Energy

the ability to do work or cause change Defining Energy Energy the ability to do work or cause change We will address what work is later Energy can come in many forms, many of which you are probably already familiar with

Forms of Energy Electrical Energy – the energy of “charged” attraction example would be the energy we get from a household circuit Magnetic Energy – the energy of polar attraction a magnet picks up other “magnetic” substances using magnetic energy

Forms of Energy (continued) Mechanical Energy – the energy of machines doing work a turning motor is an example of mechanical energy Heat (Thermal) Energy – the energy of burning calories, usually noticed through changes in temperature Example would be warming a cup of water until the water undergoes a phase change.

Forms of Energy (continued) Light (Radiant) Energy – the energy from radiating waves UV rays from the sun give off light energy Chemical Energy – the energy of breaking bonds in chemical reactions an example is a battery that turns a chemical reaction inside of itself into energy

Forms of Energy (continued) Nuclear Energy – the energy of atomic reactions in the nucleus of atoms example is radioactive decay, like at nuclear power plants Gravitational Energy – the energy from using the pull of gravity water from a waterfall has gravitational energy

Classifying Energy kinetic energy potential energy Despite the many different forms of energy, all energy can be classified as either: kinetic energy or potential energy Kinetic Energy – the energy of motion Potential Energy – the energy of position The different forms of energy can be kinetic or potential depending on what is going on in the system an electric circuit can have potential energy if the switch is open, and no current is flowing. Once, the switch is closed, and current flows, the electric current has kinetic energy

Measuring Energy Energy is measured in the unit of Joules (J) 1 Joule = 1 Newton x meter 1 J = 1 N m = 1 kg m/s2 x m = 1 kg m2/s2 When calculating energy, because we use Joules, mass must always be in kilograms!!! (kg)

PE = mass x gravity x height = mgh Potential Energy Potential energy exists whenever an object which has mass has a position within a force field (that force could be gravity, electric, magnetic, etc. . .) Potential energy is stored energy, it is not being released -- in this class, we will focus on what is known as gravitational potential energy, which is the stored energy due to height above the Earth We can calculate potential energy using the following formula: PE = mass x gravity x height = mgh m = mass of the object (kg) g = acceleration due to gravity (9.8 m/s2) h = height of object above the earth (m) PE is measured in Joules (kg m/s2 m)

KE = (1/2) mass x velocity2 = (1/2)mv2 Kinetic Energy The stored energy of potential energy is released when it is converted to kinetic energy. Kinetic energy exists whenever an object which has mass is in motion with some velocity. It is the energy of motion. We can calculate kinetic energy using the following formula: KE = (1/2) mass x velocity2 = (1/2)mv2 m = mass of the object (kg) v = velocity of the object (m/s) You must remember to square the velocity before solving the rest of the formula v2 is in (m/s)2 = m2/s2 KE is in Joules (J) = kg m2/s2

Practice Problems A 100-kg boulder is sitting on top of a hill that is 50 meters above the ground. What is the potential energy of the boulder? A 85-kg jogger is running at a velocity of 3 m/s. What is the kinetic energy of the jogger? m = 100 kg h = 50 m g = 9.8 m/s2 PE = ? PE = m x g x h PE = 100 kg x 9.8 m/s2 x 50 m PE = 49000 kg m2/s2 PE = 49000 J Click For the Answer m = 85 kg v = 3 m/s KE = ? KE = (1/2) x m x v x v PE = .5 x 85 kg x 3 m/s x 3 m/s PE = 382.5 kg m2/s2 PE = 382.5 J Click For the Answer

Challenging Practice Problems What is the velocity of a 2500-kg car that is traveling with a kinetic energy of 2,250,000 J (2250 kJ)? 4. A 3,000-kg car is stopped at a red light. The light turns green and the car accelerates for 5 seconds until its kinetic energy reaches 1,200,000 J, when it smashes into a guard rail. How much force did the car hit the guard rail with? m = 2500 kg KE = 2250000 J v = ? KE = (1/2) x m x v2 v = sqrt 2KE/m v = sqrt (2 x 2250000 J/2500 kg v = 42.43 m/s v = 42.43 m/s Click For the Answer CHALLENGE – DO THIS ONE ON YOUR OWN AND BRING IT IN

Conservation of Energy Much like momentum, energy is a conserved quantity The principle of conservation of energy says that in a closed system, all energy is conserved. Think about an object in free fall: -- as the object is at rest at a height, h, all of its energy is potential. When the object falls and accelerates due to Earth’s gravity, potential energy is converted to kinetic energy. Velocity increases, so KE increases and height decreases, so PE decreases. Right before the object hits the ground, its height = 0, so its PE is zero. All of its energy has been converted to kinetic energy.

Energy Conversions We can track the energy conversions between different types of energy. Example Questions: An object is raised off of the ground and stopped. Kinetic Potential The raised object falls to the ground Potential Kinetic Not all conversions have to be between kinetic and potential energy A firefly (lightning bug) uses a chemical reaction in its hind quarters to glow at night Chemical Radiant (Light)