Energy

Energy is a word we hear frequently—whether it's about feeling energetic, saving energy, or generating power. However, in physics, energy carries a very precise meaning. It is defined as the capacity to perform work or bring about change. Everything around us involves energy in one form or another.

From moving cars to the food we eat, energy is constantly at play. Broadly, energy can be categorised into two main types: kinetic energy, related to motion, and potential energy, related to position or condition. Here, we’ll explore the concept of energy and look into various forms it can take.

Units of Energy

The SI unit of energy is the joule (J), named in honour of physicist James Prescott Joule. One joule is defined as the amount of work done when a force of one newton moves an object through a distance of one meter. Besides the joule, energy can also be measured in other units such as calories, kilowatt-hours, kilocalories, and ergs.

Dimensional Formula of Energy

The dimensional formula for energy is [ML²T⁻²].

Different Types of Energy

Energy comes in various forms, but they can all be grouped into two primary categories:

1. Kinetic Energy (K.E)
2. Potential Energy (P.E)

1. Kinetic Energy

Kinetic energy is the energy an object has because of its motion. It depends on both the object's mass and its velocity—the greater the mass or the faster it moves, the higher its kinetic energy. In simple terms, anything that is moving possesses kinetic energy.

Kinetic Energy Formula,

Suppose an object of mass 'm' moves with a velocity of 'v' then the formula use to calculate the kinetic energy of the object is,

K.E = 1/2mv²

Units of Kinetic Energy

• SI unit of K.E. is Joule which is equal to 1 kg.m².s^-2.
• CGS unit of K.E. is erg.

Read More, Kinetic Energy

Different Types of Kinetic Energy

The different types of kinetic energy include:

• Radiant Energy
• Sound Energy
• Electrical Energy
• Thermal Energy
• Mechanical Energy

Radiant Energy:

• Radiant energy is the energy carried by electromagnetic waves. This includes visible light, ultraviolet light, infrared radiation, radio waves, and X-rays.
• It travels through space in the form of waves and does not require a medium to move.
• A common example of radiant energy is sunlight, which provides light and heat to the Earth. Other sources include lamps, lasers, and microwaves.
• Radiant energy is a type of kinetic energy because it involves the movement of electromagnetic waves.

Sound Energy:

• Sound energy is the energy produced by vibrating objects and transmitted through a medium (such as air, water, or solids) in the form of sound waves. These waves are mechanical in nature, meaning they need a medium to travel and cannot move through a vacuum.
• When an object vibrates, it creates disturbances in the surrounding particles, which then pass the vibrations from one particle to the next. This movement of particles carries energy, which we perceive as sound.
• For example, when you pluck a guitar string, it vibrates and produces sound energy that travels through the air to your ears.

Electrical Energy:

• Electrical energy is the energy caused by the movement of electric charges (usually electrons) through a conductor, such as a wire. It is a form of kinetic energy since it involves moving particles.
• This energy powers many devices and systems in our daily lives, such as lights, computers, appliances, and electric vehicles. Electrical energy can be generated from various sources, including batteries, solar panels, generators, and power plants.
• For example, when you plug in a fan and turn it on, electrical energy flows through the circuit and is converted into mechanical energy to spin the blades and produce air movement.

Thermal Energy:

• Thermal energy is the energy that comes from the movement of particles within a substance. The faster the particles move, the more thermal energy the substance has. It is often felt as heat.
• Thermal energy is a form of kinetic energy, because it results from the motion of atoms and molecules. It can be transferred from one object to another through conduction, convection, or radiation.
• For example, when you heat water on a stove, the thermal energy increases as the water molecules move faster, eventually leading to boiling.

Mechanical Energy:

• Mechanical energy is the energy possessed by an object due to its motion or position. It is the sum of two types of energy: kinetic energy (energy of motion) and potential energy (stored energy due to position or shape).
• Mechanical energy allows objects to move or do work. It is commonly seen in machines, moving vehicles, or even a swinging pendulum.
• For example, a moving car has mechanical energy because of its motion (kinetic), and a stretched rubber band has mechanical energy due to its position (potential).

2. Potential Energy

When work is done on an object, the energy is stored within it, and this stored energy is known as potential energy. Potential energy depends on the position or condition of the object. For instance, a stretched rubber band stores energy in the form of elastic potential energy, which is one type of potential energy.

Potential Energy Formula,

The energy of an object due to its position is called the potential energy of the object. Suppose an object of mass 'm' is placed at height 'h' against the gravitational acceleration 'g' then the work done is equal to the gain in potential energy of the object. Then the potential energy formula for the same is,

P.E. = m.g.h

Units of Potential Energy(P.E.)

• SI unit of P.E. is Joule which is equal to 1 kg.m².s-².
• In CGS system unit of P.E. is erg.

