chapter 21 electromagnetic waves. general physics exam ii curve: +30
TRANSCRIPT
Chapter 21Chapter 21
Electromagnetic WavesElectromagnetic Waves
General Physics
Exam IIExam II
Curve: +30
General Physics
Electromagnetic WavesElectromagnetic Waves
Ch 21, Secs 8–12Ch 21, Secs 8–12
General Physics
James Clerk MaxwellJames Clerk Maxwell 1831 – 18791831 – 1879 Electricity and magnetism Electricity and magnetism
were originally thought to be were originally thought to be unrelatedunrelated
In 1865, James Clerk Maxwell In 1865, James Clerk Maxwell provided a mathematical provided a mathematical theory that showed a close theory that showed a close relationship between all relationship between all electric and magnetic electric and magnetic phenomenaphenomena
Electromagnetic theory of lightElectromagnetic theory of light
General Physics
Maxwell’s Starting PointsMaxwell’s Starting Points
Electric field lines originate Electric field lines originate on positive charges and on positive charges and terminate on negative terminate on negative chargescharges
Magnetic field lines always Magnetic field lines always form closed loops – they do form closed loops – they do not begin or end anywherenot begin or end anywhere
General Physics
Can electric fields Can electric fields form closed loops?form closed loops?
1 2
50%50%1.1. YesYes
2.2. NoNo
10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30
General Physics
Maxwell’s Starting PointsMaxwell’s Starting Points
Magnetic fields are generated Magnetic fields are generated by moving charges or by moving charges or currents (Ampère’s Law)currents (Ampère’s Law)
A varying magnetic field A varying magnetic field induces an emf and hence induces an emf and hence an electric field (Faraday’s an electric field (Faraday’s Law)Law)
General Physics
Turning Faraday’s Law upside Turning Faraday’s Law upside down, Maxwell hypothesized down, Maxwell hypothesized that a changing electric field that a changing electric field would produce a magnetic would produce a magnetic field (Maxwell-Ampère’s Law)field (Maxwell-Ampère’s Law)
Maxwell’s HypothesisMaxwell’s Hypothesis
General Physics
Maxwell EquationsMaxwell Equations
closed surface enclosed charge closed surface no mag. charge
• Conservation of energy
closed loop linked current + flux
• Conservation of charge
Lorentz force law
closed loop linked flux
General Physics
Maxwell’s PredictionsMaxwell’s Predictions Maxwell concluded that visible light and all Maxwell concluded that visible light and all
other electromagnetic (EM) waves consist of other electromagnetic (EM) waves consist of fluctuating electric and magnetic fields, fluctuating electric and magnetic fields, with each varying field inducing the otherwith each varying field inducing the other
Accelerating charges generate these time Accelerating charges generate these time varying E and B fieldsvarying E and B fields
Maxwell calculated the speed at which Maxwell calculated the speed at which these electromagnetic waves travel in a these electromagnetic waves travel in a vacuum – speed of light c = 3.00 x 10vacuum – speed of light c = 3.00 x 1088 m/s m/s
General Physics
Hertz’s Confirmation of Hertz’s Confirmation of Maxwell’s PredictionsMaxwell’s Predictions
1857 – 18941857 – 1894 First to generate and First to generate and
detect electromagnetic detect electromagnetic waves in a laboratory waves in a laboratory settingsetting
Showed radio waves Showed radio waves could be reflected, could be reflected, refracted and diffractedrefracted and diffracted
The unit Hz is named for The unit Hz is named for himhim
General Physics
Hertz’s Experimental Hertz’s Experimental ApparatusApparatus
An induction coil is An induction coil is connected to two large connected to two large spheres forming a spheres forming a capacitorcapacitor
Oscillations are initiated Oscillations are initiated by short voltage pulsesby short voltage pulses
The oscillating current The oscillating current (accelerating charges) (accelerating charges) generates EM wavesgenerates EM waves
General Physics
Hertz’s ExperimentHertz’s Experiment Several meters away Several meters away
from the transmitter is from the transmitter is the receiverthe receiver This consisted of a single This consisted of a single
loop of wire connected loop of wire connected to two spheresto two spheres
When the oscillation frequency of the When the oscillation frequency of the transmitter and receiver matched, energy transmitter and receiver matched, energy transfer occurred between themtransfer occurred between them
General Physics
Hertz’s ConclusionsHertz’s Conclusions Hertz hypothesized the energy transfer was in Hertz hypothesized the energy transfer was in
the form of wavesthe form of waves These are now known to be electromagnetic wavesThese are now known to be electromagnetic waves
Hertz confirmed Maxwell’s theory by showing Hertz confirmed Maxwell’s theory by showing the waves existed and had all the properties of the waves existed and had all the properties of light waves (e.g., reflection, refraction, light waves (e.g., reflection, refraction, diffraction)diffraction) They had different frequencies and wavelengths They had different frequencies and wavelengths
which obeyed the relationship v = f λ for waveswhich obeyed the relationship v = f λ for waves v was very close to 3 x 10v was very close to 3 x 1088 m/s, the known speed of m/s, the known speed of
lightlight
General Physics
EM Waves by an AntennaEM Waves by an Antenna
