The
key points to remember are:
The electric field goes from positive to
negative.
The electron carries a charge of –1.6
´ 10-19 C.
The electron is attracted to the positive side,
so goes against the field.
The proton carries a charge of +1.6
´ 10-19 C.
The proton is repelled by the positive side, so
moves in the direction of the field.
Other charged particles have
whole
number multiples of 1.6 ´
10-19 C.
Electrons can be accelerated by an electric field. This happens in any cathode ray tube, e.g. a TV set, or a VDU. Electrons are “boiled off” a red hot cathode in a process called thermionic emission. They are then attracted by a positive electrode, the anode, at a high voltage. This makes them move forward at a high speed. The electrons move parallel to the direction of the field. Most hit the anode, but some fly through a hole at the front, to hit a screen at the far end of the tube. The screen is covered in phosphor, and the energy the electrons have is converted to light (and some X-rays). Because this arrangement spits out electrons, it is called an electron gun.
The
electrons are repelled by the cathode and accelerated by the anode. Each
electron is given energy, which can be found using energy = charge × voltage, E
= QV. The energy is entirely kinetic
energy, so we can say that:
QV = Ek = ½ mv2
Ž v2 = 2QV
m
There is no reason why positive charges cannot be accelerated in the same way. Indeed in a mass spectrometer, positive ions are accelerated by an electric field and bent by a magnetic field to hit a detector. Particle accelerators work in the same way.
Normally, atoms are in the ground state. When electrons hit atoms, the atoms get excited, which means that the atoms gain energy. The atoms want to lose energy, so they get rid of the energy in the form of a photon and return to the ground state. The maximum level that an atom can get is when it is ionised, i.e. and electron is knocked off. It will recapture an electron, and the energy will be given off as a photon.
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