Magnets and TVs

21 October 2007

Ingredients

An old fashioned CRT TV or Monitor (one of the ones that are almost as deep as they are wide) One that you are not overly attached to as there is a possibility this may permanently damage the picture.

A magnet, the stronger the better the effect.

Instructions

Turn on the TV or monitor and tune it into a picture.

If you can lift the TV comfortably turn it upside down very carefully, and see if it has any effect on the picture.

The take your magnet and move it close to the TV, move it around, what effect does it have.

Warning:  The second part of this experiment may permanently damage the colour on some TVs - you have been warned so don't do it on a TV that is important to you!

Result

You will find that if you turn the TV upside down the colours will go strange.  If you turn the TV off and on while it is upside down they go back to normal.

If you put the magnet near the TV, if it is strong enough you will see strange patterns appearing on the screen, that move with the magnet.

The Monitor With a magnet Primary Colours

Explanation

To understand what is going on you have to understand how a CRT (Cathode Ray Tube) TV works.

The picture is formed by firing a beam of electrons (a sub atomic particle that is normally found wizzing around an atom) from electron guns at the back of the TV to the screen at the front. This gives the electrons lots of energy which they transfer to the screen. This energy is transferred to little blobs of phosphor on the back of the screen. Phosphors are materials which tend to give out energy as light so they glow.

A TV

Electon beam

The beams of electrons are scanned across the screen very quickly and as they do this they are turned on and off by the signal sent from the TV station. This allows them to build up a picture of your (least) favourite celebrity on the screen.In a colour TV there are 3 colours of phosphor: red, blue and green, and 3 electron guns each aiming at a different set of dots. If you sit very close to the TV you can see the individual dots of red , blue and green, but as you move away they merge together forming all the colours you can see.

What is the magnet doing?

A magnetic field deflects the electrons so they end up going to the wrong place.  This has the effect of  both moving the whole picture and sometimes moving the electron beam that should be forming the red part of the picture onto another colour, so everything looks the wrong colour.

Magnet and Beam

Magnets are so good at deflecting electron beams that electromagnets near the electron guns are used to scan the beams across the screen to form the picture.  It is very hard to get the steering electromagnets to bend the beams through a large angle accurately which is why CRT TVs have to be so deep  and take up so much space.

Why does turning it upside affect it?

The earth has a magnetic field, and the TV is set up to expect this, when you turn the TV upside down the magnetic field is coming from the wrong direction and so it is just like putting a magnet near the screen, the picture moves slightly making the picture go a bit strange.

How can magnets damage the TV.

The MaskThere is a metal mask behind the screen which has holes over each group of 3 spots of coloured phosphor. The electron guns for the different colours are in slightly different places so they hit a hole from slightly different directions. This means that although all three are focussed on the same hole in the mask they end up hitting different places on the screen  and so light up different coloured phosphors. In some TVs this mask is made of steel which can be magnetised with your magnet, permanently distorting the image and ruining the colour. Luckily most of the time turning your TV off and on again a couple of times will fix the problem.

Why does turning the TV off and on affect it?

When you turn the TV on you will probably hear a mixture between a thump and a buzz, this is caused by the degaussing coil.  This is an electromagnet which is made to apply a large magnetic field that is changing too fast for the metal in the mask to keep up with it, and has the effect of demagnetising the metal so it cancels out the earth's magnetic field.  So with any luck it will fix any damage you have done.

If you really don't like your TV you could try turning it on with the magnet near it then taking the magnet away.

What has this go to do with particle physics?

First of all electrons are sub-atomic particles, the same sort of thing that are created at CERN and other large particle physics experiments, although mostly they make more exotic particles.  Also in CERN they do exactly the same experiment as you have just done in order to identify particles coming out of the collisions.  The detectors apply a magnetic field to the particles, which makes them travel on a curved path if they are charged.  If you know how much energy they have how curved these paths are tells you how heavy the particles are and so helps you find out what they are.

Why is an old fashioned TV called a Cathode Ray tube?

Beams of electrons are emitted from the -ve electrode which is also known as the cathode so the beams are also known as cathode rays.  'Tube' is just a contraction of vacuum tube, as the whole of the TV has to have the air pumped out to stop it interfering with the electron beam.  Valves which are also a form of vacuum tube were fundamental to the first forms of electronics in the beginning of the 20th century because you can interfere with the progress of electron beams by applying electric and magnetic fields, so you can use them to amplify signals and other cunning things.  So valves were used to amplify the radio signals in both radios and TVs before transistors were invented.

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