Transcript PPT - Physics 420 UBC Physics Demonstrations

Plasma
A Basic Introduction
&
Some Cool Stuff
Matthew Wan
PHYS 420
What Is It?
• A hot ionized gas
• The ‘fourth state of matter’
– Unlike gases, solids, or liquids, plasma does not
contain molecules
– Instead, it is a gas that is composed of ions
• Composes more than 99% of the known visible
universe
So What’s In It?
• Some, or all, of the electrons in the outer
orbitals have been stripped away
• The result is a collection of ions and
electrons, which are no longer bound
together
What Did That Mean?
• Because the particles are not neutral:
– Plasma behaves differently then regular
gases
– For instance, in the presence of
electromagnetic fields
Who Found It?
• First discovered by Sir William Crookes, in
1879
• But it wasn’t called ‘plasma’ until 1928,
when Irving Langmuir coined the term
Characteristics: Temperature
• Defines two kinds of plasma: Cold and Hot
• Refers to the electron temperature
• Ion temperature may be very different (lower)
Cold Plasma
• Isn’t really cold
– Typical electron temperatures for cold plasmas are in
the thousands of degrees
• Only a small fraction of the gas molecules are
ionized (degree of ionization)
– Usually on the order of 1%
• Often created using strong electric fields
Cool
Where Can I Find Cold Plasma?
• Fluorescent Lights
• Strobe Lights
• Experimental Fusion
Research Devices
Hot Plasma
• Really hot
– Like the Sun (15,000,000° C at the core)
• The molecules are nearly if not fully
ionized
• Created by heating the molecules to
extremely high temperatures
Scorching
Where Can I Find Hot Plasma?
• In Nature, actually
– The Sun and other stars
– Lightning
– The Aurora Borealis
Characteristics: Density
• Plasma (electron)
density
– The number of free
electrons per unit
volume
• Ion density
– Related to above by
the average charge
state:
ne
ne  Z ni
Density
• Neutral Density
– In hot plasmas, this
quantity is very small,
but may still determine
important physics
• The degree of
ionization is given by:
no
ni
no  ni 
Characteristics: Potentials
• Plasmas are excellent conductors
• Simple view:
– Due to the above, the electric fields in
plasmas tend to be very small
• Quasineutrality:
– On the one hand, we can assume that
densities of positive and negative charges are
equal
– However, we can assume that electric fields
exist as needed for the physics at hand
An Application:
Let’s Get Hands On (sort of)
• Plasma Globes
– An electrode sitting inside a vessel containing
some kind of inert gas
– The electrode is energized by a high-voltage,
high-frequency power supply
– This globe uses voltages around 10 000 volts,
and frequencies ranging from a few kilohertz
to a few 10’s of kilohertz
The Power Supply:
How It Works
• The IC sends out pulses which trigger a
transistor
• This causes a pulse of voltage to pass
through the coil
• This induces a large burst of voltage on
the secondary of the coil
Which Does?
• The large voltage produced by the coil can be harnessed
to produce a cold plasma
• The light bulb contains an inert gas that is relatively easy
to ionize
• With any gas, a small fraction of the particles are always
ionized
Cosmic
• Cosmic rays that are constantly
bombarding the Earth ionize some of the
particles
• The voltage and current from the power
supply accelerate these particles
• As they move around the vessel, they
ionize other particles
Exciting
• The other particles are excited into a higher energy state
• As they come back down to their ground state, they
release the extra energy:
– Light, in the form of photons
• Because particles have unique wavelengths, the colour
of the plasma depends on the gas inside the vessel
Pressure
• The gas vessel is at a reduced pressure
• As a result, the particles are spaced out
• This gives them time to accelerate
• If the particles were tightly packed, they
would not build up enough energy to
initiate a cascade effect
BE CAREFUL
• The currents and voltages in this project
are extremely dangerous
• Touching the wrong part could seriously
harm you
• I’ve already zapped myself. It hurt.