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The SI unit of energy is Joule which is named after James Prescott Joule, which is same as the unit of power. The CGS unit of energy is erg. Energy, which is a scalar quantity, can be refrred to as the ability to do work. It can be kinetic, potential, electrical, chemical, nuclear, thermal, and many other forms, as well. Renewable energy and non-renewable energy are two types of energy.
NCERT Solutions of: Class 9 Science Chapter 11 Work and Energy
Key takeaways: Unit of energy, Joule, scalar quantity, kinetic energy, potential energy, power, electric energy.
What is the SI Unit of Energy?
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Following is the table of SI unit:
| SI unit | Joules |
The name of this international unit is kept in the name of James Prescott Joule, a British physicist. His works contributed to the establishment of the energy concept. The unit in fundamental terms, 1-N.m is equal to 1 Joule and, in terms of SI base units it can be represented as:
| 1J = 1 kg(m/s)2 = 1 kg.m2 /s2 |
What is the CGS Unit Of Energy?
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Following is the table of the unit in the CGS system:
| CGS unit | erg |
The erg is equal to 10-7 J. An erg is defined as the amount of work done by the force of one dyne at a one-centimeter distance.
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What is the Unit Of Energy In the MKS System?
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Following is the table of the unit in the MKS system:
| MKS unit | Joule |
Definition of Joule and Erg
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In the SI system (International System of Units), the unit of energy is Joule. Joule is named after the scientist James Prescott to honour him for his establishment of the concept that energy can be transferred.
\(Joules = \frac {kg * m^2} {s^2}\)
A joule is equal to a Newton meter; Newton is the SI unit for force. A Newton is the force needed to accelerate one kg of mass at the rate of one metre per second square. This can be represented in SI base units (meter, seconds, mole, ampere, Kelvin, candela, kilogram) as,
| I J = N.m = kgm2/s2 |
The MKS and SI system of units are the same, and hence Joule is the unit of energy in the MKS system.
In the CGS system(centimetre gram second system), the unit of energy is erg. Erg is a term derived from the Greek word 'ergon' which means 'work'.
The energy unit Erg is equivalent to one dyne centimeter. Dyne is the unit of force in the CGS system; one dyne is the force required to accelerate 1 gram at the rate of 1 cm per second.
| 1 erg = 1 dyn.cm = g⋅cm2/s2 |
Classification of Energy Units
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Units of energy can be classified into two types: Basic units and Source-based units. Basic unit of energy is not associated with a particular source of energy; Joule is a basic unit of energy. Source-based unit of energy is associated with a particular source of energy; A barrel of oil is a source-based unit of energy.
Types of Energy
Various Energy Units
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- A barrel of oil is a unit of energy, but this is not a SI system-based unit. This unit is used to measure the volume of liquid hydrocarbons. It is the energy produced by burning one barrel of oil, which equals 6.12 × 109J.
- Calories are the energy unit to estimate heat energy. It is the energy required to increase the temperature of one gram of water by one degree Celsius. One calorie is equal to 4.18 joules.
- A horsepower is a unit used to measure the output of a mortar or engine. It is the energy required to move 550 pounds for a distance of one foot in one second. One Horsepower is equal to 745.7 joules per second.
- Kilowatt-hour is used to measure the electric energy used; It is the commercial unit of energy. Measuring large units of energy in Joules is not sophisticated and so kilowatt-hour is used. 3.6x106 joules are equal to a one-kilowatt hour.
- The energy unit megajoule is equal to 1 million joules.
- A British thermal unit is used to measure heat content. The heat required to raise the temperature of one pound of water by one Fahrenheit is a BTU. One BTU is equal to 1055 joules.
- An electronvolt is used in atomic physics as an energy unit. The amount of kinetic energy gained by an electron accelerating from rest through an electric potential difference of one volt in a vacuum is an electronvolt. 1 eV is equivalent to 1.602 × 10-19 joules.
- The Hartree is the unit of energy in the Hartree atomic units system, named after the physicist Douglas Hartree. One Hartree is equal to 4.3597×10-18 joules
- The Rydberg is used to calculate the energy levels in the hydrogen spectrum. One Rydberg is equal to 2.179872 x 10-18 joules.
