topic 8 Flashcards
1
Q
nucleon number
A
total number of protons and neutrons in a nucleus
2
Q
evidence of nuclear model - alpha scattering
A
- most of the alpha particles went straight through; atom is mostly empty space
- few were deflected; due to central charge
- very few were deflected at an angle greater than 90; most mass concentrated at centre
3
Q
why was a lead block used in the experiment
A
to keep alpha rays in (because alpha particles get absorbed in air)
4
Q
process of thermionic emmision
A
- when a metal is heated, free electrons gain thermal energy
- this gives them enough energy to emit from the surface
5
Q
where are electrons accelerated by electric and magnetic fields
A
in an electron gun
6
Q
how are electrons accelerated by electric and magnetic fields
A
- heating coil heats the metal cathode
- electrons emitted accelerate towards anode by the electric field
- some electrons pass through a little hole in the anode, making a narrow electron beam
- electrons in the beam move at a constant velocity because there’s no field beyond the anode
7
Q
how are particles accelerated in a linac
A
- a long straight tube containing a series of electrodes
- high-frequency alternating current is applied to the electrodes so the charge continuously changes between + & -
- alternating current is timed so that the particles are always attracted to the next electrode and repelled by the previous one
- particles speed increases each time it passes an electrode
- high-energy particles leaving linac collide with fixed target at the end of the tube
8
Q
why is there a vacuum in linear accelerators
A
- to avoid collisions with air molecules which would scatter the electron beam
9
Q
where are the electric fields in a linac
A
- there is a p.d. in the gaps between the tubes - this is where the electric fields are
10
Q
why are the tubes called drift tubes
A
- particles don’t accelerate within the tubes; they drift
- only accelerate within fields
11
Q
how are particles accelerated in a cyclotron
A
- two semicircular electrodes used to accelerate protons across a gap
- cyclotron takes up much less room than a linacs
- alternating p.d. is applied between the electrodes
- as particles are attracted from one side to the other their energy increases
- magnetic field is used to keep the particles moving in a circular motion
- electric and magnetic fields make particles spiral outwards as energy increases
12
Q
what is an electron-volt
A
1 electron-volt is the kinetic energy carried by an electron after
it has been accelerated through a potential difference of 1 volt
13
Q
A
14
Q
A
15
Q
A
