Turning Points

• When a potential difference is applied across a discharge tube with a low pressure gas inside of it, the tube will begin to glow with it glowing brightest at the cathode. This glow was called the cathode ray, and scientists were unsure as to what it was made up of until Thomson showed that cathode rays:

  ● Have a mass, which he measured.

  ● Have a negative charge.

  ● Have the same properties no matter what gas is used in the discharge tube.

  ● Have a very large charge to mass ratio.
• The process by which the discharge tube begins to glow is outlined below:

  1. The high potential difference across the discharge tube will pull electrons off the gas atoms, forming ion and electron pairs.
  2. The positive gas ions are accelerated towards the cathode and when they collide with it they release even more electrons.
  3. The electrons are accelerated along the tube (because the gas is at low pressure, the electrons are accelerated to high speeds) and collide with gas atoms causing them to become excited. The atoms will quickly de-excite and release photons of light.
• The glow is brightest at the cathode because here the gas ions and electrons can recombine and emit photons of light.
• Thermionic emission is where a metal is heated until the free electrons on its surface gain enough energy and are emitted.
• Electron guns use a potential difference in order to accelerate electrons, which are released from the cathode by heating it (thermionic emission). The electrons are accelerated towards the anode, which has a small gap, the electrons which pass through this gap form a narrow electron beam which travels at a constant velocity beyond the anode.
• The work done on a charged particle in an electric field is given by: Work done = Charge of an electron multiplied by the potential difference
• 1 eV is equal to the kinetic energy of an electron accelerated across a potential difference of 1 V.
• As the electron moves from the cathode towards the anode, its electrical potential energy is converted into kinetic energy and so the electron speeds up. Once the electron reaches the anode, its kinetic energy will be equal to the work done on the electron by the electric field.
• Fine beam tube - this piece of apparatus contains a low pressure gas and has a uniform magnetic field passing through.
• 1. Electrons are accelerated using an electron gun and enter the fine beam tube perpendicular to the direction of the field.
  2. The magnetic force on the electrons acts perpendicular to their motion, and therefore the electrons move in a circular path because the magnetic force acts as a centripetal force.
  3. As the electrons move through the fine beam tube, they collide with gas atoms causing them to become excited, the gas atoms then de-excite releasing photons of light meaning the path of the electrons is visible, so the radius of their circular path can be measured.
• Thomson’s determination of specific charge of the electron was significant because it showed that the specific charge was constant whatever gas was used to produce the electrons (cathode rays), demonstrating that all atoms contain electrons.
• The specific charge of the electron is around Ckg-1 .76 01 × 1 , whereas the specific charge of a 11 hydrogen ion (proton) is Ckg-1 .58 09 × 1 , meaning that the specific charge of an electron is around 1800 times larger than that of a proton.
• Millikan's oil drop experiment was formed in order to calculate the charge of an electron
• An atomizer is used to spray tiny droplets of oil, which are negatively charged due to friction.