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Energy is the capacity of a physical system to perform work. Energy can be converted in various forms but not created or destroyed. The various forms of energy are solar, wind, hydro, nuclear, thermal, sound, chemical etc. There is no absolute measure of energy because energy is defined as the ability to do work on objects. The law of conservation of energy states that the total energy of a system can only change if energy is transferred into or out of the system. The SI unit of energy is the joule (J) or newton-meter (N * m).


We use energy to do work in our day to day life. Energy from the sun gives us light during the day. Energy lights our cities, Energy powers our vehicles, trains and airplanes. Energy warms our homes, plays our music, cooks our food, and gives us pictures on television. Energy powers machinery in factories and tractors on a farm. We can explore the rate at which energy is used: a one-minute charge of the battery via solar cell will run a flashlight for several seconds but will run a clock for a comparatively longer time. Everything we do is connected to energy in one form or another. It's easy to make energy conservation part of your daily existence to avoid a serious crisis in the coming decades.


What is Energy?

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Energy is defined as the ability to do work. Let’s consider a classic example of energy from our daily life – When we eat, our bodies transform the energy stored in the food into energy to do work. When we run or walk, we "burn" food energy in our bodies.

Similarly machinery, vehicles, generators, light bulbs etc. also transform energy into work.
Energy is defined as an ability to perform work or act.
A system possesses energy if it has the ability to do work. This above Definition about Energy is same not only for a Human being but further extends to all kinds of living things, non-living things and natural objects. For example, a horse has energy to drag a cart, a piece of coal has the energy to produce heat and a storm has the energy to uproot a tree. It may be realized that energy is a hidden ability which varies from each case. A man’s walk corresponds to one type of energy and burning of coal corresponds to another type.

Points to be remembered:

Energy is…
  • A scalar quantity
  • Intangible and cannot always be perceived
  • Work shifts energy from one system to another
  • All forms of energy are either kinetic or potential
  • All forms of energy are associated with Motion
  • The Joule is the SI unit of energy

Energy has a number of different forms all of which measure the ability of an object to do work on another object. There are two main types of energy which are:

Kinetic Energy: It is an energy which is in Motion. Ex: moving wind, moving water, machines, sound, baseball flying in the air etc. The Kinetic energy of a non- rotating object of mass ‘m’ travelling at a speed (velocity) ‘v ’ is

Kinetic Energy = $\frac{1}{2}$ m $\times\ v^2$

Potential Energy: It is an energy which is stored and measured by the amount of work done. Ex: roller coaster, water wheel, oil in a barrel, water in a lake, book on a table etc. Potential energy is calculated where ‘m’ is the mass in kilograms, ‘g’ is the acceleration due to gravity and ‘h’ is the height in meters.

Potential Energy = m × g × h

Other types of Energy:

Mechanical Energy is the energy of motion that does the work. An example of mechanical energy is the wind as it turns a windmill, wave energy or sound energy.

Thermal or Heat energy is energy that is pushed into motion by using heat. An example is a fire in your fireplace, a hot cup of coffee.

Chemical Energy is energy caused by chemical reactions. A good example of chemical energy is food when it is cooked, glucose in your body.

Electrical Energy is when electricity creates motion, light or heat. An example of electrical energy is the electric coils on your stove.

Gravitational Energy is motion that is caused by gravity. An example of gravitational energy is water flowing down a waterfall, roller coaster and reservoir.

Other types of energy also include: Atomic Energy, Nuclear Energy, Electromagnetic Energy, Magnetic Energy and Solar Energy.
→ Read More Energy conservation refers to reducing energy consumption through using less of an energy service. Energy cannot be created or destroyed, but only changed from one form into another or transferred from one object to another.
The change in energy of an object is due to the transformation and is equal to the work done on the object or by the object for the transformation.

For example, when an object is at a height, a potential energy is stored by virtue of its height. When the same object is dropped the height decreases. But because of the reduction in height, the potential energy is not destructed but it is only transformed into kinetic energy as visible from its velocity during the fall.

