Chapter 8--Light & Electromagnetism
8.1 Waves: Something Else that travels
- Waves: A disturbance that travels through a medium and transfers
energy without transferring matter.
- Medium: material through which a disturbance travels
- In quantitative terms:
o Wavelength (λ): Distance between corresponding points on a
wave pattern. Crest to crest or trough to trough
Crest
Trough
o
Frequency (ƒ): Number of vibrations/waves per second
ƒ=1/T [1/s- HZ]
o
*Hertz (HZ)- Unit for ƒ
Amplitude: Width of a disturbance; distance of maximum
height/depth
λ
Amplitude
Wave speed
o
Wave Speed: Speed at which the disturbance moves
through the medium
o
Wavelength (λ), frequency (ƒ), and wave speed (s) are
related:
v = [m/s] = λ/t [m/s] = λƒ [m· 1/s = m/s]
v = λƒ
8.2 Interference: A behavior unique to waves
- Wave Interference: Effects that occur when 2 waves of the same
type are present at the same time. Interference is constructive or
destructive.
-
Destructive interference: Equal waves of opposite directions
interfere, cancelling each other out
Before
During
After
-
Constructive Interference: Equal waves of the same orientation
interfere, reinforcing each other
8.3 Light: A particle or Wave?
1801→Thomas Young gives evidence that light is a wave
Double-Slit Expt. →Confirms wave theory of light
→Light is a wave but has no medium
8.4 Electricity: Part of Electromagnetic Force
- Light is electromagnetic wave, traveling through electromagnetic force
- Electric Force: Electric part of electromagnetic force
-
Electric Force Law: Electrically charged objects exert forces on each
other, even when they are some distance apart. Objects may be
charged positively or negatively.
→Like charges repel
→Unlike charges attract
8.5 Magnetism: The Other Part
- Magnet’s ends are North and South magnetic poles
Like → Repel
Unlike → Attract
-
-
Magnetism of a bar magnet permanent
Magnetic Force Law: Charged objects that are moving exert and feel
an additional force beyond the electric force that exists when they are
at rest. This force is magnetic.
→All magnetic forces are caused by the motion of charged
Objects
Electromagnetic Force: The total (Electric and Magnetic) force
between charges
8.6 The Electric Atom
- Planetary Model
Atom is → almost entirely empty
→ Divisible
→ made up of many parts
Nucleus→ tiny center, made up of protons and neutrons,
surrounded by electrons
Electrons
Protons
Neutrons
e- = -1.6 x 10-19C
e+= +1.6 x 10-19C
No Charge
me= 9.11 x 10-31kg
mp= 1.67 x 10-27kg
mn= 1.67 x 10-27kg
Excess of Electrons → Negative Charge
Deficit “
“
→ Positive Charge
8.8 Force Fields: Disturbance of Space
- Gravitational Field: Exists throughout any region of space where an
object would feel a gravitational force if an object were placed there.
→ “Possibility” of a g-force
-
Electromagnetic Field: Exists everywhere any other charged object
would (if it were present) feel EM forces
→ “Possibility” of EM-force
-
Electric Field: Wherever any charged object would feel an electric
force
Magnetic Field: Exists wherever any moving charged object would feel
a magnetic force
-
Electric Field Lines
→ Point outward from +q’s
→
“
inward from – q’s
+q
-q
-
# of lines indicates amount of charge
E-Field strength is greater where lines are closer together
Direction of E-field lines shows direction of force on a +q
-
Maxwell’s Equations
1. Charges produce E-Fields (Gauss’s law)
2. Moving charges produce M-fields (law for magnetism)
3. Changing M-fields produce changing E-fields (Faraday’s Law)
4. Changing E-fields produce changing M-field
-
Electric Force Law-In terms of fields
An E-field surrounds every charged object and charged objects feel
forces due to the E-fields of other objects
-
Magnetic Force Law-in terms of fields
An M-field surrounds every moving charged object and charged
objects feel forces due to the M-field of other moving charges