The Nature of Energy

What is Energy? It is related to matter. When we eat more food (matter), we have more energy. Can you touch it and feel it like matter?

What is Energy? No. We can only describe it as we see it at work. Energy is the ability to do work, not a substance. So we do not know what energy is, just what it can do.

What is Energy? We do know some things about energy: It moves everything. It is a scalar quantity. It is operationally defined (described).

What is Energy? Work is done when the energy produces a force which pushes on an object and moves it. Work and energy are interchangeable. They are measured in the same unit, the joule.

Joule a unit of energy 1 joule = kg • m2 s2 1

Joule Work is the product of a force (newtons) exerted on an object times the distance it moves (m). Remember Newton’s 2nd Law of Motion. F = ma N = kg • m s2

work = force • distance = N • m kg•m • m = s2 kg • m2 = = Joule s2

2 Main Kinds of Energy Potential energy (PE) is stored energy. It can be stored by position or condition. Kinetic energy (KE) is energy of motion. It depends on the mass and speed of matter.

Energy is power. momentum. the ability to do work. force.

Which of the following is energy of motion? Potential Kinetic Gravitational Elastic

Classification of Energy

We will return to this form of energy later. Mechanical Energy the motion of objects We will return to this form of energy later.

Thermal Energy the sum total motion of all atoms and molecules in an object Changes in thermal energy are called heat.

Acoustic Energy vibrations in matter that produce sound

Electrical Energy Charged objects exert field forces. (Remember the four fundamental forces?) These field forces can do work on other charged objects by attracting or repelling.

Magnetic Energy objects with magnetic fields This is closely related to electricity.

Radiant Energy This is the direct combination of electricity and magnetism. Our sun emits radiant energy as visible light, infrared, UV, X-rays, and much more.

Chemical Energy Remember that chemical change occurs when atoms combine in new groups. To do this, chemical bonds are formed and energy is released.

Nuclear Energy Nuclear change also releases energy, but it is millions of times greater than the amount released by chemical energy.

Nuclear Energy Nuclear energy is released by two different processes. Fission—a large nucleus splits into pieces Fusion—small nuclei combine together

Mass Energy This is the largest potential source of energy in the universe. This would be achieved by converting mass to energy as in the equation E=mc2.

Sample Problem 1 This amount of energy is equivalent to that released by the nuclear bomb dropped over Nagasaki, Japan near the end of WWII—the equivalent of 20,000 tons of TNT. How much energy could be obtained from a 1 g ice cube? E = mc2 E = (0.001 kg) (3.00 × 108 m/s)2 E = 9 × 1013 Joules

Mechanical Energy the motion of objects two categories Kinetic Energy Potential Energy Kinetic Energy Gravitational Elastic

Gravitational Potential Energy (GPE) The higher an object is above a surface, the greater the GPE. The surface is usually the ground.

Note that the units are kgm2/s2 = J. Gravitational Potential Energy (GPE) GPE = mgh = wh mass (kg) weight (N) height (m) 9.81 m/s2 Note that the units are kgm2/s2 = J.

Sample Problem 2 What is the GPE of a 1 kg mass held 1 m off the ground? GPE = mgh GPE = (1 kg)(9.81 m/s2)(1 m) GPE = 9.81 J

Elastic Potential Energy (EPE) When you stretch a rubber band, you are storing energy. The more you stretch it, the greater the force it exerts, so the greater the energy that is stored.

Elastic Potential Energy (EPE) This also works with compressing or stretching a spring.

Note that the units are again kgm2/s2 = J. Kinetic Energy (KE) KE = 1 2 mv2 mass (kg) speed (m/s) Note that the units are again kgm2/s2 = J.

Sample Problem 3 What is the KE of a 1 kg mass rolling on a surface at 10 m/s? mass = 1 kg speed = 10 m/s

Sample Problem 3 1 mv2 KE = 2 1 (1 kg) (10 m/s)2 = 2 1 (1)(100) kgm2 = = 50 J

Kinetic Energy (KE) Did you notice that the formulas for KE and momentum are similar? They are not the same. If you double the speed, momentum is doubled, but KE increases fourfold.

