13 Option D: Turning points

Question 1

When was the cathode ray first used?

Answer 1

1876

Question 2

How were cathode rays used in 1876?

Answer 2

They were used to describe what causes the glow that appears on the wall of a discharge tube when a high potential difference is applied across the terminals.

Question 3

What is the set-up of a cathode ray discharge tube?

Answer 3

The cathode is connected to the negative terminal of the battery and becomes negatively charged as electrons flow from the battery to the cathode. The anode is connected to the positive battery terminal and becomes positively charged

Question 4

In 1897, who ended the debate of what cathode rays were made of?

Answer 4

J.J.Thomson

Question 5

In 1897, what did J.J.Thomson demonstrate about cathode rays (4 things)?

Answer 5

1 they have energy, momentum and mass.
2 They have a negative charge
3 They have the same properties, no matter what gas is in the tube and what the cathode is made of.
4 They have a specific charge much bigger than that of hydrogen ion. So they either have a tiny mass, or a much higher charge - Thomson assumed they has the same size charge as hydrogen ions and a tiny mass.

Question 6

What did Thomson conclude from his discovery of cathode rays?

Answer 6

Thomson concluded that all atoms contain these ‘cathode ray particles’, or electrons as they were soon known - cathode rays are just beams of electrons. He had discovered the first subatomic particle.

Question 7

What is thermionic emission?

Answer 7

When you heat a metal, its free electrons gain kinetic energy. Give them sufficient energy and they’ll break free from the surface of the metal - this is called thermionic emission.

Question 8

How can emitted electrons be accelerated by an electric field?

Answer 8

in an electron gun

Question 9

How do electron guns work?

Answer 9

1 A heating coil heats the metal cathode. The electrons that are emitted are accelerated towards the cylindrical anode by the electric field set up by the high voltage.
2 The glass tube in an electron gun has to be evacuated of air so that the electrons can travel freely in the electric field
3 Some electrons pass through a little hole in the anode, making a narrow electron beam. The electrons in the beam move a constant velocity because there’s no field beyond the anode so there’s no force.

Question 10

Where are cathode ray tubes used?

Answer 10

Old-fashioned TV screens and computer monitors

Question 11

What is the equation for work done in moving a charge?

Answer 11

work done in moving a charge = the charge being moved in C x electric potential difference that the charge is moved through in V

Question 12

How do you apply the equation for work done in moving a charge to the situation using cathode rays?

Answer 12

Q is the charge of a single electron and V is the potential difference between the cathode and anode. You get the work done in accelerating an electron through a p.d. is W=eV

Question 13

When accelerating an electron through a p.d., how are work done and kinetic energy linked? What do you have to assume for this equation to be correct?

Answer 13

The kinetic energy that the electron will have as it leave the anode (through the hole in the electron gun) is equal to the work done in accelerating through the potential difference between the cathode and anode. However, you have to assume the initial velocity of the electron is negligible.

Question 14

What is the definition of the electronvolt?

Answer 14

1 electronvolt is the kinetic energy carried by an electron after is has been accelerated from rest through a potential difference of 1 volt.

Question 15

What is specific charge?

Answer 15

The charge-to-mass ratio of a charged particle. The charge per unit mass

Question 16

Who used the specific charge of an electron equation to prove that subatomic particle exist?

Answer 16

Thomson

Question 17

When did Thomson prove that subatomic particles exist?

Answer 17

1897

Question 18

What is the experiment to find the specific charge of an electron?

Answer 18

1 A beam of electron from an electron gun is passed through low-pressure hydrogen gas.
2 The electrons in the beam collide with the hydrogen atoms long its path and transfer some of their energy, causing the electrons in the atoms to move into higher energy levels. This is known as excitation.
3 As the electrons in these excited hydrogen atoms fall back to the ground state, they emit light. The electron beam is seen as a glowing trace through the gas.
4 Two circular magnetic field coils either side of the tube generate a uniform magnetic field inside the tube. The electron beam is fired at right angles to the magnetic field, so the beam curves round a circle.
5 The circular motion of the electron beam means that the magnetic force of the electron is acting as the centripetal force.
6 (manipulation of equations)

Question 19

What was the significance of Thomson’s findings about the specific charge of an electron?

