Turning Points Flashcards
(37 cards)
How was the electron discovered?
Cathode rays: gases at low pressure in discharge tubes conduct electricity and emit light of a certain colour. Charged particles move through the gas when it conducts electricity.
Who discovered the electron? What did he prove?
J.J. Thompson proved that cathode rays have energy momentum and mass; have negative charge and have the same properties regardless of gas used.
Describe the process of emission of light from a discharge tube.
If a high voltage is applied to the tube some gas atoms are ionised. Positive ions are attracted to the cathode surface causing free electrons to be emitted.
Glowing gas near the cathode are because of the photons emitted when the positive ions and electrons recombine.
Some electrons move towards the anode and the glowing gas there is due to the deexcitation of the gas atoms.
What is thermionic emission?
When a metal is heated some free electrons gain enough energy to leave the metal surface. The electrons can be accelerated through an electric field.
How is an electron beam deflected?
By an electric or magnetic field. The force on each electron in the electric field is constant. The field is uniform so the electron curves in a parabolic path.
By using a uniform magnetic field. Electrons experience force because the magnetic field is perpendicular to the direction of motion. No work is done by the magnetic field so speed is constant and electron moves in circular path.
Deflection can be cancelled out by applying magnetic and electric fields.
How is the specific charge of an electron worked out using a fine beam tube?
It makes the beam visible and collisions between some electrons in beam and a small amount of gas. If the magnetic field is strong enough the beam will be a complete circle.
What was the significance of thompsons discovery?
He showed that the electron specific charge was 1860 times larger than hydrogen was.
How was the charge of an electron discovered?
Milikan’s oil drop experiment. He controlled the motion of the droplet using the electric field between charges plates. He made the charged droplet stationary by adjusting the pd between the plates until the electric force was equal and opposite to the weight of the droplet.
What was the significance of Milikan’s experiment?
He measured the mass of each droplet by measuring the terminal speed. He found the charge was always a multiple of 1.6x10^-19.
Stokes law was developed.
Describe Newton’s corpuscular theory of light.
Light was made up of tiny particles (corpuscles).
Reflection- corpuscles bounce off mirror without loss of speed and the force pushed the particles away from the surface.
Refraction- when light passes through a transparent substance from air the corpuscles are attracted to the substance so they move faster.
Explain Hygen’s wave theory of light.
Assumed light waves travel slower in a transparent substance than air.
Light should diffract around tiny objects and 2 coherent light sources should interfere with each other.
Why did scientists prefer Newtons theory?
Newton had a stronger scientific reputation.
The wave theory was considered in terms of longitudinal waves so couldn’t explain polarisation of light.
It wasn’t possible to measure the speed of light.
What is the significance of youngs double slit experiment?
Shows interference- a wave property and challengers corpuscular theory.
Produced an interference pattern of equally spaces light and dark fringes.
Corpuscular theory couldn’t explain fringes but wave theory could (newton only predicted 2 bright fringed).
Later realised light was a transverse wave explaining polarisation.
How were electromagnetic waves discovered?
Maxwells theory of electromagnetic waves showed waves are transverse and electric waves are in phase with and perpendicular to magnetic waves.
An alternating current in the wire creates alternating electric and magnetic fields that radiate from the wire.
Showed the speed of electromagnetic waves in free space.
What did Maxwells theory show?
Light consists of electromagnetic waves and predicted electromagnetic waves outside the known spectrum.
How did Hertz discover radio waves?
Radio waves produced by the spark gap transmitter spread out from the spark gap and pass through the detector loop. The waves passing through the detector loop cause voltage to be induced in the detector loop which makes sparks jump across the detector gap.
Radio waves do not pass through they reflect.
Insulators do not stop radio waves.
How do you measure the wave length and speed of radio waves?
Hertz produced stationary radio waves by using a flat metal sheet to reflect the waves back towards the transmitter.
He measured the wavelength and calculated frequency of the waves using charge and capacitance.
Using radio waves of know frequency he calculated the speed.
Concluded radio waves travel at the same speed as light and that they are electromagnetic waves.
How was the transverse nature of radio waves discovered?
Hertz discovered that when a reflector and dipole detector were parallel to the transmitter a Steph signal was observed.
The detector signal decreased and became zero at 90•.
Conclude radio waves were polarised and signal was zero because dipole was perpendicular to the plane of polarisation.
What did Fizeu discover and how?
He determined the speed of light.
Passed a beam of light between 2 cog teeth to a reflector. The cog was rotated at exactly the right speed so that the reflected beam passed through the next gap in the cog teeth. He used the frequency of rotation and the number of gaps to calculate the time taken for the light to travel to the reflector and back.
What is the ultraviolet catastrophe?
All glowing objects emit electromagnetic radiation due to their temperature. The radiotions consists of a continuous range of wavelengths. A black body is a body that absorbs and emits all wavelengths of electromagnetic radiation.
Each black body curve has an intensity peak at a particular wavelength that depends on the temperature which couldn’t be explained by the wave theory. Wave theory suggests the intensity becomes infinite at smaller wavelengths- ultraviolet catastrophe.
Planck used his quantum theory to explain black body curves.
Describe the photoelectric effect.
Metals emit electrons when supplied with sufficient energy illuminating the metal with light. Threshold frequency- minimum frequency of light needed to emit electrons. Photoelectrons have a range of kinetic energies from zero up to a maximum value. The number of photoelectrons emitted is proportional to intensity.
The wave theory couldn’t explain this- light of any frequency should cause emission. It couldn’t explain threshold frequency, instant emission and max kinetic energy.
What was Einsteins photon theory?
Electromagnetic radiation consists of wave packets of energy he referred to as photons. The work function is the minimum amount of energy needed to emit an electron. In order for an electron to escape it must absorb a single photon and use energy equal to the work function. Stopping potential is energy needed to stop fastest moving electrons.
What is the significance of the photon theory?
Shows the wave packet nature.
Particle like- photoelectric effect
Wave like- diffraction, interference
What did de Broglie do?
He said all particles have a wave like nature and remained a hypothesis until a beam was diffracted when passed through a thin metal foil.
Photos of the diffraction patterns were similar to using X-rays and confirmed his hypothesis.
