chapter 3 physics for junior high school viii. energy definition energy is the ability to do a...

CHAPTER 3

PHYSICS FOR JUNIOR HIGH SCHOOL VIII

WORK AND ENERGY


ENERGY DEFINITION

Energy is the ability to do a work.

Energy is acquired by oxidation (combustion) of food substances that are digested inside our body.

Energy unit in International Unit calorie, erg, andSystem is Joule (J).

The units of energy in other unit systems are kalori, erg, and kWh (kilo watt hours). The ratio between Joule and calorie :

1 Calorie = 4,2 Jouleor

1 Joule = 0,24 Calorie


THE FORM OF ENERGI

a. Chemical Energy

Chemical energy is the energy that is kept in chemical compound. Foods contain much energy that is very important for human body

b. Electrical Energy

Electrical energy occurs by various ways, e.g. :1.) using river or water fall that has kinetic energy, (2) utilising wind energy that is used for turning the windmill, (3) using accu (chemical energy), (4) using steam energy to turning the electric generator, (5) to utilize diesel energy, and (6) nuclear power.


THE FORM OF ENERGY

c. Heat Energy

The total of given heat energy can be formulated as follows :

Q = m x c t calorie, where Q = explains the total of heat energy in calorie

m = to explain the mass/ the substance that receives heat energy

c = to explain the specific heat/the substance that receives heat

t = to explain the temeprature change

d. Light energy

light energy, ecpecially sunlight is absolutely needed by greenery.

FISIKA SMP KELAS VIII

THE FORM OF ENERGY

e. Sound energy

sound can be defined as vibration, so sound energy means also vibration.

f. Resilience

all elastic or flexible things must have resilience. E.g. spring, archer’s bow, slingshot, and trampoline.

g. Nuclear energy

nuclear energy is the result of fission-reaction that occurs in the atomic nucleus.

h. Mechanical energymechanical energy consisted of potential and kinetic energy.



THE FORM OF ENERGY

Energy mechanics :

a. Kinetic Energy

Kinetic energy is energy that is existing in one object when it is moving. the total of kinetic is proportional with the mass and the velocity-square. So, kinetic energy can be stated in an equation as follows:

b. Potential Energy

Potential energy is energy that is existing in one object because of its position to the earth. E.g. a mango falling down to the earth, it has potential energy because its motion. The equation for potential energy :


THE CONVERSION OF ENERGY

Electrical energy to heat energy

The example of this conversion occurs on the room-heater machine, electric stove, electric iron, heater, electric blanket, and solder.

Mechanical energy to heat energyThis conversion could be found at the friction of two objects, e.g. if you are fiddling your two hands, you will feel hot.



Mechanical Energy to sound energy

this conversion occurs when we are clapping hand or if we hit two objects strongly

Chemical Energy to electrical energy

The conversion in battery accumulator is the kind of this conversion.



Electrical Energy converted to light and heat energythis conversion occurs on blazing-lamp. As we have mentioned, light energy is commonly followed by another energy, like heat energy. Try to bring nearer your hands to a blazing-lamp! You would feel hooter and hooter..

Light energy to chemical energyThis conversion could be observed in a photograpic process until the picture done.


THE CONSERVATION LAWS

The number of energy converted to other forms is equal to the number of energy reduced, so that the total o energy in the system is constant. Therefore, we can conclude that energy can neither be created nor destroyable, energy can be only converted to other forms. This statement then recognized as the conservation laws.


WORK One object considered committing work if there is a

force given, or the object gives a force that changes its position.

Mathematically, work can be stated as follows : W = F × s

If one object has a work F = 1 N, and changes its position for1 m, the work and its unit can be counted as follows :W = F × s = 1 N × 1 m = 1 NmNext, known that1 Nm = 1 Joule.


THE RELATIONSHIP BETWEEN WORK AND ENERGY

The number of work that is required for removing a ball is equal to the difference of both kinetic and potential energy.

So, we can conclude that the number of work is equal to total energy conversion.


THANK YOU

chapter 3 physics for junior high school viii. energy definition energy is the ability to do a...

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