Transcript Review 29:008 Exam 2

Review
29:008
Exam 3
Ch. 12 Electrostatic Phenomena
Electric Charge
Atom is electrically neutral.
Rubbing charges objects by moving electrons
from one to the other.
Charge on the electron:
Induced Charge; the Electroscope
The electroscope can be charged either by
• conduction (direct contact of two
conductors
• or by induction.
Coulomb’s Law
Coulomb’s law:
This equation gives the magnitude of
the force.
Vector addition review:
The Electric Field
The electric field is the
force on a small charge,
divided by the charge:
Field Lines
The electric field can be represented by field
lines. These lines start on a positive charge
and end on a negative charge.
Field Lines
Electric dipole: two equal charges, opposite in
sign:
Electrostatic Potential Energy and
Potential Difference
Electric potential is defined as potential
energy per unit charge:
Unit of electric potential: the volt (V).
1 V = 1 J/C.
Ch. 13 Electric Circuits
Electric Current
Electric current is the rate of flow of charge
through a conductor:
Unit of electric current: the ampere, A.
1 A = 1 C/s.
Electric Current
A complete circuit is one where current can
flow all the way around. Note that the
schematic drawing doesn’t look much like the
physical circuit!
Electric Current
In order for current to flow, there must be a
path from one battery terminal, through the
circuit, and back to the other battery
terminal.
Only one of these circuits will work:
Ohm’s Law: Resistance and Resistors
Experimentally, it is found that the current in
a wire is proportional to the potential
difference between its ends:
Ohm’s Law: Resistance and Resistors
The ratio of voltage to current is called the
resistance:
Electric Power
The unit of power is the watt, W.
Electric Power
For resistors:
Resistors in Series
A series connection has a single path from
the battery, through each circuit element in
turn, then back to the battery.
Resistors in Series
equivalent resistance (that single resistance
that gives the same current in the circuit).
Resistors in Parallel
A parallel connection splits the current; the
voltage across each resistor is the same:
19.2 Resistors in Parallel
The equivalent resistance:
Ch. 14 Magnets &
Electromagnetism
Magnets and Magnetic Fields
Magnets have two ends – poles – called
north and south.
Like poles repel; unlike poles attract.
Magnets and Magnetic Fields
However, if you cut a magnet in half, you don’t
get a north pole and a south pole – you get two
smaller magnets.
Magnets and Magnetic Fields
Magnetic fields can be visualized using
magnetic field lines, which are always closed
loops.
Magnets and Magnetic Fields
The Earth’s magnetic field is similar to that of a
bar magnet.
Note that the Earth’s
“North Pole” is really
a south magnetic
pole
Electric Currents Produce Magnetic Fields
an electric current produces a magnetic field.
Electric Currents Produce Magnetic Fields
The direction of the
field is given by a
right-hand rule.
Magnetic Field
Unit of B:
tesla, T
1 T = 1 N/A·m
Force on Electric Charge Moving in a
Magnetic Field
If a charged particle is
moving perpendicular
to a uniform magnetic
field, it experiences a
force that is
perpendicular to its
velocity v
Right hand rule;
opposite for neg
charge
Force between Two Parallel Wires
If two wires carry currents I1
& I2
They experience a mutual
force (per unit length of wire)
F 2 k ' I1 I 2

l
r
r
Flux
Magnetic flux:
A = area of loop of wire.
Unit of magnetic flux: 1 T·m2
Induced EMF
A voltage is induced (it’s called an ‘emf’, has
symbol e) when the flux through a loop of wire
varies with time
Faraday’s Law
Faraday’s law
[1 loop]
[N loops]
Transformers and Transmission of
Power
A transformer consists of two coils.
A changing emf in one induces an emf in the
other.
The ratio of the emfs is equal to the ratio of
the number of turns in each coil:
(21-6)
Transformers
This is a step-up
transformer – the emf
in the secondary coil
is larger than the emf
in the primary:
Ch. 15 Waves
Wave Motion
Wave characteristics:
• Amplitude, A
• Wavelength, λ
• Frequency f and period T
• Wave velocity
(11-12)
Types of Waves: Transverse and Longitudinal
The motion of particles in a wave can either be
perpendicular to the wave direction (transverse) or
parallel to it (longitudinal).
Types of Waves: Transverse and Longitudinal
Sound waves are longitudinal waves:
Interference
The superposition principle says that when two waves
pass through the same point, the displacement is the
arithmetic sum of the individual displacements.
In the figure below, (a) exhibits destructive interference
and (b) exhibits constructive interference.
Interference
These figures show the sum of two waves. In (a)
they add constructively; in (b) they add
destructively; and in (c) they add partially
destructively.
Standing Waves
Standing waves occur
when both ends of a
string are fixed.
There are:
• nodes, where the
amplitude is always
zero,
• antinodes, where the
amplitude varies from
zero to the maximum
value.
Standing Waves
The frequencies of the
standing waves on a
particular string are called
fundamental and
harmonics.
Air Columns
A tube open at both ends (most wind instruments)
has antinodes, at the ends.
Air Columns
A tube closed at one end (some organ pipes) has
a node
Doppler Effect
The Doppler effect occurs when a source of
sound is moving with respect to an observer.
Doppler Effect
As can be seen in the previous image:
• a source moving toward an observer
has a higher frequency and shorter
wavelength;
• the opposite is true when a source is
moving away from an observer.
Ch. 16 Light waves & color
Electromagnetic Spectrum
Electromagnetic Spectrum
Visible light: 400 nm (violet) to 750 nm (red)
1 nm = 10-9 m
Speed of light
The wavelength and frequency of
electromagnetic waves are related:
Double-Slit
If light is a wave,
there should be
an interference
pattern.
Double-Slit
The interference occurs because each point on
the screen is not the same distance from both
slits.
Depending on the path length difference, the
wave can interfere constructively (bright spot) or
destructively (dark spot).
Double-Slit
x
d
d
Diffraction pattern on screen
has bright fringes separated
by a distance  x
x
d
24.3 Interference – Young’s Double-Slit
Experiment
Between the maxima and the minima, the
interference varies smoothly.
Diffraction by a Single Slit or Disk
Light will diffract around a single slit (or around
a circular obstacle).
screen
d
x
Diffraction by a Single Slit or Disk
The resulting pattern of light and dark stripes is
called a diffraction pattern.
y
Position y of the first dark fringe,
measured from the central bright spot
y
x
d
Interference by Thin Films