3.12 - Turning Points in Physics

Question 1

How are electron beams formed in a discharge tube?

Answer 1

Cathode connected to -ve of a DC power supply and anode connected to +ve. A low pressure gas is inside this tube. The high potential difference applied pulls electrons from the gas atoms, forming ion and electron pairs. Positive gas ions are accelerated towards the cathode, causing free electrons of the cathode to be emitted. These electrons accelerate along the tube to the anode, colliding with gas ions, where they become excited. They quickly de-excite, releasing photons of light. The brightest glow is at the cathode, where the gas ions and electrons recombine and emit light photons.

Question 2

What did the production of cathode rays show about the particles being investigated?

Answer 2

their mass, it has negative charge, same properties no matter what gas was used, very large charge to mass ratio

Question 3

What is the equation for work done on an electron accelerated through a potential difference?

Answer 3

W=eV

Question 4

How to electron guns work?

Answer 4

Potential difference accelerates elecrons released from the hot cathode (thermionic emission). They accelerate towards the anode, which has a small gap. Electrons pass through this, forming a narrow electron beam travelling beyond the anode.

Question 5

What happens to electrons when accelerated through a pd and what equation does this lead to?

Answer 5

the kinetic energy is equal to the work done on the electron by the electric field
1/2mv^2=eV

Question 6

How does Tomson’s crossed fields determine the specific charge of an electron?

Answer 6

Magnetic and electric fields perpendicular to one another, deflecting electrons in opposite directions. Electrons accelerated using e- gun, entering apparatus perpendicular to both fields. electric field deflects e- up, and magnetic field deflects e- down. For the e- beam to pass straight through, electric force = - magnetic force.

Question 7

What is the significance of determining the electron SC?

Answer 7

Showed that elctron SC is constant
Beginning of atomic physics: showing electrons have mass, energy, momentum
E/Me was 1800x hydrogen (proton) SC; either electron mass is much smaller or charge is much larger; 1899: estimated e around 10^-19 C, so same charge as proton but much smaller mass.

Question 8

What is the purpose of Millikan’s oil-drop experiment?

Answer 8

To calculate the charge of an electron

Question 9

What apparatus did Millikan use?

Answer 9

Two metal plates with a high d.c. voltage put between them - creates an electric field between the plates. Above the plates, oil droplets are sprayed from an atomiser, where some fall through a hole in the upper plate. A gamma source is directed at the oil droplets that fall in between the plates, in order to ionise them (giving them charge).

Question 10

What happens to oil droplets when the electric field is then turned on?

Answer 10

They move upwards, pulled by the electric field, with a new terminal velocity

Question 11

What are the forces on an oil droplet when the electric field is off? Where does its terminal velocity go?

Answer 11

Force of weight downwards and equal magnitude of drag upwards; terminal velocity downwards

Question 12

What are the forces on an oil droplet when the electric field is on? Where does the terminal velocity go?

Answer 12

Forces of weight + drag downwards and equal magnitude of electric force upwards; terminal velocity upwards

Question 13

What dies Stoke’s law give us?

Answer 13

The magnitude of force on a spherical object due to viscous drag

Question 14

What is the equation to find terminal velocity when the field is off?

Answer 14

mg=6πrη(v1)

Question 15

What is the equation to find terminal velocity when the field is on?

Answer 15

mg + 6πrη(v2) = qV/d

Question 16

What is so significant about Millikan’s findings?

Answer 16

The smallest value of q was 1.6x10-19C and all the others were multiples of this; he found that CHARGE IS QUANTISED

Question 17

What did Huygens believe light could be fully explained by?

Answer 17

light as a wave

Question 18

What is Huygens’ Theorum?

Answer 18

Every point on a wavefront can be assumed to be a point source of secondary wavelets that spread out forwards, in the same speed of the wave. The new wavefront is the surface that is tangential to all of these secondary wavelets.

Question 19

What did Newton believe about light?

Answer 19

That light is explained by thinking of it being made up of many tiny particles, called corpuscles.

Question 20

What was Young’s experiment and what did it show?

Answer 20

Double-slit experiment.
Showing that light interferes, which is a wave property, just as water waves do in a ripple tank.

