Unit C Energy Flow in Technological Systems

 Every object involved in energy transfers (system) must use some form of energy  Energy is the ability to do work  Measured in Joules (J) or kilojoules (kJ)

 Open system-energy and matter can flow into and out of  Closed system-energy but not matter can flow into  Isolated system-neither energy or matter can flow in or out

 Total Input Energy  Energy that goes INTO a system  Examples ▪ Fuel (chemical potential energy) ▪ Batteries (chemical potential energy) ▪ Springs (elastic potential energy)  Useful Output Energy  Desirable form of energy that EXITS a system  Examples ▪ Heat/thermal (baseboards) ▪ Light (flashlight) ▪ Kinetic (golf club)

 Chemical  Nuclear  Light  Sound  Mechanical  Heat  Electrical

 Energy is the ability to do work  Two categories that all forms of energy can be put into 1. Kinetic Energy- energy of motion 2. Potential Energy-stored energy

 Occurs whenever a force moves an object any distance  2 conditions must be met  Movement  Push or Pull/Force

Work = force x distance the object travels W=Fd Joule = newton x metre

 If 10 N of force lift a textbook off the floor 2.4 m, how much work is done?  What force is applied when a box is moved 15 m and J of work are done?

 Graphs provide visual representation of work and shows relationship between two variables  1 type of graph to describe Work 1. Force-Distance Graph What does it look like?  Plot Distance in meters on the x axis and Force in newtons on the y axis What does it tell us?  Shows us direct relationship between distance traveled and force it takes to move it  The area under the line represents the amount of work done ▪ The area can often be divided into two or more simple geometric shapes ▪ A = Length x Width (rectangle) ▪ A = ½ Base x Height (triangle)

 Energy due to motion or moving objects  E k  Examples include  swinging a golf club  Walking  Pushing/pulling

Kinetic energy = ½ (mass) x (velocity) 2 E k = ½ mv 2 J = kg x (m/s) 2

Determine the kinetic energy of a 1000 kg roller coaster car that is moving with a speed of 20.0 m/s. Missy Diwater, the former platform diver for the Ringling Brother’s circus has a kinetic energy of J just prior to hitting the bucket of water. If Missy’s mass is 50 kg then what is her speed?

 Latent or stored energy, the potential of an object to do work due to its position or condition  E p  Different forms/Examples  Chemical –stored in food  Electrical – stored in electrical appliances  Stored Nuclear- stored in nuclei  Gravitational

 Force applied against gravity (usually) the weight of the object results in stored energy  Weight= force of gravity acting on mass  Weight - Fg=ma  Acceleration due to gravity (g) is 9.81 m/s 2

Potential energy = mass x acceleration due to gravity x height E p = mgh J = kg x m/s 2 x m Acceleration due to gravity is a constant and always equal to 9.81 m/s 2

Melody was grabbing a 600 g can of soup off a 1.8m shelf, when it suddenly fell to the floor. What is the gravitational potential energy of the can? A child with a mass of 25.0 kg is at the top of a slide. The gravitational potential energy of the child is 981 J. What is the height of the slide the child is on?

 Law of Conservation of Energy  Total amount of energy in a given situation remains constant Ep = Ek mgh = ½ mv 2  Energy can be converted from one form to another but it never changes Ep + Ek = Total Energy mgh + ½ mv 2

 Potential energy at the start of the swing will equal the potential energy at the end of the swing  Kinetic energy in mid swing equal to potential energy at the start of the swing

A 50.0 kg rock is dropped over the edge of a cliff, 30.0m above the surface of a lake. What is the speed of the rock just before it strikes the surface of the lake?

 Law of conservation of energy assumes that all machines are perfect and 100% efficient  In reality there are so many energy conversions between total energy input and final useful energy output energy that this is not true  Means how much of the initial energy going into a system comes out at the end in the form we want  Most ‘waste’ energy is lost to the system/surroundings as heat

 Total Input Energy  Energy that goes INTO a system  Joules (J)  Examples ▪ Fuel (chemical potential energy) ▪ Batteries (chemical potential energy) ▪ Springs (elastic potential energy)  Useful Output Energy  Energy at end of conversion that is used  Joules (J)  Examples ▪ Heat/thermal (baseboards) ▪ Light (flashlight) ▪ Kinetic (golf club)

Efficiency = final useful energy OUTPUT total energy INPUT X 100 %

When a 100 W light bulb is on for 1.0 h, it uses 360KJ of electrical energy. During that time, the light bulb emits 19 kJ of light. What is the efficiency of the light bulb in transforming electrical energy into light energy? A bobcat uses 393 kJ of chemical potential energy stored in the fuel to lift 2750 kg of dirt 3.2 m straight up, to dump it in a dump truck. What is the efficiency of the bobcat in converting chemical potential energy into gravitational potential energy?

 Units, sig digs, scientific notation  Forms of Energy and Energy Conversions  Work  Math  Graph ▪ Force-Distance  Kinetic vs Potential Energy  Math ▪ Ek ▪ Ep ▪ Total Energy  Graph ▪ Energy-Time  Law of conservation of energy and Efficiency of a system