I. Energy

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# I. Energy - PowerPoint PPT Presentation

I. Energy. The ability of an object to produce a change. Energy is measured in Joules. In this unit we cover 3 types of energy: 1. Potential (stored) energy 2. Kinetic (motion) energy 3. Thermal (heat) energy. Types of KE and PE. Potential Energy Kinetic Energy

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Presentation Transcript
I. Energy
• The ability of an object to produce a change.
• Energy is measured in Joules.
• In this unit we cover 3 types of energy:

1. Potential (stored) energy

2. Kinetic (motion) energy

3. Thermal (heat) energy

Types of KE and PE
• Potential Energy Kinetic Energy
• Gravitational Mechanical
• Elastic Thermal
• Chemical Electrical
• Nuclear Sound
• Electrostatic
http://www.nsf.gov/news/special_reports/olympics/snowboarding.jsphttp://www.nsf.gov/news/special_reports/olympics/snowboarding.jsp
II. Work
• Work is the transfer of energy by mechanical means.

1. This is why work and energy are both measured in Joules 1 J = 1 kgm2 / s2

• Other units for energy

1. Calorie (1 cal = 4.184 J)

• A food calorie is actually 1000 calories

2. British Thermal Unit (1 BTU = 1055 J)

III. Kinetic Energy
• Energy of motion.
• Any mass that is moving has kinetic energy.
• KE depends on the amount of mass and the velocity at which it is moving. (Both have a direct relationship with KE.)
IV. Potential Energy
• PE is stored energy.
• There are many types of PE:

1. Gravitational Potential Energy.

ex. Anytime an object has height.

2. Elastic/Spring Potential Energy.

ex. Rubber band, bouncy ball

3. Chemical Potential Energy.

ex. Bonds in food and gasoline

V. Gravitational Potential Energy
• Stored energy because an object has height.

1. You must set a zero height that we call our reference level.

• Most of the time the surface of the Earth is our reference level.
VI. Other types of Potential Energy
• Elastic Potential Energy - Energy stored in the bending or stretching of an object

1. Ex: rubber band, Trampoline

• Chemical Potential Energy - Energy stored in chemical bonds

1. Ex: gasoline, food

• Electric Potential Energy - Energy stored because of an electric charge

1. Ex: static electricity, magnets

Use this principle to determine the blanks in the following diagram. Knowing that the potential energy at the top of the tall platform is 50 J, what is the potential energy at the other positions shown on the stair steps and the incline?

VII. Example Problems
• A shot-putter heaves a 7.26-kg shot with a final velocity of 7.50 m/s. What is the kinetic energy of the shot? (204 J)
• A 2.00-kg text book is lifted from the floor to a shelf 2.10 m above the floor.

1. What is the gravitational potential energy relative to the floor? (41.2 J)

2. What is the gravitational potential energy relative to the head of a 1.65 m tall person? (8.8 J) (hint, the person’s head is now the reference point).

VIII. Work-Energy Theorem
• The net work done an object is equal to the change in energy of that object.
• If the work done is positive, the object gains energy.

If the work done is negative, the object loses energy.

p. 259: Work is the transfer of energy by mechanical means.

http://www.physicsclassroom.com/Class/energy/U5l2a.cfm

IX. Example Problems
• A 150-kg roller-coaster car climbs to the top of a 91-m high hill. How much work is done to lift the car to the top of the hill? (133770J)
• You do 13500 J of work in accelerating a 45-kg wagon from rest. What velocity does the wagon reach? (24.5 m/s)
X. Rest Potential Energy
• Einstein discovered that mass, by its very nature, is energy. This energy is called rest energy.
• To calculate the rest energy of an object E=mc2
• E == energy
• M == mass
• C == speed of light
• C = 3.0 x 108 m/s
• Rest energy usually deals with masses on an atomic level.
• http://www.pbs.org/wgbh/nova/einstein/experts.html
XI. Conservation of Energy
• Within a closed, isolated system, energy can change form, but the total amount of energy in the system does not change.
• Example:

A ball thrown into the air has energy that changes from kinetic to potential at the top and then back to kinetic at the bottom.

http://glencoe.mcgraw-hill.com/sites/0078458137/student_view0/chapter11/rollercoaster_applet.html

Conservation of Energy
• Ex.
• The initial height of a roller coaster must be more than the loops so that there is enough energy converted into GPE to reach the height of the loop and enough KE to continue through the loop. This is assuming that only GPE is the source of the initial energy.
• Pendulums – GPE at the top of a pendulum or swing equals the KE at the lowest point (assuming no outside forces). At the top there is only GPE at the lowest point there is only KE.

http://www.physicsclassroom.com/mmedia/energy/cs.cfm

Equation

• Falling down
• GPEtop = KEbottom
• Throwing up
• KEbottom = GPEtop
• Drawing
• http://www.jason.org/digital_library/4851.aspx
XII. Example Problem
• A 35-kg child is riding on a swing that rises to a maximum height of 0.8 m. Neglecting friction,

1. What is the child\'s gravitational potential energy at the top of the swing? (274.4 J)

2. What is the child’s velocity at the bottom of the swing? (4.0 m/s)

• A pendulum bob with a mass of 0.5 kg swings to a maximum height of 1.0 m. What is the velocity when the pendulum bob is at a height of 0.40 m? (3.42 m/s)
XIII. Elastic and Inelastic Collisions
• Elastic – kinetic energy is conserved.

