Turning points

Question 1

What is the term for the glow observed in a discharge tube with low pressure gas?

Answer 1

Cathode ray

The glow is brightest at the cathode due to the recombination of gas ions and electrons emitting photons.

Question 2

What properties did Thomson show about cathode rays?

Answer 2

• Have a mass
• Have a negative charge
• Same properties regardless of gas used
• Very large charge to mass ratio

These properties led to the conclusion that cathode rays are electrons.

Question 3

What happens when a high potential difference is applied across a discharge tube?

Answer 3

• Electrons are pulled off gas atoms
• Positive gas ions are accelerated towards the cathode
• Electrons collide with gas atoms causing excitation and photon emission

This process results in the glow of the discharge tube.

Question 4

What is thermionic emission?

Answer 4

The emission of free electrons from a heated metal surface.

This process is used in electron guns.

Question 5

What is the formula for work done on a charged particle in an electric field?

Answer 5

ΔW = QΔV

Where Q is charge and ΔV is potential difference.

Question 6

What is measured in electron volts (eV)?

Answer 6

The kinetic energy of an electron accelerated across a potential difference of 1 V.

1 eV is equal to the energy gained by an electron when accelerated through 1 volt.

Question 7

What is the specific charge of an electron?

Answer 7

Approximately 1.76 x 10^11 C/kg

This specific charge is about 1800 times larger than that of a proton.

Question 8

What is the principle behind Millikan’s oil drop experiment?

Answer 8

To calculate the charge of an electron by balancing the weight of oil droplets with electric force.

The experiment showed charge is quantized.

Question 9

What does Young’s double slit experiment demonstrate?

Answer 9

The wave nature of light through diffraction and interference patterns.

It disproved Newton’s corpuscular theory of light.

Question 10

What is Huygens’ principle?

Answer 10

Every point on a wavefront acts as a source of secondary wavelets.

This principle explains reflection and refraction of light.

Question 11

What are electromagnetic waves composed of?

Answer 11

Alternating magnetic and electric fields travelling in phase and at right angles to each other.

The direction of wave travel is perpendicular to the oscillations of the fields.

Question 12

Who discovered radio waves and how?

Answer 12

Hertz discovered radio waves using high voltage sparks to produce them.

The radio waves could be detected using a dipole receiver or a loop of wire.

Question 13

What is the formula for the speed of electromagnetic waves in a vacuum?

Answer 13

c = 1 / √(μ0 ε0)

Where μ0 is the permeability of free space and ε0 is the permittivity of free space.

Question 14

True or False: Newton’s corpuscular theory of light could explain diffraction.

Answer 14

False

Diffraction could not be explained by Newton’s theory, leading to the acceptance of wave theories.

Question 15

Fill in the blank: The charge of an electron is approximately _______.

Answer 15

−1.6 x 10^−19 C

This is the smallest possible magnitude of charge.

Question 16

What is the relationship between electric and magnetic forces in Thomson’s crossed fields experiment?

Answer 16

They are equal and opposite when the electron beam passes undeflected.

This occurs when the strengths of the electric and magnetic fields are adjusted appropriately.

Question 17

What does a wire with a gap detect?

Answer 17

It detects the waves’ alternating magnetic field, inducing a potential difference that causes a spark to cross the gap

This occurs due to the change in magnetic flux through the loop.

Question 18

What is formed when radio waves are reflected back onto themselves?

Answer 18

Stationary waves are formed

This can be used to find the distance between adjacent nodes to calculate the wavelength.

Question 19

How did Hertz confirm that radio waves were electromagnetic waves?

Answer 19

By calculating the speed of radio waves, which matched Maxwell’s predicted value

This was a significant confirmation of electromagnetic wave theory.

Question 20

What happens to the signal when the receiver is rotated 90°?

Answer 20

The signal varies from maximum to minimum

This is because the plane of the detector becomes parallel to the oscillations of the electric/magnetic field.

Question 21

What is the first step in Fizeau’s method to measure the speed of light?

Answer 21

A pulsed beam of light is passed through a gap in a toothed wheel rotating at a slow speed

The light reflects off a mirror and returns through the same gap.

Question 22

What does the speed of rotation of Fizeau’s wheel determine?

Answer 22

It determines when the light beam can no longer be seen due to being blocked by a tooth

Doubling the speed allows the light to be visible again.

Question 23

What does the equation E = hf represent?

Answer 23

The energy of electromagnetic waves in discrete packets called quanta

This equation shows that energy is directly proportional to frequency.

Question 24

What is the threshold frequency in the context of the photoelectric effect?

Answer 24

The frequency at which photon energy equals the work function of the metal

Below this frequency, no photoelectrons are emitted.

Question 25

What does increasing the intensity of light in the photoelectric effect do?

Answer 25

It increases the number of photoelectrons emitted per second ## Footnote Intensity relates to the number of photons released, not their individual energy.

Question 26

What is the stopping potential related to in the photoelectric effect?

Answer 26

The potential difference needed to stop photoelectrons with maximum kinetic energy ## Footnote This is measured to find the maximum kinetic energy of photoelectrons.

Question 27

What is the de Broglie hypothesis?

Answer 27

All particles have both wave-like and particle nature ## Footnote The wavelength of a particle can be calculated using λ = h/p.

