Millikan's experiment

(NB: variable power supply)







When light of a frequency above the threshold frequency is shone on the cathode, photoelectrons are ejected and then attracted to the annode. Electrons are also moving along tthe wire to provide a corrent. If the powersupply isconnected such that the cathode is +vely charged, some of the electrons will be attracted back to it, and -ve current will reduce. The faster photoelectrons will reach the annode. If the voltage is then increased, a point will be reached when even the fastest electrons will not reh the annode and the current will become zero. This voltage is called the stopping potential, Vs.

This gives us a methid for working out the minimum kinetic energy of the photoelectrons:

Max KE = Electric energy



The Measurments are reported for different frequencies of light, enabling us to measure the different maximum kinetic energies

Photoelectric Effect Notes

The Intensity of the light/radiation affects the number of protoelectrons emmitted. More intense - More photoelectrons.

The Frequency of the light/radiation affects the kinetic energy of the ejectrons.

Threshold frequency is the minimum frequency required to eject electrons from different metals. F0

E=HF0 = Θ = work function.
The work function is the minimum amount of energy required to eject electronsfrom the nucleus.

The energy of the ejectrons depends on the KE they had before the ejection.























For the threshold requency :



above the threshold frequency :








UNIT - Electron volt: eV

1 electron volt (eV) = the energy required to move 1 electron through a PD of 1V

1eV = e * 1V

= 1.6*10^-19 J


(theta) caesium = 3.11*10^-19 J = 1.94 eV
(theta) copper = 7.44*10^-19 J = 4.65 eV

Photoelectric Effect

The reason the charge is lost is that the electrons are gaining sufficient energy to be ejected from the sorface of the zinc plate. Electrons would be ejected from the +ve gold leaf on the Gold Leaf Electroscope but would be immediately attracted back.

Albert Einstein tried to explain these observations. His first suggestion was that the enegy supplied by the light to the electrons is related to the frequency of the light rather than the intensity. His next suggestion was that light arrives as particles, or packets of energy rather than a continuous stream or flow of energy. If it was a wave supplying continuous energy, eventually the electrons would absorb enough ro be ejeted. Each ejectron (photoelectron) recieves its energy from a single packet , otherwise an intense beam of low energy packets could supply sufficient energy. The energy of these packets is proportional to the frequency such that E=hf. These packets are called photons.

E=hf
unit = Js
h=planck's constant = 6.63*10^-34 (in formula booklet)

Polarisation: Plane polarised waves.

TRANSVERSE WAVES ARE POLARISABLE. LONGITUDINAL WAVES ARE NOT. (vibration in the same plane as difrection of propogation)

Here is an unpolarised wave.
It has many planes of disturbance.
They are all perpendicular to the direction of propogation, at any angle around the axis.

Here is a Polarised wave.

It has ONE plane if disturbance, perpendicular to the direction of trave, at ONE angle about the axis.

Note: For EM waves, we consider the E field when describing the plane of polarisation.

Some waves are generated as polarised waves, and others can be polarised.

Rotating the metal rods through 90 degrees causes the probe not to pick up any microwaves since none are geting through the rods as the generator generates polarised waves.

Rotating the probe through 90 degrees would have the same effect, none would still be recieved.



Light from lightbulbs is not polarised as it relies on random excitement of electrons, rather than specific motions of electrons. Light can be polarised using a polaroid.


Applications.

SUNGLASSES - reduce glare. Reflected rays from shiny surfaces are polarised. Sunglasses are intended to absorb the radiation

OPTICAL ACTIVITY - some solutions rotate plane of disturbance. Different concentrations affect this by different amounts.

STRESS ANALYSIS - Clear plastic models are made of structures which are placed between polaroids, stressed, and stress patterns observed.
LCD DISPLAYS - Polarised light passes through the liquid crystal and is rotated by 90 degrees. It is reflected and is re-rotated. A PD across areas of the liquid reduces the roration, thus no light can get through, causing the dark patches.

Things about standing waves.

Standing waves only occur when there are two boundaries.
They will store energy.
They occur in musical instruments, e.g. guitar string.
They are important in tv/radio waves.
Important in making aircraft wings; you don't want a standing wave along the wing.

Using Standing Waves

1) Measure frequency of microwaves.

AS we move the probe towards the source, we encounter weak signal, then strong signal. This is because we have set up a standing wave, so we have nodes and antinodes. When we are near R, the nodes and antinodes are much more distinct than towards S. This is because as the waves travel, they lose energy so the amplitudes get smaller. At R, the waves have travelled similar distances, so the nodes and antinodes are distinct. At S, they have travelled different distances, so the amplitudes of the two waves are different causing less distinct nodes and antinodes.

We find a node, then record the distance. Then go 5 nodes along and record the distance. divide the difference by 2 to find λ.

c=λf

3*10^8 = (0.104 / 3) f

f = 8.65*10^9 /s
= 8.65*10^9 Hz


2) Measuring Speed of Sound.

This is very similar, but using a signal generator and microphone instead of a microwave source and probe, and a CRO instead of an amp.

λ= 2* node to node.
f=whatever pitch is set on the generator.

c=λf

Harmonics

The seperation of the two fixed ends has to be a multiple of 1/2λ