Bài giảng Energy

Energy 
Energy is a property that enables something to do work 
Energy has various forms (kinetic, potential, rest energy) 
Kinetic energy is the energy of a moving object 
 m v 2 
KE = --------- 
 2 
It takes ~1000 N (or ~ 225 lbs) to hit a nail and drive it 5 mm into a surface. 
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Potential Energy 
Potential energy is a capacity of doing some work 
Potential energy is the energy of position 
Gravitational Potential Energy 
W = F d = m g h = PE (potential energy) 
PE is relative! 
PE of a 1000-kg car at the top of a 50-m multilevel parking lot is: 
m g h = (1000 kg)(9.8 m/s2)(50 m)=490 kJ 
Rest Energy 
Mass and Energy are related to each other and can be converted into each other. 
The rest energy of a body is the energy equivalent of its mass. 
E 0 = m 0 c 2 
E 0 ( m =1 kg) = 1 kg x (3 10 8 ) 2 (cm/s) 2 ~ 10 17 J 
PE ( m =1 kg, h =9 km) = m g h = 1 kg x 9.8 m/s 2 x 9000 m ~ 10 5 J 
Energy Transformations 
Many mechanical processes involve interchanges between KE, PE, and work. 
Energy exists in some other forms: chemical energy, heat energy, radiant energy, etc. 
Conservation of Energy 
Energy cannot be created or destroyed. 
It can only be changed from one form to another. 
The above statement is called the law of conservation of energy 
Chapter 5 
Light and Atomic Structure 
Light and its properties 
Atomic structure 
Interaction between Light and Matter 
Spectrum 
Light in Everyday Life 
Light is a form of energy, radiative energy 
1 Watt = 1 Joule/sec 
 Light has color 
A prism split light into a spectrum (rainbow of colors) 
Light travels with a speed of c = 300,000 km/s 
Rainbow 
Interaction of Light and Matter 
 Emission 
 Absorption 
 Transmission (passing through) 
 Reflection (scattering) 
Properties of Light 
Light behaves as both a particle and a wave 
Light particles are called photons , which can be counted individually. 
Light is also an electromagnetic wave 
The wavelength is the distance between adjacent peaks of the electric or magnetic field 
 1 nm (nanometer) = 10 –9 m 1μm (micron) = 10 –6 m 
The frequency is the number of peaks that pass by any point each second, measured in cycles per second or Hertz (Hz). 
light demo 
Light is an electromagnetic wave 
Light consists of many individual photons. 
Each travels at the speed c and can be characterized by a wavelength and a frequency. 
Many Forms of Light 
The spectrum of light is called the electromagnetic spectrum 
Different portions of the spectrum are called: 
The visible light - what we see with our eyes 
The infrared light - beyond of the red end of rainbow 
The ultraviolet light - beyond the blue end 
Radio waves - light with the longest wavelengths 
X rays - wavelengths shorter than ultraviolet 
Gamma rays - the shortest wavelength light 
Electromagnetic spectrum 
Electromagnetic spectrum 
Frequency units – Hertz 
1 Hz = 1 c  1 
Light and Matter 
The amount of light is called intensity 
Studying spectra of celestial bodies one can learn a wealth of information about them 
Atomic Structure 
92 chemical elements have been identified in the Universe. 
Nearly 20 more have been created artificially. 
Each chemical element is made from a different type of atom. 
Atoms are made from particles called protons , neutrons , and electrons . 
Protons and neutrons form the nucleus in the center of the atom. 
Electrons surround the nucleus. 
Atomic Structure 
Positively charged protons are hold together by the strong force , which overcomes electrical repulsion. 
Negatively charged electrons are attracted to the nucleus. 
The number of protons in an atom is called the atomic number , which is unique for different chemical elements. 
The combined number of protons and neutrons in an atom is called the atomic mass number . 
Atoms of the same element with different number of neutrons are called isotopes . 
Absorption and Emission in Gases 
Since electrons in atoms can have only specific 
 energies, the atoms can absorb or release energy 
 only in these amounts (quanta) 
Electron gets energy, jumps to an excited state, 
release the energy, and falls back down 
The energy is emitted as a photon of light 
The photon has exactly the same energy that 
the electron has lost 
Types of Spectra 
Emission line spectrum consists of photons emitted as each electron falls back to lower levels 
Absorption line spectrum appears when photons 
 are absorbed, causing electrons to jump up in energy 
Each element or molecule produces its own distinct 
set of spectral lines 
Emission by Hydrogen 
Hydrogen lines in the visible 
Examples of Spectra 
Thermal Radiation 
``Complex’’ objects - planets, stars, people - 
produce thermal radiation 
Its spectrum depends only on the object’s 
temperature 
Hotter objects emit more total radiation per unit surface area 
The radiated energy is proportional to the fourth power of the temperature 
Hotter objects emit photons with a higher average energy 
Temperature and Color 
Temperature and Intensity 
Reflected light 
When the light (for example, sunlight) strikes an 
object (ground, clouds, people), we see only the 
wavelengths of light that are reflected 
Different objects (fruits, rocks, atmospheric gases) 
reflect and absorb light at different wavelengths 
The Doppler Shift 
Radial motion of a distant object can be determined due to the Doppler effect 
The Doppler effect causes shifts in the wavelengths 
of light 
If an object is moving toward us, its entire spectrum is shifted to shorter wavelengths 
Because shorter wavelengths of the visible light are bluer, the Doppler shift of this object is called a blueshift 
The Doppler shift of a moving away object - redshift 
Doppler Effect 
Demo Doppler effect 
Summary 
Spectral information gives us more knowledge about the objects (composition, surface temperature, moving properties) 
Visible light is only a small portion of the electromagnetic spectrum 
The Doppler effect tells us how quickly light is moving toward or away from us