Answer 19

The largest specific charge that had ever been measured before was the specific charge of a hydrogen + ion. Using his own method, Thomson found in 1897 that the specific charge of an electron is much greater than the specific charge of a hydrogen + ion, meaning that it either has a much greater charge or is much lighter. He assumed that electrons had the same charge and they were very light. It turns out that the specific charge of an electron is about 1800 times greater than the specific charge of an hydrogen ion or proton. And the mass of a proton is about 1800 times greater than the mass of an electron - Thomson was right, electron and protons do have the same size charge.

Question 20

What is Stokes’ Law and the theory behind it?

Answer 20

When you drop an object into a fluid, like air, it experiences a viscous drag force. This force acts in the opposite direction to the velocity of the object, and is due to the viscosity of the fluid. Viscosity is just how thick the fluid is.

Question 21

How can you calculate the viscous force on a spherical object?

Answer 21

Stokes’ Law
viscous force = 6 x viscosity of the fluid x radius of the object x velocity of the object

Question 22

What did Millikan’s Oil-drop experiment find?

Answer 22

The charge of an electron

Question 23

What was the set-up for Millikan’s oil-drop experiment?

Answer 23

The atomiser created a fine mist of oil drops that were charged by friction as they left the atomiser (positively if they lost electrons, negatively if they gained electrons). Some of the drops fell through a hole in the top plate and could be viewed through the microscope.
When he was ready, Millikan could apply a potential difference between the 2 plates, producing a field that exerted a force on the charged drops. By adjusting the p.d., he could vary the strength of the field, and therefore vary the magnitude of the force exerted on the oil drops.

Question 24

During Millikan’s Oil-drop experiment, what are the forces acting on an oil drop with no electric field between the plates?

Answer 24

1 the weight of the drop acting downwards
2 the viscous force from the air acting upwards

Question 25

During Millikan's Oil-drop experiment, what are the forces acting on an oil drop with an electric field between the plates?

Answer 25

Millikan's next step was to apply a p.d. across the plates, creating an electric field. The field introduced a third major factor - an electric force on the drop. He adjusted the applied p.d. until the drop was stationary. Since the viscous force is proportional to the velocity of the object, once the drop stopped moving, the viscous force disappeared. The forces acting on the oil drop are... 1 the weight of the drop acting downwards 2 the force due to the uniform electric field acting upwards

Question 26

What was the significance of Millikan's oil-drop experiment?

Answer 26

Millikan concludes that charge can never exist in smaller quantities than 1.6x10-19. He assumed that this was the charge carried by an electron. Later experiments confirmed that both these things are true. This discovery meant that the mass of an electron could be calculated exactly, proving that it was the lightest particle ever discovered (at the time).

Question 27

What does it mean by the quantisation of electric charge?

Answer 27

Charge is 'quantised'. It exists in 'packets' of size 1.6x10-19 coulombs - the fundamental unit of charge. This is the size of the charge carried by one electron.

Question 28

What did Newton publish in 1671?

Answer 28

He published his New Theory about Light and Colours. In it, he suggested that light was made up of a stream of tiny particles that he called 'corpuscles'.

Question 29

What was Newton's corpuscular theory?

Answer 29

One of his major arguments for light being a particle was that light was known to travel in straight lines, yet waves were known to bend in the shadow of an obstacle (diffraction). Experiments weren't accurate enough then to detect the diffraction of light. Light was known to reflect and refract, but that was it. His theory was based on the principles of his laws of motion.

Question 30

What is Huygens' principle?

Answer 30

Every point on a wavefront may be considered to be a point source of secondary wavelets that spread out in the forward direction at the speed of the wave. The new wavefront is the surface that is tangential to all of these secondary wavelets.

