is kept on the top of the mobile tower as shown in the picture below:

The total mass of the transmitter is 1200 kg and it is kept at the height of 250 metres from the surface.

Therefore, the potential energy possess by the transmitter = Mass of the transmitter  Gravity  Height of the transmitter from the ground $=1200\times 9.8\times 250=2940000$.

(ii) Elastic Potential Energy: Elastic potential energy is a form of energy stored in an elastic material due to the stretching or compression. A stretched bow possess elastic potential energy. Elastic potential energy is also stored in the spring of a gun, stretched rubber band, etc. The amount of elastic potential energy possess by an elastic device depends on the amount of the stretched elastic device. The amount of elastic potential energy possess by an elastic device is transferred to the related device. For example, the amount of elastic potential energy possess by a spring of a gun is transferred to the bullet of the gun and bullet strikes on the target. Therefore, the amount of elastic potential energy possess by a device depends on the amount of force by which the device is stretched. Less stretched elastic device possess less amount of elastic potential energy and more stretched possess more energy. The amount of elastic potential energy possess by a stretched spring (stretched from its rest) $=\frac{1}{2}\,k{{x}^{2}}$.

Where, k = spring constant or force constant and  stretched distance. According to the Hook's law, the applied force on a spring is the product of force constant or spring constant and stretched distance by the applied force.

Hence, $F=kx$ or $k=\frac{F}{x}$. If a spring is at rest and no force is applied on the spring, then the spring does not possess any elastic potential energy. In this condition, when no force is applied on the spring, the spring constant or force constant is zero. Therefore, the elastic potential energy of the spring = $\frac{1}{2}\times 0\times {{x}^{2}}=0$. Hence, the spring does not possess elastic potential energy, either spring constant (force constant) or stretched distance is zero.

Let a spring is stretched from its rest to 2 metres by applying a force of 219 N as shown in the picture below.

The spring constant or force constant, $k=\frac{F}{x}=\frac{219}{3}=73N/m$ and $x=2$ metres (given).

Therefore, the elastic potential energy possess by the spring at the given distance

(B) Kinetic Energy: From the explanation of potential energy, we have learned that, a body possess potential energy even if the body is at rest and exerts a force on itself at rest stage. The force can be due to the gravity of the earth or applied force for stretching a spring. The concept of kinetic energy is different than that of potential energy in this way that, a body possess potential energy at rest, whereas, a body possess kinetic energy due to its motion. For example, a moving car, a running boy, moving planets, a rolling stone possess kinetic energy due to their motion.

The kinetic energy of a moving object depends on the mass and (velocity) speed. Therefore, the amount of kinetic energy possess by a body is more if the mass of the body is more. If the speed of the body is increased than the kinetic energy of the body will also be increase.

The kinetic energy possess by a body is the half of the product of its mass and square of its speed or velocity $=\frac{1}{2}\times$ Mass x (Speed)2.

Let the mass of the earth is about, 5.9736 x io24 kilograms and its average orbital speed is about $29.78\,km/s=29.78\times 1000=29780m/s$

Therefore, the amount of kinetic energy possess by the earth =

$\frac{1}{2}\times \left( 5.9736\times {{10}^{24}} \right)\times {{(29780)}^{2}}=264883880112\times {{10}^{22}}$ Joule.

Light Energy

During the photosynthesis, water and carbon dioxide react to form carbohydrates and release oxygen in the presence of light energy radiated by the Sun. Light energy helps the molecules of water and carbon dioxide to break down into the individual atom to form a complex molecule of carbohydrates. An image on the photographic film is formed when light particles fall on the film for the chemical change.

Heat Energy

If a water filled pot is heated by a burner, the water near the lower level of the pot is heated first .This water goes upwards and cold or less heated water comes downward. The movement of amount of water inside the pot is due to the heat energy passed by the burner.

Electrical Energy

A conducting wire is used to pass the electrical energy from one point to another point. The generator of electrical energy passes the energy to the electrons of the conducting wire and wire passes these energy of electrons to the load which is connected at the end of the wire.

Nuclear Energy

The heavier elements like Radium, Uranium, etc. split their nuclei to become a stable and lighter element. During this process the energy is released, which is called nuclear energy.

Magnetic Energy

When a piece of iron comes in contact with a magnet, it gets attracted to the magnets. This attraction is due to the magnetic energy of the iron. The electric motors, electric fans etc work because, the electrical energy is converted into magnetic energy by the coil and magnetic material inside.

Sound Energy

Microphone works on the sound energy. When a body produces sound, the sound waves get mixed with the air. Air, mixed with sound waves, strikes on the diaphragm of the microphone and vibrate the diaphragm according to the amplitude and frequency of the sound waves. The coil inside the mircrophone vibrates according to the vibration of diaphragm and produces a small amount of electromotive force in the wire, connected at the both ends of the coil. An amount of current is produced by the generated electromotive force and current reaches at the coil of the loudspeaker.

Consider the following statements:

Statement 1: The amount of potential energy possess by an object when placed at a height depends on the mass of the object.

Statement 2: The amount of potential energy possess by an object when placed at a height depends on the height from the surface of the object.

Which one of the following is correct about the above statements?

(a) Statement 1 is true and 2 is false

(b) Statement 1 is false and 2 is true

(c) Both statements are false

(d) Both statements are true

(e) None of these

Explanations

The amount of potential energy possessed by an object when placed at a height depends on the mass and height from the surface of the object.

Therefore, option (D) is correct and rest of the options is incorrect.

A body is capable of doing greater work /if it possesses greater energy.

Heat, electrical, sound etc. are the forms of energy.

The unit of work is Joule.

Energy is the capacity to do the work.

Potential energy is the product of mass, gravity and raised height.

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