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