Paper 3 - Option Module - Turning Points - 3.12.1 The discovery of the electron Flashcards
3.12.1 The Discovery of the electron
3.12.1.1 - Cathode Rays
Sketch a discharge tube.
In page 8 of notes
What are cathode rays?
Beams of electrons.
What could cathode rays do to the paddle wheel placed in the middle of the tube?
Radiation from the direction of cathode will rotate the wheel due to energy transfer.
Explain how electrons being accelerated towards the anode can cause the formation of charged particles in the discharge tube.
- Electrons accelerated towards anode.
- Collisions with gas atoms occurs.
- Ionisation (charged particles) form.
Explain how a high p.d. applied across the discharge tube can cause the formation of charged particles in the tube.
- High p.d. = strong uniform electric field.
- Electrons are pulled out of gas atoms.
- This forms ions = charged particles.
How can light be produced in the discharge tube when electrons from the cathode are accelerated by a high p.d.?
- electrons are accelerated towards anode.
- Gas atoms are ionised by collision.
- negative glowing gas occurs due to photon emission.
- photon emission due to gas atoms recombining in an excited state.
How does a positive column of glowing gas light produced in a discharge tube occur?
Some electrons move towards anode.
Excitation occurs by collision.
Dexcitation occurs due to emission of photon from excited gas atoms.
3.12.1.2 - Thermionic emission of electrons
Describe thermionic emission.
- Cathode metal surface is heated.
- Produces heated cathode or heated filament.
- electrons in heated cathode gain energy in KE store.
- This energy is enough to cause emission and move to anode.
Using the work done on a charged particle formula, how can the speed of an electron accelerated via a p.d. of V be found?
Set Ek = Work done formula.
Rearrange whole formula for speed, v.
3.12.1.3 - Specific charge of the electron
What electron beams be deflected by?
electric fields and magnetic fields.
How can the amount of deflection of the electron beam be altered?
By adjusting electric and magnetic field strengths.
Electrons can be deflected by a uniform electric field.
Initially the electrons are travelling perpendicular to the direction of the field. Sketch the set-up of this.
In page 9 of notes.
Electrons can be deflected by a uniform electric field.
Initially the electrons are travelling perpendicular to the direction of the field.
What quantities can be used to calculate the amount of deflection of the electron beam?
- acceleration of each electron to the plate (during deflection) = a.
- time taken by each electron to cross the field, t.
- Deflection of the electron (in metres, m).
Electrons can be deflected by a uniform electric field.
Initially the electrons are travelling perpendicular to the direction of the field.
What quantities can be used to calculate the amount of deflection of the electron beam?
Now, what are the equations to calculate each of those quantities?
- a = eV/md for acceleration.
(e = charge, V = p.d. between plates, m = mass, d = distance of separation between plates). - t = L/v.
(t = time, L= length of plate, v = horizontal component of velocity). - y = 1/2 x a x t^2 (y = amount of deflection, in m).
Electrons can be deflected by a uniform magnetic field.
Describe the path that is taken by the electrons on the screen.
- Magnetic force acts perpendicular to motion of the electrons.
- Each electron undergoes circular motion.
- The beam traces a circular arc.
Electrons can be deflected by a uniform magnetic field.
They undergo circular motion.
Which equations can be used to calculate amount of deflection?
- Centripetal acceleration equation(s).
- Centripetal force = magnetic force (set Bev = mv^2/r).
- radius of beam curvature -> r = mv/Be
- Force on each electron, F = Bev.
What will happen to path of the electrons if the electric force and magnetic force are equal and opposite?
Sketch the set-up diagram of this.
The electrons pass through undeflected.
Diagram of set-up in page 10 of notes.
If the electric force and magnetic force on the electron beam are equal and opposite, what equation can we use for when these forces are balanced?
Bev = eV/d.
or Bev = eE (as E = electric field strength)
(magnetic force on electron = electric force on electron)
If the electric force and magnetic force on the electron beam are equal and opposite, what equation can we use for when these forces are balanced to calculate the speed of the undeflected electron beam?
speed, v = V/Bd.
V = p.d.
B = magnetic flux density (in T).
d = distance of separation between plates.
How can we determine the specific charge of the electrons if we switch of the magnetic field?
Apply the equations:
t = L/v.
y = 1/2 x a x t^2.
a = 2y/t^2.
Then for e/m = ad/V