Chapter 21 Alternating Current circuits and electromagnetic waves
21.1 Resistors in an ac Circuit (Skip)
21.2 Capacitors in an ac Circuit (Skip)
21.3 Inductors in an ac Circuit (Skip)
21.4 The RLC Series Circuit (Skip)
21.5 Power in an ac Circuit (Skip)
21.6 Resonance in a Series RLS Circuit (Skip)
21.7 The Transformer
• The effect in which a changing current in one circuit induces an emf in another circuit is called mutual induction.
• A transformer is a device for increasing or decreasing an ac voltage.
• Figure 21.14
• The effect in which a changing current in one circuit induces an emf in another circuit is called mutual induction.
• The induced emf in the primary coil is (due to self-induction)
V1
= - N1
Where 
is the magnetic flux
through each turn.
• The induced emf in the secondary coil is (due to mutual induction)
V2
= - N2
Where 
is the magnetic flux
through each turn.
• The transformer equation is
V2/V1 = N2/N1
• Examples
21.8 Maxwell’s Prediction
• James Clerk Maxwell showed that an electric field and a magnetic field fluctuating together can form a propagating electromagnetic wave. (Maxwell’s equations)
• Basis for Maxwell’s equations
1) A point charge creates the electric field around it, and Coulomb’s law describes the relation between the point charge and the electric field.
2) There is no magnetic monopole.
3) A varying magnetic field induces an emf and hence an electric field. (Faraday’s law – Chapter 20)
4) Magnetic fields are generated by moving charges (current). (Amphere’s law – Chapter 19)
21.9 Hertz’s Discoveries
• Heinrich Hertz first generated and discovered electromagnetic waves in a laboratory setting.
21.10 Production of Electromagnetic Waves by an Antenna
• Figure 21.19 (Electric field set up by an antenna)
• Figure 21.21 (the electromagnetic wave by an antenna)
• An electromagnetic wave is a transverse wave; (1) the electric and magnetic fields are both perpendicular and (2) both fields are perpendicular to the direction in which the wave travels.
21.11 Properties of Electromagnetic Waves
• Electromagnetic waves can travel through a vacuum or a material substance, since electric and magnetic fields can exist in either one.
• Maxwell determined theoretically that electromagnetic waves propagate through a vacuum at a speed given by
c
= 
= 2.997 92 x 108
m/s
where eo = 8.85 x 10-12 C2/N-m2 (the electric permittivity of free space) and mo = 4p x 10-7 T-m/A (the magnetic permeablity of free space).
• The speed of light is c = 299 792 458 m/s (a measured value). Since the electromagnetic waves travel at a speed that is precisely the same as the speed of light in vacuum, light is an electromagnetic wave.
• The ratio of the electric field to the magnetic field in an electromagnetic wave equals the speed of light.
= c
• Electromagnetic waves carry energy as they travel through space.
Average
power per unit area = 
= 
= 
• Electromagnetic waves carry momentum as they travel through space.
p = 
(complete absorption)
p = 
(complete reflection)
21.12 The Spectrum of Electromagnetic Waves
• An electromagnetic wave
has a frequency f and a wavelength 
that are related
to the speed v of the wave by v = f 
. For a
electromagnetic waves traveling through a vacuum (air), the speed is that of
light (c = 3.00 x 108 m/s)
c = f 
• The ordered series of electromagnetic wave frequencies (wavelength) is called the electromagnetic spectrum.
(Figure 21.23)
• Radio waves
• Microwaves
• Infrared waves
• Visible waves
• Ultraviolet (UV) waves
• X-rays
• Gamma rays
• Examples
21.13 The Doppler Effect for Electromagnetic Waves
• When electromagnetic waves and the source and the observer of the waves all travel along the same line in a vacuum, the single equation that specifies the Doppler effect is
f’
= f (1 
) if u
<< c
where f’ and f are the observed and emitted frequencies, respectively, u stands for the speed of the source and the observer relative to one another, and c is the speed of light.
• Examples