Question 31

How did Huygens apply his principle to explain reflection and refraction?

Answer 31

He predicted that light should slow down when it entered a denser medium, rather than speed up. He also predicted that light should diffract around tiny objects and that two coherent light sources should interfere with each other.

Question 32

At the time, why was Newton's corpuscular theory of light preferred over Huygens's principle? (4 reasons)

Answer 32

1 Newton's corpuscular theory was much more popular at the time because imagining light as a stream of particle explained reflection and refraction in a way that more intuitively fitted in with the existing understanding of physics. It couldn't explain diffraction, but the equipment of the time wasn't capable of demonstrating diffraction in light. 2 Scientists thought double refraction couldn't be explained by thinking of light as a wave. Newton's corpuscular theory explained it in terms of the corpuscles having 'sides'. 3 There was no experimental evidence to support Huygens's theory that light was a wave until Young's interference experiments more than 100 years later. 4 Over time, Newton's reputation grew as his idea on maths, gravity, forces and motion revolutionised physics. By the time of Thomas Young a century later, he was a figure scientists didn't want to disagree with.

Question 33

What is double refraction?

Answer 33

A polarisation effect, where shining light through certain crystals make two images instead of one

Question 34

What was the significance of Young's double-slit experiment?

Answer 34

Diffraction and interference are both uniquely wave properties. If it could be shown that light showed interference patterns, that would help decide once and for all between corpuscular theory and wave theory.

Question 35

How did Young solve the problem of needing coherent sources to produce a clear interference pattern?

Answer 35

The problem with showing light interfering was getting 2 coherent light sources, as light is emitted from most sources in random bursts. Young solved this problem by using only one point source of light (a light source passed through a narrow slit). In front of this was a slide with 2 narrow slits in it - a double slit. Light spreading out by diffraction from the slits was equivalent to 2 coherent point sources.

Question 36

What did Young's double-slit experiment prove about Newton's corpuscular theory and Huygens's wave theory?

Answer 36

Young's experiment was proof that light could both diffract (through narrow slits) and interfere (to form the interference pattern on the screen). Newton's corpuscular theory predicted that there would only be 2 fringes, corresponding to the 2 slits that the corpuscles could pass through. Young's experiment showed that this clearly wasn't happening, and Huygens's theory could explain everything.

Question 37

Following the proof from Young's double-slit experiment, why was Huygens's theory still not accepted? (3 reasons)

Answer 37

1 Newton's work had revolutionised physics, and by this point he was an established historical figure who other scientists didn't want to contradict. 2 There were also problems with Huygens's wave theory. It used longitudinal waves, but light was known to be able to be polarised - a property of transverse waves only. Also, Huygens's theory failed to explain both double refraction and why sharp shadows were formed by light. 3 It too more than a decade before Young realised that transverse waves could explain the behaviour of light. Following this, other scientists soon started agreeing with Huygens that light was a wave.

Question 38

In the second half of the 19th century, what did James Clerk Maxwell model and predict?

Answer 38

He was trying to unite the ideas of magnetism and electricity. He created a mathematical model of magnetic and electric fields. This model said that a change to these fields would create an electromagnetic wave, radiating out from the source of the disturbance. Maxwell's prediction came before any experimental evidence for the existence of EM waves. He predicted that there would be a spectrum of EM waves, travelling at the same speed with different frequencies. Maxwell's model showed theoretically that all EM waves should travel at the same speed in a vacuum.

Question 39

How are electron guns used to make cathode ray tubes (CRTs)?

Answer 39

Electron guns are combined with fluorescent screens in the cathode ray tubes (CRTs). The electron beam is directed at the screen, causing is to emit light and produce picture on the screen.

Question 40

What are CRTs (cathode ray tubes) used for?

Answer 40

They are used in old-fashioned TV screens and computer monitors.

Question 41

During the experiment to find the specific charge of an electron, why does the hydrogen gas need a low pressure?

