Turning points 1 Flashcards
What is a discharge tube
An evacuated sealed glass tube containing a cathode (negative) and anode (positive)
Why does a glow appear at the anode when a discharge tube is filled with a low pressure gas and a large pd is applied across it
- Gas is made up of free electrons and positive ions
- Pd causes free electrons to be accelerated towards anode (as electrons are -ve)
- When free electrons have reached anode they have gained enough KE to excite gas atoms
- De-excitation causes emission of visible (and UV) photons
How do positive ions in the tube make the glow at the anode to increase
- Pd causes them to be accelerated towards the cathode
- When the ions collide with the cathode, more free electrons are released from the cathode
- These additional electrons can be accelerated towards anode and cause a glow
Why does a smaller glow appear at the cathode when a discharge tube is filled with a low pressure gas and a large pd is applied across it
- Positive ions are collide with cathode causing free electrons to be released
- Free electrons and positive ions recombine causing visible light photons to be released
What were electrons causing the glow in the discharge tube know as at the time of the experiment
Cathode rays
How can a cathode rays be produced through thermionic emmision
- Heating coil heats the cathode until electrons gain enough energy to become free
- The electrons are accelerated from the cathode to the cyclical anode due to a pd applied by electron gun
How do the cathode rays produce a glow during thermionic emission
- Cylindrical anode has a small hole in it, causing a narrow electron beam to pass through
- They hit a phosphorus coating on the other end of the tube and excite and de excite atoms, causing visible photons released causing glow
How can you do calculations involving thermionic emission
As the electron is accelerated towards anode, electrical potential energy from pd is converted to KE
At the anode:
eV = 1/2 mv^2
How can you measure the specific charge of an electron
- Fire electrons through the tube with the electron gun
- There is a magnetic field acting perpendicular to tube causing electrons to travel with circular motion
- mv^2 / r = Bev
mv / r = Be
v = sqrt (2eV / m)
so m * sqrt (2eV / m) /r= Be
e/m = 2V / B^2r^2
Describe the apparatus used in Millikan’s experiment to determine the charge of an electron
- Atomiser to spray oil droplets which are charged due to friction
- 2 parallel plates with the top one having a small hole for oil droplets to fall through
- Microscope to view the oil droplets between the plates
- Variable pd supply to change pd between the plates
What are the forces acting on the oil droplet when there is no pd between the plates
- Droplet is falling due to weight (= mg)
- Resistive force due to viscous drag force of air (= 6𝜋ηrv , where η is viscosity of fluid, r is radius of droplet and v is velocity of droplet)
What are the forces acting on a stationary oil droplet when a pd is applied between the plates
- Electric force upwards = QV/d
- Weight downwards= mg
- adjust pd until electric force = weight so stationary droplet
- No viscous drag force as stationary
What was Millikan’s method for determining the charge of the electron
- Watch how oil droplets act between the plates through the microscope
- Adjust the pd until stationary droplet so QV/d = mg and solve for Q
- However, we don’t know mass of oil droplet so look at forces when no pd is being applied to find an equation for radius and sub 4/3𝜋r^3ρ in for m
How did Millikan find the radius of a droplet by looking at when there was no pd between the plates
- At terminal velocity mg = 6𝜋ηrv
so 4𝜋r^3ρg = 6𝜋ηrv
so r^2 = 9ηv / 2ρg
Using the scenarios for no pd applied and pd applied, how did Millikan determine the charge of the oil droplet
- r^2 = 9ηv / 2ρg so solve for r as η and ρ were known from other experiments and v = s/t
- QV / d = mg = 4/3 𝜋r^3ρg so solve for Q
Explain the results of Millikan’s oil drop experiment
- Charge of oil droplets was always a whole number multiple of 1.6 x 10^-19 i.e. quantised
- So 1 electron must have a this charge
