Transcript Chapter 8 ppt

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:
 Wavelength (λ): Distance between corresponding points on a
wave pattern. Crest to crest or trough to trough
 Frequency (ƒ): Number of vibrations/waves per second
ƒ=1/T [1/s- HZ] *Hertz (HZ)- Unit for ƒ
 Amplitude: Width of a disturbance; distance of maximum
height/distance
 Wave Speed: Speed at which the disturbance moves through
the medium
 Wavelength (λ), wave speed (s), and frequency (ƒ) 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 is 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


Excess of Electrons → Negative Charge
Deficit “
“
→ Positive Charge
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
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
8.8 Force Fields
+
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
8.8 Force Fields

Maxwell’s Equations
1.
2.
3.
4.

Charges produce E-Fields (Gauss’s law)
Moving charges produce M-fields (law for magnetism)
Changing M-fields produce changing E-fields (Faraday’s Law)
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
Concept Quiz
1. Which of the following is a true wave?
(a) A row of falling dominoes
(b) Ripples on the surface of a pond, extending
outward from a pebble dropped in the water
(c) A large water wave coming into the beach, one
that surfers can ride on
(d) Water rushing downstream
(a) and (c)
2. Which wave has the larger or “higher” frequency, and
which carries more energy?
a)
b)
c)
d)
(c)
The top wave has higher frequency and carries more
energy
The top wave has higher frequency but the bottom wave
carries more energy
The bottom wave has higher frequency and carries
more energy
The bottom wave has higher frequency but the top wave
carries more energy
3. Give an example of (a) a one-dimensional medium, (b) a
two-dimensional medium, (c) a three-dimensional
medium.
(a) Undisturbed Rope
(b) Surface of a lake
(c) Space in a room
4. Describe the experimental evidence supporting that claim
that light is a wave.
In Young’s double-slit experiment, a single light source was sent
through two very small and narrow parallel slits, which acted as two
light sources with synchronized vibrations. Young demonstrated the
wave nature of light by the interference pattern that was produced on
the viewing screen
5. How do we know that the medium for light waves is not
air?
We know that light does not use air as its medium for travel
because we receive light from the sun and there is no air in space
between Earth and the sun.
6. Give two reasons that the force between bar magnets
cannot be the electric force.
(1) bar magnets do not interact with charged objects and (2) bar
magnets always have a pair of poles, both north and south; whereas,
charged objects are either only positively or negatively charged. Not
both types of charge on each end.
7. What is meant by an electric current?
An electric current is a flow of charged particles.
8. Name and briefly describe the three kinds of subatomic
particles found in atoms.
The three subatomic particles found in atoms are the proton, neutron,
and electron. The proton has a positive charge and is located in the nucleus
of the atom. Neutrons have no charge and are also in the nucleus. Electrons
are negatively charged and orbit the nucleus.
9. Which force fields would be felt by a stationary charged
object: gravitational, electric, or magnetic? By a moving
uncharged object? By a moving charged object?
All three objects would feel an imperceptible gravitational force due to
their mass. Both the stationary and moving charged objects would feel an
electric force due to their charge. Only the moving charge would feel a
magnetic force due to the magnetic properties of moving charges
10. What does Faraday’s law say about magnets and wires? What does
it say about magnetic fields and electric fields?
Faraday’s law says that when either a magnet or loop of wire is in
motion with respect to the other object, a current is induced in the loop of
wire. Stated in terms of fields: A changing magnetic field creates an electric
field.
11. Find the wavelength of an AM radio wave whose frequency is 1000
kHz (1 kHz = 1000 Hz).
 12. Two small electrically charged objects are placed a certain
distance apart, where they exert an electric force of 4 N on each
other. Suppose the charge on object #1 is halved. What happens to
the force? What if the charge on both objects is halved?
If q1 is halved the force is also halved. If both charges are
halved the force is reduced to 1/4th its original value.