Chapter 5 – Energy And Its Form

PhysicsGrade 7

Contents

• What is Energy?• Forms of Energy• Mechanical Energy• Potential Energy• Kintetic Energy• Energy Conversion• Work• Examples• Remember• Tips• Exercises• Backup

Module Objectives

• Under stand the concepts of Energy and Its Forms• Concepts and Components of Mechanical Energy • Energy Conversion• Concept of Work

What is Energy? • Energy is needed for everything we do• Need energy to Walk to School, to Play, to cook food, to run a

Computer etc..• Energy can be defined as the ability to do work • Energy is measured by the work that a body can do• Different kinds of Energy exist in the Universe• Energy can do different things• It can be found in many things and takes many forms

Forms of Energy• Energy exists in many forms: Heat, Light, Sound, Wind,

Electricity, Chemical Energy, Solar Energy, Magnetic Energy, Nuclear Energy etc.

• It primarily exists in the Mechanical form which makes the body to work in the virtue of its Position or Motion

Mechanical Energy• Mechanical energy is the energy possessed by an object due to its

motion or its position• Mechanical energy can take the form of either kinetic energy or

Potential energy (comes from Latin word – to be able)• Kinetic Energy is the energy due to an object's motion (comes from

Greek word - motion)• Potential energy is the stored energy due to an object's position• The position and a change in position of a body leads to Potential

energy and Kinetic energy respectively

Potential Energy

• Potential Energy is the energy possessed by a body by virtue of it’s position

• Activities to understand Potential Energy:1. Catapult Activity: Take a Catapult. Hold the rubber sling at it’s

end, and stretch it as shown in the figure. Do you feel a pull back? Yes, the sling tends retain the original position.

A Catapult acquires Potential energy at a stretched position

Potential Energy Activities (contd)

2. Compressed Spring Activity: Take a spring. Put it vertically on a table and compress the spring vertically downward. Do you feel a push back? Yes, it tends to regain it’s original position. Generally a spring acquires Potential energy ata compressed or stretched position. When this energy is released an equivalent amount of work is done. The potential energy of a compressed spring is used to fire a bullet from a gun

Potential Energy: Think…

• Elasticity or a position sustainity of a body is the real cause of its potential energy

• Know This:• An object at rest can have potential energy due to its raised position• The higher an objec t is raised above the ground level, the greater is

its potential energy• Greater the mass of the body, the greater is the potential energy

Potential Energy and Height

• Expression of Potential Energy of a body at height ‘h’:

Potential Energy: Know this..

Kinetic Energy

• Kinetic Energy is the energy possesed by a body by virtue of it’s motion

• It is measured by the amount of work the body can do before coming to rest

Kinetic Energy: Know This

• In hydro power station the kinetic energy of the falling water rotates the turbines connected to an electric generator. Thus, the electric energy is generated by the Kinetic energy of the water.

Kinetic Energy: Activities

1. Let us play the game of carrom

Kinetic Energy: Activities (contd)

2. Have you played Cricket?

Kinetic Energy: Activities (contd)

Kinetic Energy: Activities Conclusion

• Bodies in motion have Kinetic Energy• If two bodies of same size and different mass are travelling at

the same speed then the body of greater mass has greater Kinetic Energy

• If the speed of a body increases, its kinetic energy also increases and if the speed of the body decreases its Kinetic energy also decreases

Kinetic Energy: Expression

Kinetic Energy: Expression (contd)

Kinetic Energy: Practical Activity

Energy Conversion

• Conversion between potential and kinetic energy that occur when you shoot a basketball in a basket• When the ball is positioned in your hands it has full potential energy.

As you move and the ball is thrown at the basket, the potential energy is converted to kinetic energy, by the transfer of kinetic energy from your arms, to your hands, to the ball. When the ball hits the basket some of its kinetic energy was converted into potential energy and then back into kinetic energy as it hit the ground.