Answer 41

So that there are as few hydrogen atoms as possible to inhibit the electron beam. You want just enough hydrogen atoms for the beam to be visible, but not enough to stop the beam.

Question 42

How did Newton use his corpuscular theory of light to explain relfection?

Answer 42

Newton believed that reflection was due to a force that pushed the particles away from the surface - just like a ball bouncing off a wall.

Question 43

How did Newton use his corpuscular theory of light to explain refraction?

Answer 43

He thought refraction occurred because the corpuscle travelled faster in a denser medium, like glass compared to air.

Question 44

What were the results from Young's double-slit experiment?

Answer 44

Light from the double-clit was projected onto a screen and bright and dark 'fringes' were formed where light from the 2 slits overlapped each other. These fringes were created from constructive and destructive interference patterns.

Question 45

In terms of light interference from Young's double slit experiment, what does it mean by constructive interference?

Answer 45

Produce bright fringes on the screen and are formed when the path difference between the waves and the screen is 0 or any integer multiple of the wavelength.

Question 46

In terms of light interference from Young's double slit experiment, what does it mean by destructive interference?

Answer 46

Produce dark fringes on the screen and are formed when the waves from the 2 slits are exactly out of phase and cancel each other out. Their path difference is n+1/2 wavelengths, where n is an integer.

Question 47

What equation did James Clerk Maxwell use to calculate the speed of an electromagnetic wave in a vacuum??

Answer 47

Speed of the wave =1/ the square root of the impermeability of free space x the permittivity of free space

Question 48

What does the permeability of free space relate to?

Answer 48

It relates to the magnetic flux density due to a current-carrying wire in free space

Question 49

What does the permittivity of free space relate to?

Answer 49

It relates to the electric field strength due to a charged object in free space

Question 50

What does the unit H mean?

Answer 50

It stands for 'henry' and it is the standard unit for inductance

Question 51

When did Hippolyte Fizeau measure the speed of light?

Answer 51

mid 1800s

Question 52

How did Fizeau measure the speed of light?

Answer 52

1 He passed a beam of light through the gap between 2 cog teeth to a reflector about 9km away. 2 The cog rotated at exactly the right speed so that the reflected beam was blocked was the next cog tooth. 3 Using the frequency of rotation and the number of gaps, Fizeau was able to calculate the time taken for the light to travel to the reflector and back. 4 Because Fizeau knew both the time taken and the distance travelled, he could use these toc calculate an estimate for the speed of light.

Question 53

How did Fizeau's experiment provide evidence that light is an electromagnetic wave?

Answer 53

When Maxwell calculates his value of c, he found that it was very close to the value measured by Fizeau years earlier. So this provided very strong evidence that light (as well as UV and infrared radiation beyond the visible spectrum) is an electromagnetic wave.

Question 54

Who produced and detected radio waves using electric sparks in the late 1880s?

Answer 54

Heinrich Hertz

Question 55

How did Heinrich Hertz produce radio waves?

Answer 55

1 He used an induction coil and a capacitor to product a high voltage, and showed that radio waves were produced when high voltage sparks jumped across a gap of air. 2 He detected these radio waves using a loop of wire with a gap in it in which sparks were induced by the radio waves. The fact that a p.d. was induced in the loop showed that the waves had a magnetic component because a changing magnetic field is needed to induce a p.d. 3 You can show that the radio waves have an electric component by replacing the wire loop with a second dipole parallel to the first. The radio waves will create an alternating current in the second dipole.

Question 56

How did Heinrich Hertz adapt his original experiment set-up to create stationary radio waves?

Answer 56

He used a flat metal sheet to create stationary radio waves, showing that they could both be reflected and show interference.

Question 57

How did Heinrich Hertz measure the velocity of radio waves?

Answer 57

he used stationary radio waves

Question 58

What is meant by a stationary wave?

Answer 58

The superposition of 2 progressive waves with the same frequency, wavelength and amplitude, moving in opposite directions

Question 59

What is the easiest way to create a stationary wave?

