Option D: Turning points Flashcards
When was the cathode ray first used?
1876
How were cathode rays used in 1876?
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.
What is the set-up of a cathode ray discharge tube?
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
In 1897, who ended the debate of what cathode rays were made of?
J.J.Thomson
In 1897, what did J.J.Thomson demonstrate about cathode rays?
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.
What did Thomson conclude from his discovery of cathode rays?
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.
What is thermionic emission?
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.
How can emitted electrons be accelerated by an electric field?
in an electron gun
How do electron guns work?
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 Electrons are tiny compared to the smallest atom, so they are easily stopped or deflected by atoms. So 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.
4 Electrons guns are combined with fluorescent screens in cathode ray tubes (CRTs). The electron beam is directed at the screen, causing it to emit light and produce a picture on the screen.
Where are cathode ray tubes used?
Old-fashioned TV screens and computer monitors
What is the equation for work done in moving a charge?
work done in moving a charge = the charge being moved in C x electric potential difference that the charge is moved through in V
How do you apply the equation for work done in moving a charge to the situation using cathode rays?
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
When accelerating an electron through a p.d., how are work done and kinetic energy linked?
The kinetic energy that the electron will have as it leave the anode (through the hole) 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.
What is the definition of the electronvolt?
1 electronvolt is the kinetic energy carried by an electron after is has been accelerated from rest through a potential difference of 1 volt.
What is specific charge?
The charge-to-mass ratio of a charged particle. The charge per unit mass
Who used the specific charge of an electron equation to prove that subatomic particle exist?
Thomson
When did Thomson prove that subatomic particles exist?
1897
What is the experiment to find the specific charge of an electron?
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)
What was the significance of Thomson’s findings about the specific charge of an electron?
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.
What is Stokes’ Law and the theory behind it?
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.
How can you calculate the viscous force on a spherical object?
Stokes’ Law
viscous force = 6 x viscosity of the fluid x radius of the object x velocity of the object
What did Millikan’s Oil-drop experiment find?
The charge of an electron
What was the set-up for Millikan’s oil-drop experiment?
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.
During Millikan’s Oil-drop experiment, what are the forces acting on an oil drop with no electric field between the plates?
1 the weight of the drop acting downwards
2 the viscous force from the air acting upwards
During Millikan’s Oil-drop experiment, what are the forces acting on an oil drop with an electric field between the plates?
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
What was the significance of Millikan’s oil-drop experiment?
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).
What does it mean by the quantisation of electric charge?
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.
What did Newton publish in 1671?
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’.
What was Newton’s corpuscular theory?
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.
Newton believed that reflection was due to a force that pushed the particles away from the surface - just like a ball bouncing back off a wall. He though refraction occurred because the corpuscles travelled faster in a denser medium like glass.
What is Huygens’ principle?
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.
How did Huygens apply his principle to explain reflection and refraction?
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.
At the time, why was Newton’s corpuscular theory of light preferred over Huygens’s principle? (4 reasons)
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.
What is double refraction?
A polarisation effect, where shining light through certain crystals make two images instead of one
What was the significance of Young’s double-slit experiment?
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.
How did Young solve the problem of needing coherent sources to produce a clear interference pattern?
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.
What did Young’s double-slit experiment prove about Newton’s corpuscular theory and Huygens’s wave theory?
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.
Following the proof from Young’s double-slit experiment, why was Huygens’s theory still not accepted? (3 reasons)
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.
In the second half of the 19th century, what did James Clerk Maxwell model and predict?
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.
