Download Revision of Electrons Photons and Waves File

Electrons , Photons and Waves
Module 1: Electric current
Module 2: Resistance
Module 3: DC circuits
Module 4: Waves
Module 5: Quantum physics
define the coulomb
• The SI unit of electrical charge. One Coulomb
is defined as the amount of charge that passes
in 1 second when the current is 1 ampere.
define potential difference (p.d.)
• Energy transfer per unit charge from electrical
to other forms
define the volt
• 1 Volt is equal to 1 Joule per Coulomb (JC-1)
The potential difference across a component
is 1 volt when you convert 1 joule of energy
moving 1 coulomb of charge through the
component.
• Or
• A pd of 1V is the voltage required to produce a
current of 1A in a resistor of resistance 1 Ohm
define electromotive force (e.m.f.)
• Energy transfer per unit charge from
chemical/other to electrical form. Measured
in V or JC-1
define resistance
• Resistance = Potential difference/current .
• ratio of voltage to current; voltage per (unit)
current (VA-1)
• resistance = p.d./current
define the ohm
• A component has a resistance of 1 ohm if a
potential difference of 1 volt makes a current
of 1 amp flow through it.
define resistivity of a material
• ρ=RA/l
define the kilowatt-hour (kW h)
• A unit of energy equal to 36 MJ or the energy
used by a 1kW for 1h
Define displacement, amplitude,
wavelength, period, phase difference,
frequency & speed of a wave
Displacement-how far a point on the wave has
moved from its equilibrium position
Amplitude- Maximum displacement
Wavelength-Distance between neighbouring
identical points
Period-Time taking for one complete
oscillation of a particle
Define displacement, amplitude,
wavelength, period, phase difference,
frequency & speed of a wave
Phase Difference- The fraction of a cycle
between the oscillations of two particles
Frequency-Number of waves passing a point
per unit time
Speed-Distance travelled by the wave per unit
time
define the terms nodes and antinodes
• Node-When the amplitude is always zero
• Antinode-When the amplitude is always at its
maximum possible value
define fundamental mode of vibration
• Simplest pattern of movement and has the
lowest possible frequency band and the longest
wavelength
define harmonics
• Harmonics are different modes of vibration of a
wave with increasing frequency and decreasing
wavelength
define the electronvolt (eV)
• The Electronvolt is defined as the kinetic
energy gained by an electron when it is
accelerated through a potential difference of 1
volt.
define work function
• Work function- the minimum energy required to
release an electron from the surface of a material
define threshold frequency
• Threshold frequency- the minimum frequency of
a photon that will cause and electron to be
emitted from the material.
Define the term intensity
• intensity is the incident energy per unit area
per second
Remember I α Amplitude2
State what is meant by the term mean
drift velocity of charge carriers
• The average distance travelled by the charge
carriers along the wire per second
State Ohm’s law
• Provided the temperature is constant, the
current through an ohmic conductor is directly
proportional to the potential difference across
it.
State Kirchhoff’s second law
• sum of e.m.f’s = sum of p.d.s around a closed
loop in a circuit
Energy is conserved
State Kirchhoff’s First law
• The sum of the currents into a junction is equal to
the sum of the currents out of the junction
Charge is conserved
State the principle of superposition of
waves
• When two or more waves meet at a point and
interfere, The resultant displacement equals the
vector sum of the displacements of each wave.
Q=It
1mA = 1 x 10-3 A
I = nAve
Electric Current is the rate of
flow of charge.
1μA = 1 x 10-6 A
Kirchoff’s first law states that
the sum of the current into a
junction is equal to the sum of
the currents flowing out of the
junction. An example of
conservation of charge
Conventional current
n conductors >> n semiconductors >> n insulators
v insulators >> v semiconductors >> v conductors
Electrons , Photons and Waves
Module 1: Electric current
Module 2: Resistance
Module 3: DC circuits
Module 4: Waves
Module 5: Quantum physics
I-V Characteristics
Interpreting I-V Graphs
I
V
Interpreting I-V Graphs
I
V
Interpreting I-V Graphs
I
V
Drawing Graphs
Work out where it starts
Get the shape right
Consider if it has to go
through any particular
point ( often calculated in
a previous part of a
question)
Resistivity
A
Calculate the area
carefully ( use m) .
Is it a circle ?
B
Which of these two wires has the greatest resistance ?
Resistivity
Remember if you are
asked what happens if
l doubles and r halves
you can try out an
answer with numbers.
The Fuse
Explain how a fuse works?
When a current bigger than the fuse
rating flows through, the fuse gets
hot, melts and cuts off the power.