Few Examples of Conservation of Energy:
  • When you push a book across the table, the energy from your moving arm is transferred to the book from your body, causing the book to move.
  • Water can produce electricity. Water falls from the sky, converting potential energy to kinetic energy. This energy is then used to rotate the turbine of a generator to produce electricity. In this process, the potential energy of water in a dam can be turned into kinetic energy which can then become electric energy.
  • Potential energy of oil or gas is changed into energy to heat a building.
→ Read More

Energy Resources

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Energy is all about performing work. Animals and human beings require energy resources in order to function, and machines do not function any differently, they require energy resources to work as well. Conservation of energy helps us a lot in energy solutions. We are able to predict the results whenever energy is transformed to another form. Work applied by a conservative force reduces the potential energy converting it to kinetic energy. Energy Resources can be classified into two groups – Renewable energy resources and Non Renewable energy resources. Examples of these types of energy include geothermal energy and hydro electric energy.

Sun is the source of all energy. The solar energy in the form of solar heat is widely used in various types of solar heaters. Water and wind are among the important natural agencies which are considered as important energy resources. Water can be stored at high altitudes in the form of dams and its potential energy can be converted into kinetic energy by streaming the water through pen stocks. The turbines coupled to electrical generators are fitted below the pen stocks to convert the kinetic energy to electrical energy. Almost the same principle is used in wind mills which uses the wind forces to generate electrical power. The energy resources of earth are items like coal, petroleum oils and other mineral products. These products have high calorific values and release high thermal energy which in turn is transformed for useful purpose.

Energy Solutions

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In the context of this article, energy solutions mean that energy equations arise due to conservation of energy. We will explain such energy solutions with examples.

Solved Examples

Question 1:
An object of 5 kg is placed at an height of 10 meters. Suppose the object is freely dropped, what is the velocity of the object when it hits the ground?

First let us calculate the potential energy P of the object when it was at a height of 10 meter.
It is calculated as,
P = 5 $\times$ 9.8 $\times$ 10 = 490kgm/s2
Let ‘v’ meter per second be the velocity of the object when it hits the ground. The kinetic energy K acquired by the object at this point is given by
K = $\frac{1}{2}$(5)(v2) kgm/s2

When the object hits the ground, the entire potential energy has become 0 because the height has become 0. But as per conservation of energy this must be equal to the kinetic energy acquired. Therefore, E = P and hence,
$\frac{1}{2}$(5)(v2) kgm/s2 = 490kgm/s2

From the above equation ‘v’ can be solved as 14. That is, the object hits the ground with a velocity of 14 meters per second.


Question 2:
100 kg of water is heated on a 3 kw electric heater for 1 hour. If the initial temperature of the water was 20°C, what would be its final temperature?

First let us calculate the electrical energy E consumed by the heater. It is given by,
E = 3kw $\times$ 1hr = 3 kwh
= $\frac{3}{2.78\times10^{-7}}$ joules
= $\frac{3}{2.78}$ $\times$107$\times$ 2.39 $\times$ 10-4 kilo calories
= 2580 kilo calories

The thermal energy H acquired in kilo calories by the water is given by,
H = M $\times$ s $\times$ T,
where 'M' is mass, 's' is the specific heat, which is 1 in this case and 'T' is the temperature rise in oC.
    = 100$\times$1$\times$ T = 100T kilo calories.

As per conservation of energy, H = E and therefore,
100T = 2580, which gives the temperature rise as 25.8°C
Therefore the final temperature of water would be (20 + 25.8) °C = 48.8 °C


More topics in Energy
Conservation of Energy Types of Energy
States of Matter Sources of energy
Renewable energy Non Renewable Energy
Inexhaustible Energy Radiation
Convection Conduction
Law of Conservation of Energy Thermal Energy
Power Energy Conversions
Related Topics
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