Magnetism. What do you remember? Pg Syllabus cgrahamphysics.com 2015

Transcription

1 Magnetism What do you remember? Pg yllabus

2 tarter Have you ever wondered...how does a Maglev Train work?

3 Magnets repel and attract MagneJc material: iron, nickel, cobalt Magnets are able to amract these materials A magnet has a north and a south pole à mono poles have not found yet Equal poles repel, opposite poles amract Permanent magnets are hard to magnejze but they stay magnejze for a long Jme. They are called hard magnets à made from steel Temporary magnets are easy to magnejze, but they easily lose their magnejc properjes. They are called so/ magnets. à made from iron MagneJsm can be induced in a magnejc material by leaving it in a magnejc field

4 What happens?

5 How can you test for a magnet? The only true test for a magnet is repulsion. ince and R both amracted to Q, the rod is only made of magnejc material, but is not a magnet Only U repels Q, therefore only rod 3 is a magnet

6 How does a magnet become a magnet? Inside magnejc metals are group of atoms, domains, which act like Jny magnets Domains The iron on the lez is un- magnejzed. You can magnejze these materials by stroking them in one direcjon with a magnet, this is to line up the domains Methods of de- magnejzing: you need to scramble the domains by: - Hammer it - Heat it - Wrap a coil around it and pass an alternajng current through it

7 An iron or steel bar which has not been magnetised. o magnetic force field Atomic Magnets all jumbled pointing at each other: Their fields cancel each other and this iron or steel bar would not be magnetic

8 Unmagentised steel or iron bar

9 The magnetic force field from the permanent magnets line up the atomic magnets and will make the steel or iron bar magnetic.

10 The magnetic force field from the permanent magnets lines up the atomic magnets and will make the steel or iron bar magnetic.

11 The magnetic force field from the permanent magnets lines up the atomic magnets and will make the steel or iron bar magnetic.

12 The magnetic force field from the permanent magnets lines up the atomic magnets and will make the steel or iron bar magnetic.

13 The magnetic force field from the permanent magnets lines up the atomic magnets and will make the steel or iron bar magnetic.

14 The blue magnet now has its own magnetic field E J Vine.

15 TEMPORARY MAGET ARE MADE OF IRO : When the domains are lined up the bar becomes magnetic. The domains in pure iron are easily lined up, iron is easy to magnetise. However, even the warmth of the room gives the domains enough energy to jumble up again and the bar easily looses its magnetic field. E J Vine.

16 Heat from the room is sufficient to give the domains energy, this destroys the orderly lines, the magnejc field will disappear. E J Vine.

17 The magnetic force field gradually disappears.

18 o field left!!!!

19 Permanent MAGET ARE MADE OF teel plus impurity atoms like Al aluminium, i nickel, Co cobalt They are called AliCo Magnets Al i Co The impurity atoms make it difficult to magnetise Al i Co magnets but they also lock the domains in position once they are lined up.

20 Permanent MAGET ARE MADE OF teel plus impurity atoms like Al aluminium, i nickel, Co cobalt They are called AliCo Magnets Al i Co

21 Magnetic ;ields Magne2c field is a region in which a magnejc force can be detected. MagneJc field lines can be drawn with the aid of a compass. The compass will point in the direcjon of the field, OT the north pole A normal compass points in the direcjon of the - pole They are called plo`ng compass

22 Another method You can see the shape of the magnejc field around a magnet by using iron fillings trong magnejc field lines are close together MagneJc field lines never cross MagneJc field lines are symmetric MagneJc field lines point from to outside the magnet but inside the magnet they go from to If two bar magnets are placed next to each other, their fields affects each other

23 Different magnetic ;ields

24 Electromagnetism When a current flows through a wire a magnejc field is created egajve current Zero current PosiJve current

25 Electrons flowing in a wire Produce a magnetic field Wire Electron flow Magnetic field lines

26 The right hand rule #1 With the thumb of your right hand poinjng in the direcjon of the current, your fingers will curl in the direcjon of the field The strength of the field can be increased by - increasing the current in the wire - wrapping the wire into a solenoid

27 Magnetic coils If you bend a wire round to make a coil, the circular magnetic fields produce a new pattern which is rather like that of a bar magnet!! An electric current on circuit diagrams shows.a direction which is OPPOITE to the electron flow!!!! + Current Current : from + to - Electrons Actually go the other Way! Current - Electron flow

28 Magnetic Field of a Current Loop olenoids produce a strong magnejc field by combining several loops. A solenoid is a long, helically wound coil of insulated wire. The strength of the field around a solenoid can be increased by Increasing the current through the solenoid Increasing the number of turns of the solenoid Wrapping the solenoid around magnejcally soz core such as iron

29 Right hand rule #2 You can work out the polarity of the solenoid by Wrap you right hand around it so that the fingers point in the direcjon of the current Your thumb will point in the direcjon of the - pole

30 Field pattern due to a ;lat coil If the direcjon of the current is reversed, so are the poles

31 Example ketch the magnejc field based on the current shown:

32 Example Draw the current based on the magnejc field shown

33 Example Draw the electron flow that would cause the labeled polarity

34 Match the currents and magnetic ;ields