The fuse prevents fires ..
33
The Kilowatt Hour
An iron that operates at a power of 3 kW for
4 hours uses electricity that costs 8p per
unit. How much does it cost for the
electricity used by the iron in that time?
Number of kWhrs:
number of units of electricity = number of kilowatt hours
= 3 kW x 4 h
= 12 kWh
Cost of electricity:
cost = number of kWhrs x cost per unit
= 12 kWhrs x 8 p
= 96p
Electrons , Photons and Waves
Module 1: Electric current
Module 2: Resistance
Module 3: DC circuits
Module 4: Waves
Module 5: Quantum physics
Circuit
Circuit Analysis
What is the current flowing
in this circuit and the voltage
across each resistor ?
Circuit
Circuit Analysis
What is the current flowing
in this circuit and the voltage
across each resistor ?
Using a thermistor
Resistance of Thermistor
decreases as temperature
increases.
V across thermistor
decreases
so V increases across fixed
resistor since voltages must
add up to the supply
voltage.
Internal Resistance
Electrons , Photons and Waves
Module 1: Electric current
Module 2: Resistance
Module 3: DC circuits
Module 4: Waves
Module 5: Quantum physics
Polarisation of Radio Waves
Path Difference
Light Interference
Use the formula to work
out what is happening if
a, x, D or λ changes.
The Diffraction Grating
The spectral pattern seen from a monochromatic source such as a sodium lamp
Stationary Waves
The apparatus used for setting up a stationary wave in a closed tube
Quantum Physics
• Energy of a photon
• The photoelectric effect
• Wave–particle duality
• Energy levels in atoms
Energy of a Photon
What is a Photon ?
a quantum/packet/of electromagnetic
energy
Energy of a Photon
How do you calculate the energy of a photon ?
The Photoelectric Effect
Describe the Photoelectric Effect
A photon is absorbed by an electron in a metal surface causing an electron to be
emitted.
Energy of photon = hf
Energy is conserved.
Only electrons with energy above the work function energy will be emitted.
Only light with a frequency greater than the threshold frequency will cause emission
Energy=work function + Max KE of electron.
The work function is the minimum energy required to release an electron from the
surface.
Number of electrons emitted depends on light intensity.
( Energy of Electron does not)
The Photoelectric Effect
Describe the Photoelectric Effect Experiment
A clean zinc plate is mounted on the cap of a gold leaf electroscope
where the plate is initially charged negatively .
Shine a UV light on the plate and watch the gold leaf collapse as
charge leaves the plate, indicating the emission of electrons
The Photoelectric Effect
Photoelectric Effect Graphical Analysis
The Photoelectric Effect
Photoelectric Effect Graphical Analysis
As intensity goes up
it does not affect
the energy of the
electrons.
As frequency goes
up it does not affect
the number of
electrons emitted
As intensity goes up
more electrons
emitted
The Photoelectric Effect
Photoelectric Effect Numerical Analysis
To convert from eV to J multiply by 1.6 x 10 -19
The Photoelectric Effect
Describe the LED Experiment to find h
Adjust the potential divider to low/zero voltage
Connect flying lead to one LED
Increase voltage until LED just lights/strikes
Repeat several times and average to find Vmin
Repeat for each LED
Shield LED inside opaque tube to judge light more accurately
Wave Particle Duality
When do waves behave like particles ?
In the photoelectric effect – the energy of the light does not
depend on the amplitude of the wave but on its frequency.
Wave Particle Duality
When do particles behave like waves ?
Travelling electrons are diffracted by graphite producing
a series of diffraction rings. Maximum diffraction occurs
when the de Broglie wavelength of the electrons is
similar to the gap between layers of carbon atoms.
Wave Particle Duality
What is the de Broglie wavelength for an electron ?
Travelling electrons are observed to behave as
waves/show wavelike properties
Where the electron wavelength depends on its
speed/momentum
Energy Levels in Atoms
What is an Emission Line Spectrum?
light emitted from excited isolated atoms produces a
line spectrum
a series of sharp/bright/coloured lines
against a dark background
Energy Levels in Atoms
What is an Absorption Line Spectrum?
in an absorption spectrum a series of dark lines appears
against a bright background/within a continuous
spectrum
Energy Levels in Atoms
Why are spectra important?
They reveal that electrons within
atoms can only occupy certain
energy levels.
Explaining Line Spectra
Energy Levels in Atoms
An energy-level diagram for the hydrogen
atom
Numerical and Graphical Analysis
Electrons have discrete energy
levels within atoms - they
cannot have any energy .
hf or hc/λ = E1 – E2