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What is Energy? Course Webpage www.yorku.ca/mearl/energy/energy.htm Course email [email protected] • Energy is the ability to do work • Energy can neither be created nor destroyed in only changes form • Can be measured in a number of units, calories, BTU’s, ergs.. but we will use Joules (metric system) for the most part in this course Energy • There are two main types of energy • stored energy which we call potential energy • Kinetic energy - the energy of something in motion 1 Energy types • Chemical Energy – this includes energy either used or derived by chemical reactions. In any chemical reaction the bonding patterns between molecules are changed. When bonds are broken and reformed energy is either released (exothermic or exergonic reactions) or energy is absorbed/stored (endothermic or endergonic reactions). Combustion or burning of any kind is a release of stored chemical energy. Most biological processes involve the release and storage of chemical energy. • . • Nuclear Energy – this comes in two forms. • Fission is the break down of larger atoms that releases energy. • Fusion is the amalgamation of smaller atoms to make bigger ones that also releases energy. • This is the form of energy for nuclear bombs, nuclear power plants, and fusion is the energy source for the sun. • Mechanical Energy - this is the energy of some sort of movement (type of kinetic energy) like the work done by our muscles, by wind turning a turbine etc. 2 • Thermal energy – this is the energy derived from heat (how the energy is actually extracted in electrical generating plants that either burn fossil fuels or use nuclear reactions). Here we use what is called a heat engine. • Radiant or light energy – this is the energy that comes from light (what powers solar cells and photosynthesis) • Electrical energy – this is the energy of flowing electrons. It is a major way in which we transmit energy • Gravitational Energy - this is the energy that results from objects moving in a gravitational field (generally stored as potential energy). This is where the energy for hydroelectric dams comes from 3 • With any of these forms of energy, one of the major things we try to accomplish is the transformation from one form to another. (Table 2.2). The problem with any energy transformation is that by the laws of thermodynamics they can never be 100% efficient. Some Basic physics In dealing with energy we deal often with motion.. Water moving over a turbine, wind turning a wind turbine, so we need some basic defintions Newton’s 3 Laws of motions (see special topics chapter 2) describe the rules by which objects move Motion • We can quantify motion with a few basic equations – Velocity and speed.. These are not the same thing in a strict physics definition. – Speed is the rate at which something moves – Velocity is a vector, which means it is the speed and the direction of an object 4 • In a simple sense we can apply this direction generally by saying something like up is positive and down is negative. (though this is a simplification, usually vectors involve angles for direction) • Though this concept won’t be used in the context of this course, it does often appear in really motion and physics calculations • Energy is ability to do Work • Work is the force exerted over a distance W = F•d • Force comes from Newton’s Second Law of Motion • Force is equal to mass times acceleration F = ma 5 • Mass is how much matter is present. • Weight is how much force gravity is exerting on you. • Acceleration is how fast you are changing velocity. v2 − v1 t where v 2 = final velocity v1 = initial velocity t = time a= • Velocity is rate at which your distance changes d 2 − d1 t d 2 is final distance d 1 is initial distance v= 6 Units • Units – we will always work in the metric system. • Distance is always measured in metres. (m) • d= 50m • Time is always measured in seconds. t = 200 s • Velocity is always be measured in m/s Units • Acceleration is always measured in m/s2 • Force is always kg* m/s2 is also called Newtons (N) • Mass is always measured in kilograms kg • Work and energy is always measured in Joules = N*m (J) • Some quantities will sometimes have a negative sign attached, these would include: distance, velocity, acceleration, force, and work this is because they are vector quantities. A vector has both an amount and a direction as part of the quantity. • Forward = + (positive) • Backward = - (negative) • Other quantities are scalars – direction is irrelevant, this includes energy, time and mass. 7 • When work is done – energy is either expended or stored by an object. (it is the total change of the energy in a system we will discuss this in the next lecture) Kinetic Energy • The kinetic energy of an object depends on it’s velocity and mass • KE (Ek) = ½ mv2 • Heat energy or Thermal Energy (TE) is a form of kinetic energy since it is a measure of the motion of the molecules in a substance. When we deal with heat and temperature however because we do averages, we tend to calculate as a separate quantity. Potential Energy • The other type of energy is potential energy. (PE or Ep) • This is energy that has been stored in some form or another. • One major type is gravitational potential energy • • E = m g h • PE Ep • m= mass • g= gravitational constant 9.8 m/s2 • h= height 8 Power • Power – this is the rate at which energy is used, or the rate at which work is done • P = E /t = W/t • Units are Watts (W) = J/s Sample Problems • • • • • • • Problem 1 page 59 text F = ma m = 50 kg F= 12 N a = F/m = 12 / 50 = 0.24 m/s2 Sample Problems • • • • • • • • • • Problem 9 page 73 Here m = 80 kg h= 6m t = 12 s P = E/t E = mgh since we are dealing with someone moving to a certain height E = 80(9.8) 6 = 4704 J P= 4704 /12 = 392 W 9 • • • • • • Problem 6 page 73 a = 4 m/s2 t=6s a = v/t (here initial v is zero) v=at= 4 x 6 V = 24 m/s 10