SK

Ch. 4 Energy

4.1 The Nature of Energy

What is energy?

Every change that occurs, no matter how big or small, involves energy

Change Requires Energy

When something is able to change its environment or itself, it has energy

Different Forms of Energy

Energy moves from one place to another

There are 4 different types of energy:

  • Electrical
  • Chemical
  • Radiant
  • Thermal

An Energy Analogy

Energy from the Sun that warms you and energy from the food that you eat are both energy just in different forms

Kinetic Energy

An object in motion does have energy

Kinetic energy - the energy a moving object has because of its motion

The kinetic energy of a moving object depends on the object’s mass and its speed

Kinetic Energy Equation: KE =12mv2

Joule (j) - SI unit of energy

Potential Energy

Energy does not have to involve motion; motionless objects can have energy since the energy is stored in the object. Therefore, the object has potential energy to cause change

Potential energy - stored energy due to position

Elastic Potential Energy

Elastic potential energy - energy stored by something that can stretch or compress

  • Rubber band
  • Spring

Chemical Potential Energy

Gasoline and food store energy in the same way, in the chemical bonds between atoms

Chemical potential energy - energy stored in chemical bonds

Gravitational Potential Energy

Anything that can fall has stored energy

Gravitational potential energy (GPE) - energy stored by objects due to their position above Earth’s surface

  • The GPE of an object depends on the object’s mass and height above the ground

GPE equation: GPE = mgh

Changing GPE

An objects GPE increases as its height increases

4.1 Summary

Energy

  • Energy is the ability to cause change

  • Forms of energy include electrical, chemical, thermal, and radiant energy

Kinetic Energy

  • Kinetic energy is the energy a moving object has because of its motion
  • The kinetic energy of a moving object can be calculated from this equation: KE =12mv2

Potential Energy

  • Potential Energy is stored energy due to the position of an object
  • Different forms of potential energy include elastic potential energy, chemical potential energy, and gravitational potential energy
  • Gravitational potential energy can be calculated from this equation: GPE =mgh

4.2 Conservation of Energy

Changing Forms of Energy

When the Sun warms your skin on a summer day, it involves energy changing from one form to another

Transforming Electrical Energy

Lightbulbs are a prime example of a device that you use every day that converts one form of energy to another form

Transforming Chemical Energy

Fuel stores energy in the form of chemical potential energy

The engine of a car transforms the chemical potential energy stored in gasoline molecules into the kinetic energy the car.

  • An electric spark ignites a small amount of fuel. The burning fuel produces thermal energy.
  • The thermal energy causes gases to expand and move parts of the car which produces kinetic energy.
    • Chemical energy is changed to thermal energy which then produces kinetic energy

Some energy transformations aren’t obvious because they do not result in visible motion, sound, heat, or light

Conversions Between Kinetic and Potential Energy

Bicycles, roller coasters, and swings can be described in terms of potential and kinetic energy

Mechanical energy - the total amount of potential and kinetic energy in a system

Mechanical energy equation: mechanical energy = potential energy + kinetic energy

Mechanical energy is due to the position and the motion of an object(s) in a system

Falling Objects

As an object falls, it loses height so its GPE decreases. The potential energy is transformed back into kinetic energy as the velocity of the object increases. The form of energy changes but the total amount of energy remains the same

Energy Transformations in Projectile Motion.

Energy transformations also occur during projectile motion when an object moves in a curved path

Kinetic energy and GPE are converted into each other as the object rises and falls

Energy Transformation in a Swing

As a swing rises you lose speed but gain height. In energy terms, kinetic energy changes to gravitational potential energy. Although, at the top of the swing potential energy is at its greatest

The Law of Conservation of Energy

Kinetic and potential energy simply change forms and no energy is destroyed; which is always true. Energy can change from one form to another form but the total amount of energy never changes

The law of conservation of energy - states that energy cannot be created or destroyed.