Question 28

What does electron diffraction provide evidence for?

Answer 28

It provides evidence for the de Broglie hypothesis ## Footnote It shows that particles like electrons can exhibit wave behavior.

Question 29

What is the resolving power of a microscope?

Answer 29

Its ability to distinguish structures that are close to each other ## Footnote Higher resolving power allows for viewing smaller structures.

Question 30

What are the two types of electron microscopes?

Answer 30

* Transmission electron microscope (TEM) * Scanning tunneling microscope (STM) ## Footnote Each type has different methods for imaging samples.

Question 31

What is a key feature of a Transmission Electron Microscope (TEM)?

Answer 31

Electrons pass through a set of magnetic lenses and an extremely thin sample ## Footnote This allows for high-resolution imaging.

Question 32

What limits the resolving power of a TEM?

Answer 32

* Sample thickness * Electrons traveling at a range of speeds ## Footnote These factors can cause blurring and decrease resolution.

Question 33

What principle does a Scanning Tunneling Microscope (STM) operate on?

Answer 33

Quantum tunneling of electrons ## Footnote This allows imaging of surfaces at atomic scales.

Question 34

What does the Michelson-Morley experiment aim to measure?

Answer 34

The absolute speed of the Earth through the ether ## Footnote It used an interferometer to detect shifts in light interference patterns.

Question 35

What conclusion was drawn from the Michelson-Morley experiment?

Answer 35

The speed of light is invariant in free space ## Footnote This challenged the existence of ether and led to new theories in physics.

Question 36

What was the conclusion about the ether based on the interference pattern experiment?

Answer 36

1. The ether doesn’t exist or the Earth drags the ether along with it as it moves ## Footnote This conclusion was drawn from the lack of shift in the interference pattern when the apparatus was rotated.

Question 37

What does the speed of light being invariant in free space imply?

Answer 37

The speed of light is independent of the motion of the source or the observer. ## Footnote This is a key postulate of Einstein's theory of special relativity.

Question 38

What are inertial frames of reference?

Answer 38

Frames that move at a constant velocity relative to each other. ## Footnote Acceleration or rotation disqualifies a frame from being inertial.

Question 39

What are the two main postulates of Einstein's theory of special relativity?

Answer 39

* The speed of light in free space is invariant * The laws of physics have the same form in all inertial frames ## Footnote These postulates form the foundation of special relativity.

Question 40

What is time dilation?

Answer 40

Time runs at different speeds depending on the motion of an observer. ## Footnote It is a consequence of special relativity and only occurs in inertial frames.

Question 41

What is the proper time (t0)?

Answer 41

The amount of time passed experienced by the stationary observer. ## Footnote It is always shorter than the time measured by an external observer.

Question 42

If Lucy measures 1 hour on her clock traveling at 0.9c, how much time passes on Earth?

Answer 42

2.3 hours ## Footnote This demonstrates the effect of time dilation.

Question 43

What experimental evidence supports time dilation?

Answer 43

Muon decay provides evidence for time dilation due to significant time dilation experienced by muons. ## Footnote Muons enter the atmosphere at high speeds.

Question 44

What is length contraction?

Answer 44

The length of objects moving at high speeds appears shorter to an external observer. ## Footnote This phenomenon occurs only in inertial frames.

Question 45

How is the proper length (l0) defined?

Answer 45

The length of an object as measured by an observer at rest relative to the object. ## Footnote Only the length is affected by motion; width remains constant.

Question 46

What equation relates mass and energy in special relativity?

Answer 46

E = mc² ## Footnote Where E is energy, m is mass, and c is the speed of light in a vacuum.

Question 47

What happens to an object's mass as its speed increases?

Answer 47

Its mass increases, termed relativistic mass. ## Footnote This is due to the relationship between mass and energy.

Question 48

What is the formula for kinetic energy at relativistic speeds?

Answer 48

Eₜ = Eₖ + E₀ ## Footnote Where Eₜ is total energy, Eₖ is kinetic energy, and E₀ is rest energy.

Question 49

What did Bertozzi's experiment demonstrate?

Answer 49

It provided evidence for the increase in mass of an object with speed. ## Footnote This involved measuring kinetic energy of electrons at varying speeds.

Question 50

True or False: According to special relativity, an object can reach the speed of light.

Answer 50

False ## Footnote As an object's speed approaches the speed of light, its mass and energy approach infinity, which is impossible.

Question 51

What is the expected count rate at the second detector without acknowledging special relativity?

Answer 51

4.5 s⁻¹ ## Footnote This is calculated based on half-lives without special relativity consideration.

Question 52

What is the expected count rate at the second detector considering special relativity?

Answer 52

76 s⁻¹ ## Footnote This value is much closer to the observed count rate.

Question 53

What is the formula to calculate the time taken for muons to move between detectors?

Answer 53

t = s/v ## Footnote Where s is the distance and v is the velocity of the muons.

Question 54

What is the significance of the muon half-life in the context of time dilation?

Answer 54

The muon half-life at rest is 1.5 µs. ## Footnote This value is crucial for calculating expected decay rates.

Question 55

What effect does length contraction have on muons traveling at high speeds?

Answer 55

The distance they travel appears shorter than the distance viewed by an external observer. ## Footnote This is used to calculate expected count rates.