Turning Points: Electrons

Question 1

Describe the process that releases the electrons emitted at the filament.

Answer 1

→ filament is heated due to current through it
→ e- gain enough energy to overcome the work function
→ e- leave the metal (thermionic emission)

Question 2

State the equation that relates terminal velocity and radius of a droplet falling through a substance.

Answer 2

4/3 πr^3ρg = 6πrnv

Question 3

State how Thomson’s result for specific charge of each particle within a cathode ray compared with that for the hydrogen ion and the conclusion.

Answer 3

→ specific charge of cathode rays was greater than the
hydrogen ion
→ provided evidence that cathode rays were composed of electrons which have a small mass

Question 4

Explain how cathode rays are produced in a gas discharge tube.

Answer 4

→ high PD pulls e- off of gas atoms, ionising them
→ positive gas ions are accelerated towards cathode causing e- to be emitted
→ electrons are accelerated to anode due to high voltage
( e- also collide with gas atoms, exciting them, de-excitation releases photons hence the glow)

Question 5

Describe the motion of electrons in a fine beam tube and the forces acting on them.

Answer 5

→ enter fine beam tube perpendicular to field
→ magnetic force acts perpendicular to velocity
→ this is a centripetal force therefore e- move in a circular path

Question 6

Describe the motion and the forces that act on electrons in an apparatus consisting of an electric field and a magnetic field.

Answer 6

→ e- accelerated perpendicular to both fields
→ deflected upwards by electric field (towards +ive)
→ deflected downwards by magnetic field
→ when undeflected (straight path):
magnetic force = electric force

Question 7

State and explain the effect of increasing voltage on diameter of the rings.

Answer 7

→ increasing pd increases speed electrons
→ decreases de Broglie wavelength
→ diffracted less so rings become smaller

Question 8

State and explain the effect of increasing voltage on diameter of the rings.

Answer 8

→ increasing pd increases speed electrons
→ decreases de Broglie wavelength
→ diffracted less so rings become smaller

Question 9

Explain the purpose of the two different voltages in an electron diffraction apparatus.

Answer 9

→ one supplies current to heat filament wire
→ the other produces accelerating electric field to accelerate electrons

Question 10

State what the de Broglie hypothesis states.

Answer 10

• wavelength will decrease if momentum increases