Two rods are connected to an oscillating source, charges Two rods are connected to an oscillating source, charges oscillate between the rods (a)oscillate between the rods (a)
As oscillations continue, the rods become less charged, the As oscillations continue, the rods become less charged, the field near the charges decreases and the field produced at t = field near the charges decreases and the field produced at t = 0 moves away from the rod (b)0 moves away from the rod (b)
The charges and field reverse (c) – the oscillations continue (d)The charges and field reverse (c) – the oscillations continue (d)
General Physics
EM Waves by an Antenna, EM Waves by an Antenna, finalfinal
Because the oscillating charges in Because the oscillating charges in the rod produce a current, there the rod produce a current, there is also a magnetic field generatedis also a magnetic field generated
As the current changes, the As the current changes, the magnetic field spreads out from magnetic field spreads out from the antennathe antenna
The magnetic field is The magnetic field is perpendicular to the electric fieldperpendicular to the electric field
General Physics
Electromagnetic Waves, Electromagnetic Waves, SummarySummary
A changing magnetic field produces A changing magnetic field produces an electric fieldan electric field
A changing electric field produces a A changing electric field produces a magnetic fieldmagnetic field
These fields are These fields are in phasein phase At any point, both fields reach their At any point, both fields reach their
maximum value at the same timemaximum value at the same time
General Physics
Electromagnetic Waves are Electromagnetic Waves are Transverse WavesTransverse Waves
TheThe and fields are and fields are perpendicular to each perpendicular to each otherother
Both fields are Both fields are perpendicular to the perpendicular to the direction of motiondirection of motion Therefore, EM waves are Therefore, EM waves are
transverse wavestransverse waves
E
B
Active Figure: A Transverse Electromagnetic Wave
General Physics
Properties of EM WavesProperties of EM Waves
Electromagnetic waves are transverse Electromagnetic waves are transverse waveswaves
They travel at the speed of lightThey travel at the speed of light
This supports the fact that light is an EM This supports the fact that light is an EM wavewave
1
o o
c
General Physics
Properties of EM Waves, 2Properties of EM Waves, 2 The ratio of the electric field to the magnetic field The ratio of the electric field to the magnetic field
is equal to the speed of lightis equal to the speed of light
Electromagnetic waves carry energy as they Electromagnetic waves carry energy as they travel through space, and this energy can be travel through space, and this energy can be transferred to objects placed in their pathtransferred to objects placed in their path
max
max
B
E
B
Ec
General Physics
Properties of EM Waves, 3Properties of EM Waves, 3
Energy carried by EM waves is Energy carried by EM waves is shared equally by the electric and shared equally by the electric and magnetic fieldsmagnetic fields
Average power per unit areaAverage power per unit area
0
2max
0
2max
0
maxmax
222 cB
c
EBEI
A
Pave
General Physics
Properties of EM Waves, Properties of EM Waves, finalfinal
Electromagnetic waves transport Electromagnetic waves transport linear momentum as well as energylinear momentum as well as energy For complete absorption of energy U For complete absorption of energy U
p = U/c p = U/c F = P F = Paveave/c/c For complete reflection of energy UFor complete reflection of energy U
p = (2U)/c p = (2U)/c F = 2P F = 2Paveave/c/c Radiation pressures (forces) can be Radiation pressures (forces) can be
determined experimentallydetermined experimentally
General Physics
Determining Radiation Determining Radiation PressurePressure
This is an apparatus This is an apparatus for measuring for measuring radiation pressureradiation pressure
In practice, the system In practice, the system is contained in a is contained in a vacuumvacuum
The pressure is The pressure is determined by the determined by the angle at which angle at which equilibrium occursequilibrium occurs
General Physics
Summary of Properties ofSummary of Properties ofElectromagnetic (EM) Electromagnetic (EM)
WavesWaves They travel at the speed of lightThey travel at the speed of light They are transverse wavesThey are transverse waves
E, B perpendicular to each other and velocityE, B perpendicular to each other and velocity Ratio of E and B field magnitudes: E/B=cRatio of E and B field magnitudes: E/B=c
Electric and magnetic fields carry equal energyElectric and magnetic fields carry equal energy They carry both energy and momentumThey carry both energy and momentum
Can deliver U and p to a surfaceCan deliver U and p to a surface
General Physics
The Spectrum of EM WavesThe Spectrum of EM Waves Forms of electromagnetic Forms of electromagnetic
waves exist that are waves exist that are distinguished by their distinguished by their frequency and wavelengthfrequency and wavelength c = ƒλc = ƒλ
Wavelengths for visible light Wavelengths for visible light range from 400–700 nmrange from 400–700 nm
a small portion of the spectruma small portion of the spectrum
WavelengthsWavelengths 1 km = 101 km = 10-3-3 m (radio) electronic m (radio) electronic 1 1 m = 10m = 10-6-6 m (visible, IR) m (visible, IR) 1 nm = 101 nm = 10-9-9 m (UV, X-ray) m (UV, X-ray) 1 Å = 101 Å = 10-10-10 m (X-ray) atomic m (X-ray) atomic 1 fm =101 fm =10-15-15 m ( m (-ray) nuclear-ray) nuclear