Photoelectric Effect is the
phenomenon that when light shines on a metal surface, electrons are emitted
from the surface.
Photoelectric Effect – A photon
may knock an electron out of an atom and in the process itself disappear.
Electrons should be emitted when
light shines on a metal is consistent with the electromagnetic wave theory of
light – that is the electric field of an EM wave could exert a force on
electrons in the metal and eject some of them
The process of measuring maximum
kinetic energy can be done by using a variable voltage source and reversing the
terminals so that the electrode C is negative and P is positive. The electrons emitted from P will be
repelled by the negative electrode. But if this reverse voltage is small
enough, the fastest electrons will still reach C and there will be a current in
the circuit.
If the reverse voltage is
increase, a point is reached where the current reaches zero, so no electrons
have sufficient energy to reach C. This called stopping potential or stopping
voltage
Assume a monochromatic light. Two
important properties of light are intensity and frequency. When two properties
varied, the wave theory make prediction as below:
·If the light intensity increase, numbers of
electrons ejected and their maximum
kinetic energy should be increased because – the higher the intensity,
greater electric field amplitude and greater electric field should eject
electrons with higher speed.
·The frequency of the light should not affect the
kinetic energy of the ejected electrons.
Photon theory
In monochromatic beam, all
photons have the same energy, E=hf.
Increasing the intensity of light mean increasing the number of photons.
·If the frequency remains the same, it does not affect the energy of each
photon
·An electron is ejected from the metal by a
collision with a single photon. Consequently, all the photon energy is
transferred to the electron and the forces some minimum energy W0
(Work Function) is required to get an electron out through the surface.
·hf<W0
- The photon will not have enough energy to eject any electron
·hf>W0
– The electrons will be ejected and energy will be conserved.
Consideration of photon theory:
An increase in intensity of the light beams means
more photons are incident, so more electrons will be ejected.
Since the energy of each photon is not changed,
the maximum kinetic energy of electrons is not changed by increase in intensity
If the frequency is increased, the maximum kinetic
energy of the electrons increases linearly.
Compton Effect
Compton Effect – a photon
can be scattered from an electron and in the process, lose some energy. But the
photon is not slowdown; it still
travels with speed, c but its frequency will be lower.
Compton scattered short
wavelength light, which is X-rays, from various materials. He found that the
scattered light had a slightly longer wavelength than the incident light,
therefore, there is a slight lower
frequency indication loss of energy.
Since the photon is relativistic
particle that travel with the speed of light, v= c, the momentum of
photon is
In 1911, Ernest Rutherford (1871-1937)
theorized that the atom must consist of a
tiny but massive positively charged nucleus, surrounded by electrons some distance away. The electrons would be moving in orbits
about the nucleus.
Hydrogen is simplest atom that has only one
electron orbiting its nucleus. It atomic number is 1.
In 1885, J. J. Balmer showed that the four visible
lines in the hydrogen spectrum (with wavelength 656 nm, 486 nm, 434 nm and 410
nm) fit the following formula
Later was found that this Balmer series of lines
extended to UV region, ending at = 365 nm
Figure 3:Electron transitions for the Hydrogen atom
Wave-Particle Duality
Some indicate that
light behaves like waves and the
other indicates light behaves like stream
of particles. These behaviours of light come in to conclusion as wave-particle
duality.
In 1923, Louis de
Broglie suggests that the wavelength of a particle would be related to its
momentum as in the same way with photon.
p
=
linear momentum
= wavelength
Sometimes it is called
the de
Broglie wavelength of a particle
X-ray is produced when electrons accelerated
by a high voltage strike the metal
target inside the X-ray tube. W. C. Roentgen in 1895 discovers the X-ray using
voltages of 30kV – 150 kV. H-rays scattered from a crystal did indeed show the peaks and valleys of a diffraction pattern. It was shown that
X-rays have a wave nature and the
atoms are arranged in a regular way in crystals (serve as diffraction) Today,
X-rays are recognized as electromagnetic
radiation.
The
diffraction of X-rays with wavelength, that a
reflection from a crystal as described by Bragg equation. Strong reflections are
observed at grazing angles given by,
d = distance between reflecting planes in the crystal
= angle between the face of the crystal
m =
reflected beam in order of reflection = 1, 2, 3, ...
In Figure 2, the magnetic field lines represent both
the magnitude and direction of the magnetic field vector.
The magnetic field vector at any point is tangent
to the field line and the magnitude of the field is proportional to the number of lines per unit area perpendicular to
the lines.
The bar magnet is one instant of a magnetic dipole. Magnetic dipole consists of two
opposite magnetic poles:
The end of the bar magnet where the
field line emerge is called the north pole
The lines goes back in called south pole
Magnetic field lines are all closed loops. If there are no magnetic monopole, there is no place
for the field lines to begin or end.
MAGNETIC
FORCE ON A POINT CHARGE
Electric field is defined as the electric force per
unit charge.
q = a point charge
The electric force is either in the samedirection as E or in the oppositedirection depends on the
sign of the point charge
The magnetic
force depends on the point charge’s velocity
as well as on the magnetic field.
If the charge is at rest, there is no
magnetic force. The magnitude and the direction of the magnetic force
depend on the direction and speed of the charge’s motion. The
magnetic force increases in magnitude with increasing velocity. The direction
of the magnetic force on a charged particle is perpendicular to the velocity of the particle.
Factors a charge moving
in a magnetic field:
The magnitude of the charge, q
The strength of the magnetic, H
The magnitude of the velocity of the
charge, v or the component of the
velocity perpendicular to the field
Sin , is the angle between the field lines and the
velocity, v
Magnitude
of the magnetic force on a moving point charge:
The SI unit of magnetic
field is N/A m or tesla, T.
The direction and the magnitude of the magnetic field force depend on the vector v and B. The magnetic force can be written in term of the cross product (vector product).
Let and
The vector products
have perpendicular directions to the vectors. It can be determined using the right-hand-rule.
A conductor carrying electric
current has many moving charges in it. For a current carrying conductor in
magnetic field, the magnetic forces on
moving charges add up to produce a net magnetic force on the wire.
A straight wire segment of length, L
in a uniform magnetic field B carries a current I
The mobile carriers have charge, q
The magnetic force on any one charge is
v
is the instantaneous velocity of the charge
Multiply
the average magnetic force on each charge by the number of charges
If N is the total number of carriers in
the wire, the total magnetic force on the wire is
Magnetic force in terms of current,
n
is the number of carriers per unit volume If the length, L and the cross-sectional
area is A, then
Number per unit volume x volume =nLA
In substitution, the magnetic force on
the wire is
The magnetic force on a straight segment
current carrying conductor
The current I times the cross product
L x
B gives the magnitude and direction
of the force
Figure 3: A
current-carrying conductor in a magnetic field experiences a magnetic force
(a)A wire suspended vertically between the
poles
(b)The blue x represents the magnetic
field. When there is no current, the wire remain vertical
(c)If the current going upwards, the wire
deflects to the left
(d)If the current going downwards, the wire
deflects to the right
MAGNETIC
FIELD DUE TO AN ELECTRIC CURRENT
Magnetic field due to a
Long Straight Wire
Using the right-hand-rule to find the direction
of the magnetic field due to a long straight wire:
Point the thumb of the right hand in the
direction of the current in the wire
Curl the fingers inward toward the palm
The direction that the fingers curl is
the direction of the magnetic field lines around the wire
