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Energy
• Energy is the ability to do work–Work is a transfer of energy
• Energy is measured in joules (J)– 1 joule = work done to move an object 1
meter by 1 N
Kinetic NRG• Energy of a moving object• KE = 1/2mv2
• m = mass and v = velocity• Square velocity first• Double the mass, double
the KE• Double the velocity,
quadruple the KE
Potential NRG• Stored energy• Gravitational potential
energyPE = mghm = mass g = gravity h = height
• Greater the height, more PE
• Greater the mass, more PE
• Elastic potential energy–When an object is stretched or compressed– The more stretched, the more potential
energy– The more compressed, the more potential
energy
• Can there be kinetic energy and potential energy at the same time?– Yes!– Mechanical energy = KE + PE
motion position
• Do you think energy can be created or destroyed?– No!– So the total amount of energy at the beginning
will be the same at the end
Forms of NRG
• Mechanical• Thermal• Chemical• Electrical• Electromagnetic• Nuclear
Mechanical• Energy associated with motion and
position• Sum of potential and kinetic energy
Chemical• Energy stored in chemical bonds• Bonds are broken, energy is released• Examples– Chemical compounds, such as fuels– Food–Muscles
Electrical
• Associated with electric charges• Batteries convert chemical energy
into electrical energy
Electromagnetic• Form of energy that travels through space in
the form of waves• Ex: light, X-rays, radio waves• Often used for communication because they
travel long distances
Nuclear• Energy stored in atomic nuclei• Nucleus held by strong and weak forces
(Large amount of PE)• Fission (split nuclei)• Fusion (combine nuclei)
Thermal• Potential and kinetic energy of atomic
particles• When atoms move faster, thermal energy
increases and object becomes warmer
Energy Conversions• Energy can be converted from one form
to another• What type of energy conversion takes
place when lighting a match?Chemical: rxns in muscles allow you to move the match
Kinetic: match is moving
Thermal: heat produced by friction
Chemical: rxn on match’s tip triggered by thermal energy
Thermal: heat produced
Electromagnetic: light produced
Conservation of Energy• Energy cannot be created or destroyed, it can
only be transferred• Ex: Bike slowing– Kinetic NRG becomes thermal NRG– Friction between the bike and the ground/air– Lose kinetic NRG, gain thermal NRG, total
NRG is the same• Ex: Pole vault– KE Elastic PE Gravitational PE KE
• (KE + PE)beginning = (KE + PE)end
Heat • TOTAL amount of kinetic energy of
particles• Transfer of thermal energy because of
temperature differences• Moves from hot to cold objects
Temperature• AVERAGE amount of
kinetic energy of particles
• Measure of how hot or cold something is
• Use a thermometer• Celsius and Kelvin
scales• Absolute zero = 0 K
• Object heats up, particles move faster
Thermal Contraction and Expansion
• If temp decreases, particles move slower and closer together, so object contracts
• If temp increases, particles move faster and farther away, so object expands
Thermal NRG Absorption• Depends on color, texture, and
exposed surface area
Absorb more than emit = temp increase
Emit more than absorb = temp decrease
Absorb and emit the same amount = thermal equilibrium
Specific Heat• Amount of heat needed to raise the
temperature of one gram of an object one degree Celsius
• Lower specific heat = temp rises more when given a certain amount of energy
• Q = m x c x change in T or Q = mc∆T• Q = heat gained or lost (J)• m = mass (g)• c = specific heat (SI unit: J/g • ˚C)• T = temperature (˚C)
Conduction• Transfer of thermal energy with no
overall transfer of matter• Objects must be touching to transfer
heat
• Conductors – conduct thermal energy well–Most metals are good conductors
because electrons move easily
• Insulators – conduct thermal energy poorly
Convection• Transfer of thermal energy when
particles of a fluid move from place to place
• Convection current–Warm air rises– Cool air sinks
Radiation• Transfer of energy by waves moving
through space• All objects radiate energy– Heat lamps– Sun