Factors Affecting the Work

Work: Measurment

• Note: Greater is the displacement, greater is the work

Work: Measurment (contd)

Worked Examples

Worked Examples (contd)

Remember• A body can have two types of energy – Potential and Kinetic• Potential energy and Kinetic energy together constitue the Mechanical

Energy• The energy possessed by a body by virtue of its position or shape is called

Potential energy• A moving object possess Kinetic energy• Potential energy depends on the mass of the body and the height to

which it is raised• Kinetic energy depends upon the mass of the body and its velocity• Potential Energy – P.E = mgh joule• Kinetic Energy – K.E = ½ MV2 joule• Force – F = MA newton• Work done – W = Fs joule• Potential energy of a body can be increased by raising it to more height or

bringing in to a definite shape• Kinetic energy of a body can be increased by increasing the force applied

and reducing the friction

Tips

• Do not step out suddenly from a moving vehicle. Otherwise kinetic energy acquired by your body will make you fall down

• Try to gain more kinetic energy by running fast in athletic events, such as long jump and high jump

• Do not waste energy as it can be referred as stored work

Excercises

Excercises (contd)

Excercises (contd)

Excercises (contd)

Excercises (contd)

Excercises (contd)

Excercises (contd)

Excercises (contd)

Excercises (contd)

Project Work

Backup • Thermal Energy• Chemical Energy• Electrical Energy• Sound Energy• Light Energy• Electrochemical Energy• Electromagnetic Energy• Nuclear Energy

Topic

Thermal Energy• Thermal energy is the part of the total potential energy and kinetic

energy of an object or sample of matter that results in the system temperature. It is represented by the variable Q, and can be measured in joules

• Example: Consider a hot cup of coffee. The coffee is said to possess "thermal energy", or "heat energy" which is really the collective, microscopic, kinetic and potential energy of the molecules in the coffee (the molecules have kinetic energy because they are moving and vibrating, and they have potential energy due their mutual attraction for one another - much the same way that the book and the Earth have potential energy because they attract each other). Temperature is really a measure of how much thermal energy something has. The higher the temperature, the faster the molecules are moving around and/or vibrating, i.e. the more kinetic and potential energy the molecules have

Chemical Energy • Chemical energy is the potential energy of a chemical

substance to undergo a transformation through a chemical reaction or, to transform other chemical substances.

• Example: Consider the ability of your body to do work. The glucose (blood sugar) in your body is said to

have "chemical energy" because the glucose releases energy when chemically reacted (combusted) with oxygen. Your muscles use this energy to generate mechanical force and also heat. Chemical energy is really a form of microscopic potential energy, which exists because of the electric and magnetic forces of attraction exerted between the different parts of each molecule - the same attractive forces involved in thermal vibrations. These parts get rearranged in chemical reactions, releasing or adding to this potential energy.

Electrical Energy • Electrical energy an energy made available by the

flow of electric charge through a conductor. Wind farms, on the other hand, are examples of wind turbines that convert wind energy into electrical energy.

Sound Energy • Sound energy is the energy produced by vibrating sound

waves• Sound waves are compression waves associated with the

potential and kinetic energy of air molecules. When an object moves quickly, for example the head of drum, it compresses the air nearby, giving that air potential energy. That air then expands, transforming the potential energy into kinetic energy (moving air). The moving air then pushes on and compresses other air, and so on down the chain. A nice way to think of sound waves is as "shimmering air".

Light Energy • Light energy is the only form of energy that we can

actually see directly. It is formed through chemical, radiation, and mechanical means. Light energy can also be converted into other forms of energy

Electro Chemical Energy • Electro chemical energy is the energy that involves

electricity and is stored in a chemical way• Consider the energy stored in a battery. Like the

example above involving blood sugar, the battery also stores energy in a chemical way. But electricity is also involved, so we say that the battery stores energy "electro-chemically".

Electromagnetic Energy • Electromagnetic Energy is the energy transmitted by

any source of light• Consider the energy transmitted to the Earth from

the Sun by light (or by any source of light). Light, which is also called "electro-magnetic radiation". Why the fancy term? Because light really can be thought of as oscillating, coupled electric and magnetic fields that travel freely through space (without there having to be charged particles of some kind around).

Nuclear Energy • Nuclear energy is the energy that holds the nucleus

of an atom• The energy released during nuclear fission or fusion,

especially when used to generate electricity.

Energy Conversions

Additional Informational Links

• http://www.childrensuniversity.manchester.ac.uk/interactives/science/energy/what-is-energy/