Turning points

• What happens when a potential difference is applied across a discharge tube with low pressure gas?

  The tube glows, brightest at the cathode
• What did Thomson demonstrate about cathode rays?

  Cathode rays have mass and negative charge
• What is the charge to mass ratio of cathode rays?

  Very large charge to mass ratio
• What conclusion was drawn about atoms after studying cathode rays?

  All atoms contain electrons
• What is the process by which the discharge tube begins to glow?

  1. High potential difference pulls electrons off gas atoms
  2. Positive gas ions accelerate towards the cathode
  3. Electrons collide with gas atoms, causing excitation
  4. Excited atoms release photons of light
• What is thermionic emission?

  Emission of electrons from heated metal
• How do electron guns work?

  They use potential difference to accelerate electrons
• What is the formula for work done on a charged particle in an electric field?

  W = QΔV
• What is the kinetic energy of an electron accelerated through a potential difference V?

  Kinetic energy equals eV
• What is the specific charge of an electron?

  Charge to mass ratio of an electron
• How can the specific charge of an electron be determined?

  1. Using a fine beam tube with low pressure gas
  2. Using Thomson's crossed fields apparatus
• What happens to electrons in a fine beam tube?

  They move in a circular path due to magnetic force
• What is the equation relating specific charge to other variables in a fine beam tube?

  \( \frac{e}{m_e} = \frac{2V}{B^2r^2} \)
• What did Thomson's determination of specific charge demonstrate?

  Specific charge is constant for all gases
• What model of the atom did Thomson propose?

  Plum pudding model
• How does the specific charge of an electron compare to that of a proton?

  It is about 1800 times larger
• What was the principle of Millikan's oil drop experiment?

  1. Oil droplets are negatively charged
  2. Droplets fall between parallel plates in an electric field
  3. Adjust potential difference until droplets are stationary
  4. Calculate charge using weight and electric force

  
• What does Millikan's experiment show about charge?

  Charge is quantized in discrete packets
• What is the significance of the charge measured in Millikan's experiment?

  It is the charge of an electron
• What is Newton's corpuscular theory of light?

  Light consists of tiny particles called corpuscles
• How did Newton explain reflection and refraction of light?

  • Reflection: Corpuscles collide and change direction
  • Refraction: Corpuscles speed up in denser mediums
• Why was Huygens' wave theory proposed?

  To explain diffraction of light
• What did Young's double slit experiment demonstrate?

  Interference and diffraction of light

  
• What occurs at light fringes in Young's experiment?

  Light meets in phase and interferes constructively
• What occurs at dark fringes in Young's experiment?

  Light meets out of phase and interferes destructively
• How did Young's experiment challenge Newton's theory of light?

  It showed light has wave properties, not particles
• What are electromagnetic waves composed of?

  Alternating electric and magnetic fields
• What is the formula for the speed of electromagnetic waves in a vacuum?

  c = \( \frac{1}{\sqrt{\mu_0 \epsilon_0}} \)
• What did Hertz discover about radio waves?

  They are produced by high voltage sparks
• How can radio waves be detected?

  • Using a dipole receiver for electric fields
  • Using a loop of wire for magnetic fields
• What happens when a metal sheet is placed in front of the radio wave apparatus?

  Radio waves are reflected back, forming stationary waves
• What is the relationship between the direction of wave travel and oscillations in electromagnetic waves?

  Wave travel is perpendicular to oscillations
• What is the significance of the speed of light measured in water?

  It contradicted Newton's corpuscular theory
• How did the scientific community react to Huygens' wave theory after Young's experiment?

  It gained acceptance after disproving Newton's theory
• Who discovered radio waves?

  Hertz
• What apparatus did Hertz use to discover radio waves?

  High voltage sparks apparatus
• What are the two types of detectors used for radio waves?

  • Dipole receiver: detects electric field
  • Loop of wire with a gap: detects magnetic field
• How does a dipole receiver detect radio waves?

  It detects the waves' electric field
• What does a loop of wire with a gap detect?

  Alternating magnetic field of waves
• What happens when a metal sheet is placed in front of Hertz's apparatus?

  Radio waves reflect back, forming stationary waves