Cathode Rays and Thermionic Emission Flashcards
(12 cards)
What is observed when a potential difference is applied across a discharge tube with low-pressure gas?
The tube glows, with the glow being brightest at the cathode, called the cathode ray.
What did scientists initially not understand about cathode rays?
They did not know what the rays were made of.
What did J.J. Thomson discover about cathode rays?
They have mass, they have a negative charge, they behave the same in all gases, and they have a very large charge-to-mass ratio.
What conclusion did scientists reach after Thomson’s work?
All atoms must contain cathode ray particles, which were renamed electrons.
What is the process by which the discharge tube glows?
High potential difference removes electrons from gas atoms → ions + free electrons. Positive gas ions accelerate towards the cathode, colliding with it and releasing more electrons. Free electrons accelerate and collide with gas atoms → excite atoms. Excited atoms de-excite, emitting photons of light.
Why is the glow brightest at the cathode?
Because this is where electrons and gas ions recombine, emitting light.
What is thermionic emission?
The process where electrons are emitted from a heated metal as they gain enough energy to escape the surface.
How does an electron gun use thermionic emission?
A cathode is heated → emits electrons. A potential difference accelerates electrons towards an anode with a small gap. Electrons that pass through form a narrow, high-speed beam.
What happens to the electrons’ energy as they accelerate toward the anode?
Their electrical potential energy is converted into kinetic energy, causing them to speed up.
What equation gives the work done on a charged particle in an electric field?
ΔW = QΔV
For an electron: W = eV
What is an electronvolt (eV)?
The kinetic energy gained by an electron when it is accelerated through a potential difference of 1 volt.
What is the relationship between kinetic energy and work done on the electron?
1/2 mv² = eV
