Specific Charge of Electron

Question 1

Glow in a discharge tube (Explain glow)

Answer 1

• Naturally occuring free electrons in the gas accelerate towards positive anode.
• The electrons then collide with gas atoms and ionise them, creating positive ions and more free electrons.
• It is the free electrons that are responsible for the glow in the tube (in the three areas)

Another card for Areas of glow &raquo_space;

Question 2

Negative Glow?
Positive Column?
Glowing Glass?

What are they?

Answer 2

• Negative glow: Pos Ions close to neg cathode are attracted to it and acc towards it. Some ions collide with cathode, causing more free electrons to release from cathode. Electrons acc towards anode and some collide with pos ions near cathode. They combine and emit photons.
• Positive Columns: as free electrons acc towards the anode some electrons collide with gas atoms and excite them. The excited gas atom de-excites and emits photons.
• Glowing Glass: Some of the electrons accelerating towards the anode are going so fast, even though they are attracted to it they miss it. They instead collide with the end of the glass tube and excite molecules in the glass, the glass glows as the molecules de-excite and emit photons.

acc = accelerate

Question 3

Thermionic Emission

Explain it

Answer 3

Giving off charged Particles when you heat something.

• When you heat a metal, its free electrons gain a load of kinetic energy.
• When enough energy is given them they will break free from the surface of the metal - this is called thermionic emission.
• Once the electrons have been emitted, the electrons can be accelerated by an electric field in an electron gun.

Question 4

Electron Gun

Explain it

Answer 4

• Heating coil heats metal cathode
• electrons emitted accelerate to cyclindrical anode (by electric field, set up by High V)
• Some electrons pass through small hole in anode, making narrow beam of electrons.
• Electrons (in beam) have constant v due to no field beyond anode ( i.e no force)

Question 5

Measuring specific charge of electron

explain

Answer 5

• Electron from electron gun is fired through the fine beam tube.
• They accelerate through the low pressure gas and pass hydrogen atoms that aborb energy, as the electrons in these excited hydrogen atoms fall back down to ground state and de-excite. They emit photons, leaving a glowing trace of the light beam in the tube.
• There are two coils creating a uniform magnetic field.
• The beam of electrons is initally fired at right angles to the magnetic field so that the beam curves round in a circular motion.
• This means that the magnetic force is acting on the electron as a centripetal force.

Radius given by : m_ev² / r = Bev
^This equation can be rearanged for v and subbed into 1/2m_ev²=eV

to get: e/m_e = 2V / B²r² , V is accelerating potential.