NCEA Physics Thermionic emission

Thermionic emission Aims: To understand that electrons are ‘boiled off’ hot metal filaments and this is called thermionic emission. To know that a beam of electrons is equivalent to an electric current and perform simple calculations involving the rate of flow of electrons and the current, given the electronic charge.

Thermionic emission Aims: To understand that an electron beam, or a stream of charged ink drops, can be deflected by the electric field between parallel charged metal plates. To understand the principles of a simple electron gun with a heated cathode and accelerating anode.

Thermionic emission In normal atoms orbiting electrons are held in place by the force of attraction of the positive nucleus. However when some metals are heated to high temperatures the energy gained by the outermost electrons is enough to break free of the atom. We say that the electrons have been ‘boiled off’ by thermionic emission.

Electron beams Electrons boiled off a hot metal filament by thermionic emission can be channelled into a thin beam. These beams of electrons are used in electron guns that can be found in televisions and computer monitors.

Beam deflection Here we can see that a beam of electrons is being repelled by the magnet above the tube. Moving electrons generate a magnetic field.

Picture of ink jet printer

Ink droplets Tiny droplets of ink can be easily charged. They are so small that when they fall through a large electric field they can be attracted or repelled. The electric field is usually made by two big metal plates one positive and one negative. Some ink jet printers use this movement of droplets by an electric field to place ink onto paper.

Electron guns Electrons emitted by thermionic emission can be used in a variety of ways. The emitted electrons can be attracted towards a positively charged plate called an anode. This sort of device, called an electron gun, can be used in televisions and computer monitors. Glass tube Electron gun Fluorescent coated screen

Electron guns When the electrons hit the flourescent coated screen they give off a flash of light. If there are a large enough number of flashes we can see a picture on the screen. Glass tube Electron gun Fluorescent coated screen

Controlling the gun While a high voltage is needed to accelerate the electrons a lower voltage can be used to control the path of the electrons. Magnets or plates can be used to deflect the electrons onto a screen coated with fluorescent paint.

Deflection tube

Heated cathodes emit electrons The first part of an electron gun is a small electric circuit that is capable of ‘boiling off’ electrons by thermionic emission. The voltage and current in this circuit do not necessarily have to be very large.

Cathodes are negative

Accelerated by an anode Electrons emitted by thermionic emission will soon jump back onto the atoms they came from. However a large positive voltage can be used to attract the free electrons away from the metal that they came from. This accelerating voltage needs to be very large often 100’s or even 1000’s of volts.

Anodes are positive

Deflecting the electrons A large pair of plates, one positive and one negative, can now be used to deflect the beam of electrons up and down.

Vacuum tube The electron beam is sealed inside a vacuum tube to stop ionisation and deflection of the beam by other particles.

Electronic charge Current = Charge / time I = Q / t The charge on a single electron is 1.6  10-19 Coulombs. Current = Charge / time I = Q / t Amps = Coulombs / seconds How many electrons flow to make 1A of current?

Coulombs One coulomb is equal to the number of electrons multiplied by the charge on each electron. Charge = Number of electrons × Electronic charge Charge = Number of electrons × ( 1.6 × 10-19 )

Charge example 1 How many electrons are in a charge of 2 C? Electrons = Charge  ( 6.25  1018 ) Electrons = 2  ( 6.25  1018 ) Electrons = 1.35  1019

Summary – Thermionic emission Electrons have negative charge and flow along wires in a current. Current = Electrons × Electronic charge / time Electrons can be ‘boiled off’ a hot metal filament in a process called thermionic emission. In an electron gun these electrons are attracted to an anode and can also be deflected onto a screen. In ink jet printers small drops of ink are charged and then deflected onto paper to form letters and shapes.