EnergyUnit C Energy Flow in Technological Systems

Energy

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)

Types of Systems

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

Energy Flow

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)

Forms of Energy

Chemical Nuclear Light Sound Mechanical Heat Electrical

Quantifying Energy

Energy is the ability to do work Two categories that all forms of

energy can be put into1. Kinetic Energy- energy of motion2. Potential Energy-stored energy

Work

Occurs whenever a force moves an object any distance

2 conditions must be met Movement Push or Pull/Force

Quantifying Work

Work = force x distance the object travels

W=FdJoule = newton x metre

Example

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 1450.67 J of work are done?

Analyzing Work Graphs provide visual representation of work and

shows relationship between two variables

1 type of graph to describe Work1. Force-Distance GraphWhat does it look like? Plot Distance in meters on the x axis and Force in newtons

on the y axisWhat 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)

Kinetic Energy

Energy due to motion or moving objects

Ek

Examples include swinging a golf club Walking Pushing/pulling

Quantifying Kinetic Energy

Kinetic energy = ½ (mass) x (velocity)2

Ek = ½ mv2

J = kg x (m/s)2

Example

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 15 000J just prior to hitting the bucket of water. If Missy’s mass is 50 kg then what is her speed?

Potential Energy

Latent or stored energy, the potential of an object to do work due to its position or condition

Ep Different forms/Examples

Chemical –stored in food Electrical – stored in electrical appliances Stored Nuclear- stored in nuclei Gravitational

Gravitational Potential Energy

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/s2

Quantifying Gravitational Ep

Potential energy = mass x acceleration due to gravity x height

Ep = mgh

J = kg x m/s2 x m

Acceleration due to gravity is a constant and always equal to 9.81

m/s2

Examples

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?

Total Energy

Law of Conservation of Energy Total amount of energy in a given

situation remains constantEp = Ek

mgh = ½ mv2

Energy can be converted from one form to another but it never changes

Ep + Ek = Total Energy mgh + ½ mv2

Pendulum

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

Examples

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?

Efficiency

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

Efficiency

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)

Quantifying Efficiency

Efficiency = final useful energy OUTPUT

total energy INPUT

X 100 %

Example

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?

Energy Quiz

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