Download Faraday`s Law

EMF Induced in Moving Conductor
Faraday’s Law
EMF= - ΔΦB/Δt
Motional EMF=
EMF=ΔΦB/Δt=BΔ
=BΔA/Δ
A/Δt=BlvΔ
lvΔt/Δ
t/Δt=Blv
Chapter 21
Electric Generator
Alternating Current
Transformer
Electromagnetic Waves
Έ=NBAω
NBAωsinω
sinωt
Transformer
h
l
N
S
N
Loop horizontal
EMF=max
EMF
S
¾The output emf of a transformer may be
greater or smaller than its input emf
¾A transformer operates on ac
Loop vertical
EMF=0
Emax
-Emax
t
E=BlvsinΘ => E=2NBlvsinΘ=NBAω sinωt
where Θ=ωt, v=ωr=ωh/2
Electrical Power Transmission
E1=N1Δφ/Δt
E2=N2Δφ/Δt
E1/E2=N1/N2
E1I1=E2I2 (power input=power output)
So I1/I2=E2/E1=N2/N1
• When transmitting electric power over long
distances, it is most economical to use high
voltage and low current
– Minimizes I2R power losses
• In practice, voltage is stepped up to about 230
000 V at the generating station and stepped
down to 20 000 V at the distribution station and
finally to 120 V at the customer’s utility pole
Question: Alternating current in the
secondary coil of a transformer is
induced by
(a) a varying electric field
(b) a varying magnetic field
(c) the iron core of the transformer
(d) motion of the primary
Physical Nature of EM Waves
A changing magnetic field produces a changing electric
field (Faraday’s
Faraday’s law)
law
ΔB/Δt
E
A changing electric field produces a changing magnetic field
(Maxwell’s
Maxwell’s hypothesis)
hypothesis
ΔE/Δt
Answer: b
B
Source of Electromagnetic Waves
Types of Electromagnetic Waves
Electric field
c
•
•
•
•
•
Antenna
×
×
×
×
c
c
c=E/B
Magnetic field
EM waves are produced, fundamentally,
by accelerating electric charges
Frequency f and Wavelength λ
f λ=c
or f=c/λ
Wavelength of visible light:
400 nm (Blue) to 750 nm (Red) (1 nm=10-9 m)
Or
4000 Å to 7500 Å (1 Å=10-10 m)
Question: In a vacuum, the speed of an
electromagnetic wave
(a) depends on its frequency
(b) depends on its wavelength
(c) depends on its electric and magnetic fields
(d) is a universal constant
Answer: d
Which one of the following statements concerning electromagnetic waves is
false?
X (a) Electromagnetic waves are longitudinal waves.
(b) Electromagnetic waves transfer energy through space.
(c) The existence of electromagnetic waves was predicted by Maxwell.
Which one of the following will not generate electromagnetic waves or
pulses?
X (a) a steady direct current
(d) Electromagnetic waves can propagate through a material substance.
(b) an accelerating electron
(e) Electromagnetic waves do not require a physical medium
(c) a proton in simple harmonic motion
(d) an alternating current
(e) charged particles traveling in a circular path in a mass spectrometer
Which one of the following types of wave is intrinsically different from the
other four?
Complete the following sentence: When electrons from a heated filament
accelerate through vacuum toward a positive plate,
(a) only an electric field will be produced.
(b) only a magnetic field will be produced.
X (c) electromagnetic waves will be produced.
(d) longitudinal waves will be produced.
(a) radio waves
(b) gamma rays
(c) visible light
X (d) sound waves
(e) ultraviolet radiation
(e) neither electric nor magnetic fields will be produced.
Which one of the following statements concerning electromagnetic waves is
false?
Which one of the following statements concerning the wavelength of an
electromagnetic wave in a vacuum is true?
(a) Electromagnetic waves carry energy.
(a) The wavelength is independent of the speed of the wave for a fixed
frequency.
X(b) X-rays have longer wavelengths than radio waves.
(c) In vacuum, all electromagnetic waves travel at the same speed.
(d) Lower frequency electromagnetic waves can be produced by oscillating
circuits.
(e) They consist of mutually perpendicular electric and magnetic fields that
oscillate perpendicular to the direction of propagation.
(b) The wavelength is inversely proportional to the speed of the wave.
(c) The wavelength is the same for all types of electromagnetic waves.
(d) The wavelength is directly proportional to the frequency of the wave.
X(e) The wavelength is inversely proportional to the frequency of the wave.
Complete the following sentence: The various colors of visible light differ in
(a) frequency only.
(b) frequency and wavelength.X
A cellular telephone transmits electromagnetic waves at a frequency
of 935 MHz. What is the wavelength of these waves?
(c) wavelength only.
(a) 0.0106 m
(c) 0.642 m
(d) frequency and their speed in a vacuum.
(b) 0.321 mX
(d) 1.22 m
(e) 2.40 m
(e) their speeds in a vacuum.
Note the different types of electromagnetic radiation:
An average distance between the surface of the earth and the surface
of the sun is 1.49x1011 m. How much time, in minutes, does it take for
light leaving the surface of the sun to reach the earth?
(a) zero minutes
X
(c) 8.3 min
(b) 2.9x10–3 min
(d) 74 min
(e) 500 min
(1) X-rays
(4) visible light
(2) radio waves
(5) infrared radiation
(3) gamma rays
(6) ultraviolet radiation
Which list correctly ranks the electromagnetic waves in order of increasing
frequency?
(a) 2, 3, 4, 5, 6, 1
(c) 2, 5, 4, 6, 1, 3 X
(b) 2, 5, 4, 1, 6, 3
(d) 3, 1, 6, 4, 5, 2
(e) 3, 6, 1, 4, 5, 2
X
Which one of the following colors of visible light has the highest
frequency?
(a) Yellow
(c) green
(b) Red
(d) blue
(e) violetX