Different Types of Potential Energy

The different types of Potential energy include:

• Gravitational Potential Energy
• Elastic Potential Energy
• Electric Potential Energy
• Chemical Potential Energy

Gravitational Potential Energy:

• Gravitational potential energy is the energy stored in an object due to its height above the ground. It depends on the object's mass, the height it is raised to, and the force of gravity.
• The higher an object is lifted, the more gravitational potential energy it gains. This energy can be converted into kinetic energy when the object falls.
• For example, water stored in a tank on a rooftop has gravitational potential energy. When the water flows down, that stored energy is released as it moves.

Elastic Potential Energy:

• Elastic potential energy is the energy stored in an object when it is stretched, compressed, or deformed and has the ability to return to its original shape. This type of potential energy is commonly found in elastic materials like springs, rubber bands, and trampolines.
• When you stretch a rubber band or compress a spring, you do work on it, and that work is stored as elastic potential energy. Once released, this energy can be converted into kinetic energy or other forms.
• For example, A compressed spring in a toy car stores elastic potential energy. When released, this energy helps move the car forward.

Chemical Potential Energy :

Chemical potential energy is the energy stored within a substance due to its chemical composition and the arrangement of atoms in its molecules. This energy is released or absorbed during chemical reactions when bonds between atoms are formed or broken.

For example, in an electrochemical cell (such as a battery), chemical potential energy is stored in the substances inside the cell. When the battery is connected in a circuit, a chemical reaction occurs, converting this stored energy into electrical energy to power devices like flashlights, phones, or remote controls.

Elastic Potential Energy:

Elastic potential energy is the energy stored in an object when it is stretched or compressed. This type of energy arises from the object���s ability to return to its original shape after being deformed.

For example, a stretched rubber band stores elastic potential energy. When released, this stored energy is converted into motion as the rubber band snaps back to its original form.

Also Read, Difference between Gravitational Potential Energy and Elastic Potential Energy

Energy Conversion: Transfer and Transform

It is a well-known fact that energy can be transformed from one form to another form. The transfer of energy from one form to other form is known as energy transfer. The transformation of energy generally categorised into four ways,

• Mechanical Energy Conversion
• Electrical Enegy Conversion
• Energy Conversion By Radiation
• Energy Conversion By Heating

Law of Conservation of Energy

Law of Conservation of Energy states that, "Energy can neither be created and nor be destroyed, and it can only change its form from one form to another."

In other word we can also say that, "In a closed and isolated system the total energy of the system is always conserved."

This is one of the basic law of the physics and help us to explain various astronomical and physical phenomenon.

Work-Energy Theorem

Work Energy theorem states that, the change in kinetic energy on an object is equal to the work done by the object.

⇒ W_net = (KE)_final – (KE)_initial

W_net = 1/2m(v² - u²)

In other words, we can say that, the work done by the object is equal to change in kinetic energy of the object.

What is Power?

Power is the rate of doing work or the rate of transfer of energy. It is denoted by P.

In other words, the the ratio of the work done by the object by the time taken is defined as the power of the object.

P = W/t

• SI unit is Watt (Js⁻¹).
• Commercial unit of power is kWh, i.e., energy used in 1 hour at 1000 Joules/second.

1kWh = 3.6×10⁶ J

Solved Examples on Energy Formula

Example 1: How much work is required to stop the car in 30 s if the kinetic energy of the car is 6000 J and what is its power?

Solution:

Work done to stop car = change in Kinetic energy
W = K.E. at stop - KE at start
W = 0 - 6000
W = -6000 J (negative sign means work is done against car)
Time = 30 sec (given)
Power = Work Done /Time = 6000/30 = 200 Watt

Example 2: Two passengers of mass 40 kg each sit in the car then find the Kinetic energy of car if the mass of the car is 700 kg and velocity of the car is 18 km/h.

Solution:

Given,

• Mass of Car = 700 kg
• Mass of Each Person = 40 kg
• Total Mass(m) = 700 + 40 + 40 = 780 kg
• Velocity = 18 kn/hr = 18(5/18) = 5 m/s

K.E. = 1/2 mv²
K.E. = 1/2(780)(5)²
K.E. = 9750 J

Example 3: Find the work done by the 10 N force acting on the object at the angle of 60°, which displaces the object 10 m.

Solution:

Given,

• F = 10 N
• s = 10 m
• θ = 60°

W= F.s.cosθ
cos 60° = 1/2
W = (10).(10).(1/2)
W  = 50J

Example 4: 245 × 10² J of work doneto raise a 50 kg boy above the ground. How high would he be raised? (g = 9.8 m.s^-2)

Solution:

Given,

• Work Done = 24500 J
• m = 50 kg
• g = 9.8 m.s^-2

Work Done = m.g.h
245 × 10² = 50 × 9.8 × h
 h = 50 m
The height at which the boy is raised is 50 m.