Answer 59

Reflecting a progressive wave back on itself

Question 60

What are resonant frequencies and how do they link to stationary wave?

Answer 60

If the oscillator happens to produce an exact number of waves in the time it takes for a wave to get to the end and back again, the original and reflected waves reinforce each other. The frequencies at which this happens are called resonant frequencies and it causes a stationary wave where the overall pattern doesn't move along

Question 61

What are nodes?

Answer 61

Points on a stationary wave with 0 amplitude

Question 62

What are antinodes?

Answer 62

Points on a stationary waves with maximum amplitude

Question 63

How did Heinrich Hertz use radio stationary waves to measure the velocity of radio waves?

Answer 63

1 He moved the radio wave detector between the transmitter and the reflecting sheet and measured the distance between nodes. 2 Since the distance between nodes in half a wavelength, he could work out the wavelength and then use c=f x lambda to calculate the wave speed.

Question 64

Why was Hertz experiment to find the velocity of radio waves beneficial?

Answer 64

He measured the speed of radio waves to be in the same in a vacuum as the rest of the electromagnetic spectrum, which confirmed that radio waves are electromagnetic waves.

Question 65

What is the effect of the photoelectric effect?

Answer 65

If you shine electromagnetic radiation of a high enough frequency onto the surface of a metal, it will instantly emit electrons. For most metals, this frequency falls in the UV range.

Question 66

How does the photoelectric effect work?

Answer 66

Because of the way atoms are bonded together in metals, metals contain 'free electrons' that are able to move about the metal. The free electrons on or near the surface of the metal can sometimes absorb energy from the radiation. If an electron absorbs enough energy (the radiation has a high enough frequency), the bonds holding it to the metal break and the electron is released.

Question 67

What are the electrons that are emitted from the photoelectric effect called?

Answer 67

Photoelectrons

Question 68

What is conclusion 1 of the photoelectric effect?

Answer 68

For a given metal, no photoelectrons are emitted if the radiation has a frequency below a certain value, which is called the threshold frequency

Question 69

What is conclusion 2 of the photoelectric effect?

Answer 69

The photoelectrons are emitted with a variety of kinetic energies ranging from 0 to some maximum value. This maximum value of kinetic energy increases with the frequency of the radiation, but is unaffected by the intensity of the radiation.

Question 70

What is conclusion 3 of the photoelectric effect?

Answer 70

The number of photoelectrons emitted per second is directly proportional to the intensity of the radiation.

Question 71

Why can't wave theory explain the threshold frequency within the photoelectric effect?

Answer 71

Wave theory states that if EM radiation were shone on a metal, each free electron on the surface of the metal would gain a bit of energy from each incoming wave. Gradually, each electron would gain enough energy to leave the metal. If the EM waves had a lower frequency, it would take longer for the electrons to gain enough energy but it would eventually happen. HOWEVER, electrons are never emitted unless the waves are above a threshold frequency so the wave theory can't be used to explain this.

Question 72

Why can't wave theory explain the idea of the kinetic energy of photoelectrons within the photoelectric effect?

Answer 72

Wave theory states that the higher the intensity of the waves, the more energy they should transfer to each electron - the kinetic energy should increase with intensity. HOWEVER, wave theory can't explain the fact that the kinetic energy depends only on the frequency in the photoelectric effect.

Question 73

What was Max Planck's idea of wave packets?

Answer 73

Max Planck was the first to suggest that EM waves can only be released in discrete packets, or quanta.

Question 74

What is the equation to find the energy of 1 wave packet?

Answer 74

E = hf = hc/lambda

Question 75

How did Einstein explain the photoelectric effect?

Answer 75

He suggested that EM waves can only exists in discrete packets called photons. He saw these photons of light as having a one-on-one particle-like interaction with an electron in a metal surface. Each photon would transfer all its energy to 1 specific electron, which could explain the photoelectric effect.

Question 76

What is the work function in the photoelectric effect?