On a much larger, this law means that the total amount of energy in the universe does not change

Conserving Resources

The law of conservation of energy is a universal principle that describes what happens to energy as it is transferred from one object to another or as it is transformed

Energy in = Energy out

Electrical energy = Thermal, Kinetic, and Sound energy

Is energy always conserved?

Sometimes seeing the law of conservation of energy is hard to see

In some situations you may think that is energy just destroyed or created

The Effect of Friction

After you stop pushing a swing, the mechanical energy of the swing seems to decrease, as if it were being destroyed but it isn’t. The energy goes into a different object by the same amount to keep the original amount of energy the same.

Friction and air resistance cause some of the mechanical energy of the swing to change to thermal energy

The total amount of energy always stays the same

Converting Mass into Energy

Nuclear fusion takes place when the Sun unleashes energy to light and warm the Earth from so far away

Nuclear Fission

A process involving the nuclei of atoms that converts a small amount of mass into enormous quantities of energy

  • in this process nuclei do not fuse—they are broken apart

In both fusion and fission, mass is converted into energy

In processing involving nuclear fission and fusion, the total amount of energy is still conserved if the energy content of the masses involved is included

  • then the total energy before the reaction is equal to the total energy after the reaction

The process of nuclear fission is used by nuclear power plants to generate electrical energy

The Human Body—Balancing the Energy Equation

If you raise your arm and touch your opposite shoulder, stored potential energy within your body was converted to the kinetic energy of your moving arm

Some of the chemical potential energy stored in your body is used to maintain a nearly constant internal temperature; a portion of this energy also is converted to the excess heat that your body gives off to its surroundings

Energy Conversions in Your Body

Chemical potential energy stored in your body is used to fuel the processes that keep you alive

  • your body also converts this energy to heat that is transferred to your surroundings and you use this energy to make your body move

Food Energy

After you eat breakfast your body starts breaking it down into molecules that can be used as fuel; the chemical potential energy in these molecules supplies the cells in your body with the energy they need to function

The food Calorie (C) is a unit used by nutritionists to measure how much energy you get from various foods

4.2 Summary

Energy Transformations

  • Energy can be transformed from one form to another
  • Devices such as lightbulbs, hair dryers, and automobile engines convert one form of energy into other forms
  • The mechanical energy of a system is the sum of the kinetic and potential energy in the system: mechanical energy = KE + PE
  • In falling, projectile motion, and swings, kinetic and potential energy are transformed into each other and the mechanical energy doesn’t change

The Law of Conservation of Energy

  • According to the law of conservation of energy, energy cannot be created or destroyed
  • Friction converts mechanical energy into thermal energy
  • Fission and fusion are nuclear reactions that convert a small amount of mass in a nucleus into an enormous amount of energy

Ch. 4 Study Guide (Reviewing Main Ideas)

4.1 The Nature of Energy

  1. Energy is the ability to cause change
  2. Energy can have different forms, including kinetic, potential, and thermal energy
  3. Moving objects have kinetic energy that depends on the object’s mass and velocity, and can be calculated from this equation:
    KE =12mv2
    KE =12mv2
  4. Potential energy is stored energy. An object can have gravitational potential energy that depends on its mass and its height, and is given by this equation:
    GPE =mgh
    GPE =mgh

4.2 Conservation of Energy

  1. Energy can change from one form to another. Devices you use every day transform one form of energy into other forms that are more useful
  2. Falling, swinging, and projectile motion all involve transformation's between kinetic energy and gravitational potential energy
  3. The total amount of kinetic energy and gravitational potential energy in a system is the mechanical energy of the system: mechanical energy = KE + GPE
  4. The law oof conservation of energy states that energy never can be created or destroyed. The total amount of energy in the universe is constant
  5. Friction converts mechanical energy into thermal energy, causing the mechanical energy of a system to decrease
  6. Mass is converted into energy in nuclear fission and fusion reactions. Fusion and fission occur in the nuclei of certain atoms, and release tremendous amounts of energy