3.12 - Turning Points in Physics Flashcards
How are electron beams formed in a discharge tube?
Cathode connected to -ve of a DC power supply and anode connected to +ve. A low pressure gas is inside this tube. The high potential difference applied pulls electrons from the gas atoms, forming ion and electron pairs. Positive gas ions are accelerated towards the cathode, releasing even more electrons. These electrons accelerate along the tube, colliding with gas ions, where they become excited. They quickly de-excite, releasing photons of light. The brightest glow is at the cathode, where the gas ions and electrons recombine and emit light photons.
What did the production of cathode rays show?
mass, negative charge, same properties no matter what gas was used, very large charge to mass ratio
What is the work done on an electron accelerated through a potential difference?
W=eV
How to electron guns work?
Potential difference accelerates elecrons released from the hot cathode (thermionic emission). They accelerate towards the anode, which has a small gap. Electrons pass through this, forming a narrow electron beam travelling beyond the anode.
What happens to electrons when accelerated through a pd and what equation does this lead to?
the kinetic energy is equal to the work done on the electron by the electric field
1/2mv^2=eV
How does Tomson’s crossed fields determine the specific charge of an electron?
Magnetic and electric fields perpendicular to one another, deflecting electrons in opposite directions. Electrons accelerated using e- gun, entering apparatus perpendicular to both fields. electric field deflects e- up, and magnetic field deflects e- down. For the e- beam to pass straight through, electric force = - magnetic force.
What is the significance of determining the electron SC?
Showed that elctron SC is constant
Beginning of atomic physics: showing electrons have mass, energy, momentum
E/Me was 1800x hydrogen (proton) SC; either electron mass is much smaller or charge is much larger; 1899: estimated e around 10^-19 C, so same charge as proton but much smaller mass.
