In physics, we encounter various quantities having varying attributes, and describing them becomes difficult as we talk about the terms like energy, time, work, or any physical quantity that needs to have a standard measure to distinguish. In physics, the units determine the standard measure of these quantities. Such as if I say my aunt is 36 kg and she lives 1200 km from my city. Here, kg (for weight) and km (for distance) are the units to describe these physical quantities. Other examples are Kelvin for temperature; etc.
According to the international system (SI), the unit of energy is Joule which is denoted by J.
On this page, we will learn about the following:
Unit of energy
The SI unit of energy
CGS unit of energy
The SI unit of potential energy
MKS unit of energy
The unit of energy in the MKS system
CGS unit of power
CGS unit of work
The SI unit of electrical energy
The SI unit of energy is Joule (the basic energy unit of the metric system) symbolized as J is named in honor of an English physicist James Prescott Joule (1818–1889) for his experiments on the mechanical equivalent of heat.
The unit of energy is the same as that of work (As work and energy are the two sides of the same coin).
The CGS stands for centimeter-gram-second.
The CGS unit of energy is erg equivalent to 1 g cm^2 s^ - 2.
Ergs are a lot smaller than Joules, equivalent to 10 ^ - 7 J.
The potential energy is the energy stored in the matter. Therefore, the SI unit of potential energy is the same as the SI unit of all energies given by,
1 J = 1 kg m^2 s^ - 2 where the potential energy due to height is described as,
P.E. = mgh whose MKS unit is also kg m^2 s^ - 2 = 1 Joule.
The MKS stands for meter-kilogram-second.
1 Joule is equivalent to one Newton-meter (N-m) where 1 J is the amount of work done by a newton of force to a distance of 1 meter.
Where 1 J = kg (m/s)^2 = 1 Watt-second = 1 kg m^2 s^- 2
So MKS unit of energy is kg m^2 s^- 2
A force does the work to accelerate a body of mass 1 kg at the rate of 1 ms^ - 2.
Power is defined as the rate of doing work given by,
Power = Work / time
The CGS unit of power is erg per second.
The CGS unit of work is Erg where 1 erg = 1 gm cm square/ second square.
The word erg is derived from the Greek word ergon (ἔργον) which means 'work' or 'task'.
One erg is also defined as the work done by a force of 1 dyne on a one-centimeter distance.
The movement of charged particles inside the wire produces electricity. The electrical energy is the form of energy resulting from the flow of electrical charge is called electrical energy.
The SI unit of electrical energy is Joule symbolized as J.
1 Joule is equal to the energy transferred (or work done) when applying a force of one newton through a distance of one meter (1 newton-meter or N-m),
It is also defined as the work done in passing an electric current of one ampere through a resistance of one ohm for one second.
Unit | Value equivalent to Joule |
1 Cal (IT, or international table calorie) | 4.1868 Joule |
1 Cal (Th, or thermochemical calorie) | 4.184 Joule |
1 Cal 15 (Calories from 14.5°C to 15.5°C | 4.1855 Joule |
1 mean calorie | 4.1900 Joule |
1 Hartree or Eh | 4.35975 × 10 ^ −18 Joule |
1 Btu (British thermal unit) | 1055.06 Joule |
1 Tce (a ton of coal equivalent) | 29.3076 × 109 J |
1 Toe (a ton of oil equivalent) | 41.868 × 109 Joule |
1 Mwyr (Mega-watt year) | 31.557952 TJ (Tera-Joule) |
1 KiloJoule | 1000 Joules |
1 MegaJoule | 10,000,00 Joules |
1 Kilowatt-hour | 3.6 x 10^ 6 Joules or 3.6 million Joules |
1 electronVolt | 1.602 x 10 ^ -19 Joule |
1. What is energy? List the different forms of energy.
In physics, energy is a quantity that must be transferred on to the body to perform the work or any task as energy is defined via work.
There are two main forms of energy, namely, Potential energy (P.E), and Kinetic energy (K.E.). The energy is further classified into two types, namely renewable and non-renewable energy. Other forms of energy are:
Radiant energy
Sound energy
Elastic energy
Light energy
Motion energy
Gravitation energy
Nuclear energy
Electromagnetic energy
Mechanical energy
Wind energy
Biomass energy
Tidal energy
Geothermal energy
Hydroelectric energy
Wave energy
Hydrogen energy
Thermal energy
2. What do you mean by the law of conservation of energy?
The law of conservation of energy states that the energy can never be created or destroyed but can be transformed into another form.
For example, when a box slides through a slop, at rest it has potential energy stored in itself when comes into motion that energy gets changed into the kinetic energy. The friction acts in the opposite direction. Therefore, the kinetic energy from the box gets transformed into the thermal energy heats the box and the slope.
3. How is power related to work and energy?
Work is done by energy and the rate at which any task is done is called the power. Let’s take an example, suppose you’re driving a car. The air friction is trying to slow down your car, the engine is powerful enough to accelerate the car. So here the work done is equal to the force applied by an engine to a distance. Now the rate at which the engine does this work is called the power.
4. Why do physical quantities need units?
The units are important because, without units, we can never express physical laws precisely just from qualitative reasoning. Such as I am 8 but if I say I am 8 years old so years old is the unit to define the quantity age.
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