Thermal Mechanical Thermal Mass Nuclear The sum total motion of all atoms and molecules in an object describes which form of energy? Thermal Mechanical Thermal Mass Nuclear

Radiant Electrical Magnetic Radiant Chemical This form of energy results from the direct combination of electricity and magnetism. Radiant Electrical Magnetic Radiant Chemical

What is the potential energy of a 340 What is the potential energy of a 340.2 kg piano being lowered from a window that is 21.3 m high? 71,100 J 157 J 319 J 7,250 J 71,100 J

What is the potential energy of a 340 What is the potential energy of a 340.2 kg piano being lowered from a window that is 21.3 m high? GPE = mgh GPE = (340.2 kg)(9.81 m/s2)(21.3 m) GPE = 71,085.81 J

What is the kinetic energy of a 0.00036 kg BB fired at 228.6 m/s? 9.4 J 0.04 J 0.81 J 9.4 J 26,000 J

KE = 1 mv2 2 0.00036 kg BB 228.6 m/s = 1 (0.00036 kg) (228.6 m/s)2 2 1 (0.00036)(52,257) kgm2 s2 = 2 = 9.4 J

Energy Conservation

Law of Conservation of Energy What does it mean to conserve energy? to use less of it to lose none of it

Law of Conservation of Energy The second definition is the one we will be considering. In physics, conserve means the same as preserve.

Law of Conservation of Energy Energy can be changed from one form to another, but cannot be created nor destroyed. This law is also called the First Law of Thermodynamics.

Law of Conservation of Energy Because this law is biblical, no natural exception can be found. This law started in Genesis 2:2 when God rested on the seventh day. (See also Hebrews 4:4.)

Miracles Except for miracles, no new energy is created, and none is lost.

2 Peter 3:7 But the heavens and the earth, which are now, by the same word are kept in store, reserved unto fire against the day of judgment and perdition of ungodly men.

Miracles Except for miracles, no new energy is created, and none is lost. A miracle is a temporary suspension of God’s Laws of the universe, such as the First Law of Thermodynamics.

Energy Transformation Every time energy is converted from one form to another, there is always some energy that escapes the process and becomes unusable.

Energy Transformation Efficiency is the measure of how much energy is converted from one form of energy into the desired form. If a device is 80% efficient and receives 100 J of energy in one form, it converts 80 J of energy to the desired form.

Energy Transformation As the pendulum transforms kinetic energy to potential and back again, no new energy is created and no energy is destroyed. Then why does a swing stop if you do not keep it moving?

Energy Transformation Some energy is converted to sound and some is lost due to air resistance. The energy is still there, but we can’t use it.

Collisions and Energy We have studied collisions and found that the momentum of a system is always conserved. Is the same true for the kinetic energy of a system? It depends on the type of collision of which there are three.

Elastic Collisions Kinetic energy is conserved. Objects bounce off of each other with no deformation, no friction, no sound, or other energy loss. No real-world collision of objects is perfectly elastic, but some can come close.

Elastic Collisions Examples ball bearing colliding a puck on an air hockey table a “super” bouncy ball that bounces back almost to its original height

Partially Elastic Collisions Some kinetic energy is transformed into heat, light, and/or sound. Most real-world collisions are partially elastic.

Inelastic Collisions Kinetic energy is not conserved when the objects collide and stick together. Remember that momentum is always conserved in collisions unless an external force acts on the system during the collision.

Youtube: Wile E. coyote falling of cliff

T/F The Law of Conservation of Energy states that energy cannot be changed from one form to another but must be preserved. F

T/F Miracles follow the First Law of Thermodynamics.

T/F If a device is 60% efficient, it means that 60% of the energy entering the device is successfully converted and the other 40% is destroyed. F

T/F Most collisions in the universe are elastic collisions.