7.3 Thermionic Emission Flashcards
What is thermionic emission?
The process when electrons gain enough energy through heating(thermal energy) to break free from the surface of the metal.
What does the thermal energy gained must be greater than?
Greater than the work function
What is the work function?
Minimum amount of energy required for an electron to leave a metal.
What can the process of thermionic emission be used for?
Can be used to create an electron gun.
What is an electron gun?
When the electrons released will be accelerated due to an electric field and leave through the slit, creating a beam?
How does an electron gun work?
Electrons are emitted and accelerated towards a positively charged plate (anode) by an electric field set by a high pd. Some electrons pass through the slit in the positive plate making a narrow beam.
Electrons released by an electron gun move at what velocity and direction. Why?
Constant velocity and in a straight line.This is because after passing the positive plate the electrons feel no force due to the electric field.
What can be used to change the direction of the electrons?
A magnetic field
The two plates are said to be electrodes. What are electrodes?
An electrical conductor that makes contact with non metallic part of circuit.
What are the two electrodes?
cathode and anode
What is a cathode? What does it act like in this gun?
a negative electrode ,acts as a source of electrons
What is an anode? What does it act as in this gun?
a positive electrode, attracts and therefore accelerates electrons
What are electrons guns used to create ?
electron beams
Examples of where electron guns are used?
electron microscopes - along with magnetic fields to focus on sample
cathode ray oscilloscopes
How can an electron beam be made visible?
When the beam is placed in tube with low pressure gas. The electrons ionise the atoms through collisions and when electrons return to ground state photons are emitted.