Energy Conversion Table
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| 1 joule | 1 calorie | 1 kilocalorie | 1 kilowatt hour | 1 foot pound | 1 British thermal unit | |
|---|---|---|---|---|---|---|
| 1 joule | 1 | 0.2388 | 2.388x10-4 | 2.78x10-7 | 0.7376 | 9.481x10-4 |
| 1 calorie | 4.187 | 1 | 0.001 | 1.16x10-6 | 3.088 | 0.003969 |
| 1 kilocalorie | 4187 | 1000 | 1 | 0.001163 | 3088 | 3.969 |
| 1 kilowatt hour | 3.6x106 | 8.598x105 | 859.8 | 1 | 2.65x105 | 3413 |
| 1 foot pound | 1.356 | 0.3238 | 3.238x10-4 | 3.76x10-7 | 1 | 0.001285 |
| 1 British thermal unit | 1055 | 252.0 | 0.252 | 2.93x10-4 | 777.9 | 1 |
Points to Remember
- Energy is transferred from one form to another; it is neither created nor destroyed.
- Energy is the potential to do work.
- Chemical, thermal, nuclear, electrical, mechanical are some important kinds of energy.
- Joule is the unit of energy in SI and MKS systems; SI and MKS systems of units are the same. In the CGS system energy is measured in erg, which is equal to 10-7J.
- 3.6x106 J is equal to one kilowatt hour, which is the commercial unit of energy.
- Energy units can be classified as basic units and Source-based units.
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Sample Questions on Unit of Energy
Ques: Write the difference between power and energy. (2 marks)
Ans: Rate of work done is called power. It is work done by time taken. The unit of power is watt.
Energy is the work done. It is represented as power multiplied by time. The unit of energy is joule.
Ques: What is the commercial energy unit? (1 mark)
Ans: When a huge amount of energy is utilised commercially, it is inconvenient to use joule as the unit and hence kilowatt hour is used. It is the commercial unit of energy. 3.6x106 joules is equal to one kilowatt hour.
Ques: Kinetic energy is one of the forms of energy. Explain. (1 mark)
Ans: Kinetic energy is the energy an object possesses while in motion. Examples of kinetic energy are rolling ball, pouncing kitten, walking man, swaying tree, and bullet from gun.
Ques: Mention the basic forms of energy. (4 marks)
Ans: Some basic forms of energy are:
- chemical energy
- electrical energy
- Kinetic energy
- radiant energy
- mechanical energy
- Potential energy
- Nuclear energy
- Thermal energy.
Ques: How many joules are in one kilowatt hour. (1 mark)
Ans: 3.6 x 106 joules are in one Kilowatt hour.
Ques: Define law of conservation of energy. (1 mark)
Ans: The law of conservation of energy states that energy can neither be created nor destroyed, it can only be transformed from one form to another.
Ques: Classify the Units of Energy. (1 mark)
Ans: Units of energy can be classified into two types: Basic units and Source-based units. A basic unit of energy is not associated with a particular source of energy; Joule is a basic unit of energy.
Ques: Name the units of the energy list. (4 marks)
Ans: Energy units can be categorized by various factors and there are usually different units for measuring energy. Some common units include:
- British thermal unit (BTU)
- Horsepower
- Kilowatt-hour (kWh)
- Calorie
- Electronvolts (eV)
- Hartree (the atomic unit of energy)
- Rydberg units
- Barrel of oil
Ques: What is the relation between power and energy? (2 marks)
Ans: The relation between power and energy is that power has energy units that are divided by time.
The unit of power is Watts.
Unit of energy is Joules
1 Watt = 1 Joule/1 second
Ques: When the unit of force and length is doubled what happens to the unit of energy? (3 marks)
Ans: The unit of unit energy will increase by 4 times when the unit of force and length is doubled.
The relationship between energy, force, and distance is given as:
Energy = force x distance
The units of force and distance are N and m, respectively.
Therefore,
E = N x m
Given that,
E = 2 N x 2 m
Therefore, E = 4Nm.
Ques: What is the relationship between commercial and SI units of energy? (3 marks)
Ans: The commercial unit of energy is 1 kWh. One kilowatt-hour is referred to as the amount of energy consumed by a device in one working hour at a constant rate of one kilowatt.
The SI unit of energy is Joule.
Therefore, the relationship between commercial and SI units of energy is:
1 kWh = 1kW x 1h = 1000W x 1h = 1000(J/s) x 3600 s = 3.6 x106 J
Ques: One unit of electrical energy is equal to how many Joules? (1 mark)
Ans: One unit of electrical energy is equal to 3.6 × 106 Joules.
Ques: When will a spring have maximum potential energy? (1 mark)
Ans: Spring will have maximum energy when it is either pulled out or compressed.
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