Answer 76

Before an electron can leave the surface of the metal, it needs enough energy to break the bonds holding it there, which is called the work function energy

Question 77

What does the work function energy depend on?

Answer 77

It value depends on the metal

Question 78

During the photoelectric effect, what happens if the energy gained by the electron from the photon is greater than the work function?

Answer 78

The electron is emitted.

Question 79

During the photoelectric effect, what happens if the energy gained by the electron from the photon is less than the work function?

Answer 79

The metal will heat up but an electron will not be emitted.

Question 80

With regard to the photoelectric effect, what is meant by the threshold frequency?

Answer 80

The minimum frequency a photon can have and still cause a photoelectric to be emitted.

Question 81

How do you find the maximum kinetic energy of a photoelectron than has been emitted due to the photoelectric effect?

Answer 81

The kinetic energy the emitted photoelectron will be carrying when it leaves the metal is hf - any other energy losses. The minimum amount of energy an electron being emitted can lose is the work function energy so the maximum kinetic energy = hf - work function

Question 82

Why do the emitted photoelectrons in the photoelectric effect have a range of energies?

Answer 82

Electrons from deeper down in the metal lose more energy than the electrons on the surface, so the emitted photoelectrons have a range of energies even if all the incident photons have the same energy.

Question 83

During the photoelectric effect, how would you increase the number of photoelectrons emitted?

Answer 83

By increasing the intensity, this increases the number of photons hitting the metal so therefore increases the number of photoelectrons.

Question 84

During the photoelectric effect, when do the photoelectrons have a maximum kinetic energy?

Answer 84

Electrons have this energy when they are on the surface of the metal and the only energy lost is in escaping from the material (the work function)

Question 85

During the photoelectric effect, is the kinetic energy of the photoelectrons dependent or independent of the intensity?

Answer 85

The kinetic energy of the photoelectrons is independent of intensity, as they only absorb 1 photon at a time.

Question 86

Why was Einstein's work with his photon model so significant?

Answer 86

He demonstrated that light is a stream of particles called photons, and that photons are the smallest possible unit of electromagnetic radiation - a quantum

Question 87

How was Einstein rewards for his work towards the photon model?

Answer 87

As well as winning the Nobel Prize in 1921, Einstein's photon model opened up a whole new branch of physics called quantum theory.

Question 88

What is the definition of a black body?

Answer 88

A black body is a body that absorbs all electromagnetic radiation of all wavelengths and can emit all wavelengths of electromagnetic radiation.

Question 89

What is the appearance of a black body?

Answer 89

It is an object with a pure black surface that emits radiation strongly and in a well-defined way. It absorbs all light incident in it and doesn't reflect any light.

Question 90

What is black body radiation?

Answer 90

Because they emit all wavelengths of electromagnetic radiation, they emit a continuous spectrum of electromagnetic radiation.

Question 91

What is a black body curve?

Answer 91

A graph of radiation power output against wavelength for a black body varies with temperature, but they all have the same general shape.

Question 92

With black body curves, where does the peak of the graph go?

Answer 92

The peak of the graph towards towards the shorter wavelengths as the temperature of the black body increases.

Question 93

What did wave theory predict that a black body curve would look like?

Answer 93

Wave theory could explain the slope of the black body radiation curves at long wavelengths. Classical wave theory suggested that the power radiated was proportional to (lambda)-4, but this meant that the power output was predicted to head towards infinity in the ultraviolet region, which was impossible.

Question 94

What is the ultraviolet catastrophe?

Answer 94

Wave theory (then widely accepted) had predicted something that was impossible - the incorrect shape of a black body curve. It predicted hat the power output would head towards infinite but it reaches a maximum value and then decreases, to create the peak in the graph.

Question 95

How was the shape of black body curve explained?

Answer 95

It wasn't until Einstein built on Planck's interpretation of radiation in terms of quanta and came up with the photon model of light that physic was able to explain black body curves. Using the assumption that the object absorbed and radiated discrete packets of energy, he derived a formula that correctly matched the observed black body curve.