Turning Points
No description yet. This curated deck is ready for daily study.
Card preview (24)
What were discharge tubes used for?
To study the effects of applying a high voltage across gases at low pressure
What is observed when a high potential difference is applied between two electrodes in a discharge tube?
Light is emitted Colour of light is dependent on which gas is inside the tube
What is a positive electrode called?
anode
What is a negative electrode called?
Cathode
What was used to create a high potential difference between the electrodes of a discharge tube?
An induction coil
What are cathode rays?
Can cathode rays exist outside the tube?
Yes, and they were not in the form of electromagnetic waves
What happens when a high potential difference is created between electrodes?
A strong electric field is created. This ionises the gas atoms [pulls some of the electrons from atoms] creating positively charged ions
When does thermionic emission occur?
When you heat a cathode which frees electrons in order to create cathode rays
What is work function?
Min. energy required to remove free electrons from the surface of a metal
What happens when free electrons gain a thermal energy equal to or over their work function?
They can overcome attractive electrical force bonding them to positive ions of the metal and can thus escape from the metal surface
Why is thermionic emission beneficial?
• It can produce electrons without the need of a gas to be present
Why do we have an electron gun/beam inside an evacuated tube?
The electron beam has no atoms to collide with
What is a thermionic valve?
A device that heats a cathode to release electrons in a vacuum. As a result electrical signals can be controlled [which is why it was used to make early computers, and radios]
What is a Cathode Ray Tube (CRT)?
An evacuated tube which creates a beam of high-speed electrons from thermionic electrons [this was the basis of TVs, oscilloscopes and electron microscopes] [uses an electron gun]
What is specific charge?
Describe the fine beam tube method for determining specific charge
• Use an electron gun to fire a beam of electrons into a bulb containing gas at low pressure eg hydrogen • As electron pass through the gas, they collide with gas atoms and they absorb some energy and become excited • Gas atoms are immediately de-excited and go back to ground state thereby emitting photons • In a darkened room, this emission of photons allows us to see the path of the electrons
What are the Helmholtz coils and what is their purpose in the fine beam tube method?
They are two large circular current-carrying coils -They are positioned outside of the chamber (on either side) to generate a uniform magnetic field at right angles to the electron’s motion
What is the purpose of the magnetic force in the fine beam tube method?
Magnetic force acts as a centripetal force, continually deflecting the electron beam into a circular path [BUT it does no work on the electrons ; so it doesn’t change their speed or energy] [see page 8]
What was the purpose of an atomiser in Milikan’s experiment for determining charge?
Created oil droplets which were then charged by friction as they left the atomiser
How did Milikan determine the charge of an electron when the electric field is applied?
• Released atomisers where oil droplets were created and fell into a chamber between two metal plates • He then applied a voltage between the plates, creating a uniform magnetic field [so a a magnetic force is therefore exerted] • If oil droplet’s charge is negative electric force would act upwards [bottom plate therefore negative] • He varied the voltage until the droplets motion became stationary [therefore electric force and weight are balanced] • He then switched off the electric field [Next stages are on flashcard 23]
What happens to the electron in Millikan’s experiment if there is no electric field applied?
The falling oil droplet reaches terminal speed
How did Milikan determine the charge of an electron after he turned off the electric field?
[Atp he realised needed mass and radius] • The only forces acting on the electron is its weight and viscous drag • Milikan used stokes law to determine drag to determine radius [due to known values of density and viscousity of fluid] • With radius, he then found mass [coz he can now get volume and he already has density] • He then found charge
What was the significance of Milikan’s results?
That electric charge is quantised [always a multiple of 1.6x10^-19C] AND that charge did not exist in quantities smaller than that