1. The objects bounce off each other.

a. Example: Billiard balls

• Inelastic – some kinetic energy is changed into other forms of energy (heat, sound, etc.) Almost all collisions are inelastic
• (Remember, momentum was conserved in both elastic and inelastic collisions, but KE is only conserved in elastic collisions.)
• Superelastic- Kinetic energy is gained

1. Ex. A compressed spring is released during a collision

Matter is made up of many tiny particles that are in constant motion.

1. The particles are held together by electromagnetic forces (strongest in solids, weakest in gases) (states of matter)

In a hot object, the particles move faster, therefore they have more kinetic energy. (expand with more thermal energy - thermal expansion)

XIV. Kinetic-Molecular Theory
XV. Thermal Energy vs. Temperature
• Thermal Energy is the sum of the kinetic and potential energies of the internal particles in an object.

1. Thermal energy is measured in joules.

• Temperature measures the hotness of an object on a definite scale.

1. Temperature shows a direct relationship with the average kinetic energy of the particles in an object.

XVI. Temperature Scales
• Kelvin (units are K)
• Based on absolute zero (the temperature were the particles in an object stop moving)
• Water – 273 k, 373 k
• Celsius (units are °C)
• Based on properties of water – 0, 100
• Fahrenheit (units are °F)
• Based on coldest manmade temp of time as 0 and body temp as 100 – water 32, 212
XVII. Heat
• Thermal energy will travel from hot to cold until the two objects are the same temperature. This is called thermal equilibrium.
• The energy that flows as a result of a difference in temperature is called heat (Q).

1. Heat is measured in Joules.

Thermal Equilibrium is the state in which the rate of energy flow between two objects is equal and the objects are at the same temperature.XVIII. Thermal Equilibrium
XIX. Transfer Process

A. Conduction – the transfer of thermal energy when particles collide

1. Ex: a metal spoon gets hot when sitting in boiling water.

B. Convection – the rising of hot air/liquids pushing cooler air/liquids down.

1. Ex: thunderclouds

C. Radiation – the transfer of energy by electromagnetic waves

1. Ex: sunlight heating the Earth

XX. Specific Heat
• The specific heat (C) of a material is the amount of energy that must be added to raise the temperature of a 1 kg mass 1°C.
• Temperature in the above equation can be measured in °C or K.
XXI. Example problem
• A 0.400-kg block of iron is heated from 295K to 325 K. How much heat is absorbed by the iron if it has a specific heat of 450 J/kgK?

(5400 J)

Latent Heat
• Heat of Fusion (Hf) is the amount of heat needed to melt 1 kg of a substance.
• Heat of Vaporization (Hv) is the amount of heat needed to vaporize one kilogram of a liquid.
• Latent heats can be negative if the object is moving in the other direction on the heating curve.
http://www.uwsp.edu/geo/faculty/ritter/glossary/l_n/latent_heat.htmlhttp://www.uwsp.edu/geo/faculty/ritter/glossary/l_n/latent_heat.html
• latent heatLatent heat is the heat energy involved in the phase change of water. Latent heat is gained by water molecules when water evaporates. The heat added during evaporation is used to break hydrogen bonds between water molecules and does not raise the temperature of the water body. The heat then is "hidden" or stored in the water molecule until it is released during condensation. At that point, the heat is converted into sensible heat. Latent heat released during condensation is an important source of energy to drive atmospheric systems like hurricanes and cumulus clouds.
Change of State Graphs
• Endothermic changes –
• Exothermic changes –
XXIII. Example Problem
• How much heat must be transferred to 100.0 g of ice at 0.0 °C until the ice melts and the resulting temperature of the water rises to 20.0 °C?
• (41760 J)
XXIV. Heat Engine

A. A heat engine is a machine that is able to convert thermal energy to mechanical energy.

XXV. Thermodynamics
• 1st Law

1. The total increase in the thermal energy of a system is the sum of the work done on it and the heat added to it.

2. Only restates the law of conservation of energy.

• 2nd Law

1. Natural processes go in a direction that increase the total entropy of the universe.

2. Entropy is the measure of disorder in an object