Discovery of the Electron Flashcards
What did Crookes discover?
That gases at a sufficient low temperature will conduct electricity and glow with a specific colour.
What does the magnet do in a Crookes discharge tube?
Deflects the positive column
How much P.D is applied in cathode rays?
Enough to produce a field that ionises atoms.
What do electrons do in cathode rays?
Accelerated towards the anode, exciting gas molecules and causing the glow.
What do positive ions do in a cathode ray?
Move towards the cathode, recombining to give off a negative glow.
How is an electron beam produced in thermionic emission?
Heating a cathode (via a filament);
Accelerating the beam using a high p.d anode
Evacuating the glass tube so there are no collisions between electrons and gas molecules.
What is the k.e of an electron accelerated through a p.d V?
eV = 1/2 m v^2
What did Millikan determine electronic charge e to be?
1.6 x 10^-19
What is special about electric charge?
Is quantised, so only exists in integer values of e. Suggests existence of electron.
What are the equations showing an oil drop in stasis?
F(e) = QE = QV/d = mg
What is stokes’ law, and what are the symbols?
Fd = 6 (pi) (eta) rv, r is radius, v is velocity, eta is viscosity of air.
What is m for a spherical droplet?
m = 4/3 rho pi r^3, rho is density
What is the weight of a spherical droplet?
mg, so 4/3 rho pi g r^3
When does an oil drop fall at a steady speed?
With no electric field, as mg is cancelled by an upward viscous drag Fd.
What is the radius of an oil drop, using Stokes’ law?
6 (pi) (eta) r v = 4/3 (pi) (rho) g r^3
so
r^2 = 9(eta v) / 2(rho g)
What did Thompson find?
That e/m for the electron was much larger than for that of a proton (hydrogen ion).
What did Thompson deduce from his e/m values?
That electrons were much much less massive than protons.
How does deflection by a magnetic field for a cathode ray work?
Am electron beam moves in a circle, by a magnetic field out of the diagram - no work is done as force is perp to the velocity, forms circular motion.
Do electric fields have effects on cathode rays?
No, discovered by Hertz
What equations are linked to mag. field deflection for cathode rays?
F = Bqv = Bev.
F = mv^2 / r
What do you get from combining the equations in mag. field deflection for cathode rays?
Bev = mv^2 / r
v^2 = (ber/m)^2, so
eV = 1/2 m(ber/m)^2
e/m = 2v/b^2r^2
What should you measure to find e/m for mag. field deflection for cathode rays?
Anode V
Mag. B
Radius of circle r
How does deflection by a magnetic AND electric field for a cathode ray work?
Electron beam is undeflected, as mag. and electric fields will cancel. Mag field into page, positive plate of e. field at top.
What are the equations for deflection by an electric AND magnetic field for a cathode ray beam?
Fmag = Bev
Felec = ev
E = Vplates/d (plate separation)
v = E/B = Vplates/Bd
What do you get from combining the equations for deflection by an electric AND magnetic field?
F = mv^2 / r = Bev
mv/r = Be so
e/m = v/Br
What do you have to measure to find the e/m for an electric AND magnetic field?
Vplates
d, plate separation
B, mag field
r
How does deflection by an electric field in a cathode ray work?
Electrons deflected, due to a vertical force. V should be separately measured when beam is not bent. Positive plate at top when electrons coming from left.
What are the equations for deflecting a beam of electrons by an electric field?
Felec = eV
E = Vplates / d (plate separation)
a = F/m = eVplates/md
How do you find a using equations for the deflection of a beam of electrons by an electric field?
deflection y = 1/2at^2, a = 2y/t^2
t = L/v
How do you find v using equations for the deflection of a beam of electrons by an electric field?
v = E/B = vplates/Bd
also e/m = da/Vplates
What do you have to measure to find the e/m for an electric field deflection?
d
Vplates
y
L
B