Question 21

What does it mean to say that electromagnetic waves are self-sustaining?

Answer 21

Oscillating B field produces ELECTRIC field and the oscillating E field produces the MAGNETIC field; they sustain each other - couldn’t exist on their own

Question 22

What did Maxwell show?

Answer 22

That electric and magnetic forces were different manifestations of the same force

Question 23

What did Maxwell’s formula show?

Answer 23

The speed of electromagnetic radiation in a vacuum

Question 24

What did Hertz do/show?

Answer 24

He produces radio waves using a spark transmitter; about 1m away from two charged plates with a spark gap between them, another spark was produced in the gap between two poles of a loop antenna.

Question 25

What did Hertz do in his later experiment? How did this compare to Maxwell's findings?

Answer 25

Produces standing waves by reflecting the radio waves off a metal sheet, allowing him to measure the speed of the waves. Agreed with Maxwell's findings.

Question 26

How did Fizeau measure the speed of light?

Answer 26

A ray of light passed through one of the slots on a rotating wheel, travels around 9km to a mirror and is reflected back. If the wheel is rotating at a a certain speed, the reflected beam was blocked by the next cog tooth. He could calculate the speed of light using the frequency of rotation, the number of gaps, and the distance travelled.

Question 27

How could you study the spectrum for different temperatures?

Answer 27

Put a cube in an oven, raise it to a certain temperature. Any radiation emitted from the inside walls of the cube exits through the hole to a spectrometer, which separates the different wavelengths. You can measure the intensity of these with a detector.

Question 28

How did the oven cube spectra results abide with the current wave theory?

Answer 28

They agreed well and low temperatures, but differed greatly at higher temperatures.

Question 29

What did the wave theory at the time of the UV catastrophe predict?

Answer 29

That at lower temperatures, huge amounts of small wavelength radiation would be produced

Question 30

What did Planck assume when producing the new wave theory?

Answer 30

Atoms can only have discrete amounts of energy and so the photons they emit can also only have discrete amounts of energy

Question 31

What did Einstein say about light?

Answer 31

EM waves (and their energy) can only exist in discrete packets, called photons. These have one-on-one interactions with an electron on a metal surface, where all its energy is transferred to that one specific electron.

Question 32

How did Maxwell prove his theory?

Answer 32

By producing an equation for EM wave speed, combined with experiments

Question 33

Why was Newton's theory of light preferred?

Answer 33

- His explanations of reflection/ refraction intuitively fitted with the existing understanding of physics. - No experimental evidence supported Huygens's theory for 100 years - Newton had a greater reputation as a revolutionary physicist and mathematician

Question 34

What were the implications of Fizeau's calculations?

Answer 34

Maxwell's value of c later on was very similar to Fizeau's, providing strong evidence that light is an EM wave.

Question 35

What did Planck's new equation predict? How did it become accepted?

Answer 35

that energy is quantised; it had no evidence (a mathematical solution only) but the curve it produced matched experimental evidence

Question 36

Why can classical wave theory not explain the photoelectric effect?

Answer 36

- For one frequency of light, the energy carried is proportional to the beam's intensity. - The light's energy would spread evenly over the wavefront. - each surface electron would gain a bit of energy from each incoming wave. - gradually, each electron would gain enough energy to leave the metal. - None of this explains the KE depending on frequency, or the fact of a threshold frequency.

Question 37

What did De Broglie hypothesise? Equation?

Answer 37

That electrons, like light, must also have wave properties, therefore making it have wave-particle duality. p=h/λ

Question 38

When does diffraction happen?

Answer 38

when a particle interacts with an object of approx. the same magnitude as its de Broglie wavelength

Question 39

How are images formed in a TEM?

Answer 39

A thin specimen is used. An electron beam is projected onto the specimen. The parts of the beam which pass through the specimen are projected onto a screen, forming an image.

Question 40

How are images formed in a STM?

Answer 40

A very fine probe is moved over the surface of the sample. A voltage is applied between the probe and the surface, inducing a weak electric current. The smaller the distance between these, the greater the current induced. By scanning the probe over the surface and measuring the current, you can produce an image of the sample's surface.