Introduction. Lodestone (1 st known magnets) An ore of Iron oxide (Fe 3 O 4 ) Property of attraction Definite direction (used in ships)

Transcription

1 Magnetism Std 8

2 Introduction Lodestone (1 st known magnets) An ore of Iron oxide (Fe 3 O 4 ) Property of attraction Definite direction (used in ships) Found in large in Magnesia

3 Natural Magnets The magnets (lodestone) found in nature Artificial Magnets is a magnetised piece of iron Odd shapes Weaker Desired size & shape Stronger

4 Properties of a Magnet Attractive Directive Like poles repel, unlike attract Poles exist in pair

5 Property ONE - Attractive property Property of attracting small pieces of iron towards it Ends of the magnet attracts more iron filings than the middle Near each end, there appears a point with maximum attractive property This point is called the pole of a magnet.

6 Attractive property Pole Pole Poles are slightly inside The distance between the poles - Effective length (5/6 th of length of magnet) The imaginary line joining the poles of a magnet Magnetic axis

7 Property TWO - Directive property A magnet suspended with a silk thread sets itself in geographic north-south direction The end that points towards the north is the north pole. The end that points towards the south is the south pole.

8 Property THREE - Like poles repel & unlike poles attract Two like poles (north-north or south-south) repel each other Two unlike poles (north-north or south-south) attract each other

9 Property FOUR - Poles exist in pairs If a magnet is broken into two each part is found to have attractive & directive property If further broken each piece is found to retain magnetism Each part contains N and S pole New ends formed at cut place acquires opposite polarites

10 Induced Magnetism Magnetism acquired by a magnetic material when it is kept near (or in contact with) a magnet, is called induced magnetism. Magnetic material Temporary process material acquires magnetic properties due to a magnet s presence

11 Induced Magnetism Induction precedes Attraction Polarity A magnetic pole induces an opposite polarity on the near end and a similar polarity on the farther end First Magnet s South end induces North pole on iron nail Second Attraction takes place (unlike poles attract) Temporary When magnet is removed, uppermost nail loses its magnetism and all the others and get separated

12 Lines of Magnetic Field Space around a magnet the needle of a compass rests in a definite direction Other than geographic north-south Called magnetic field lines Region near magnet where magnetic forces can be detected Magnetic field is a vector quantity

13 Lines of Magnetic Field

14 Properties of Magnetic Field Lines Closed & continuous curves North pole to south pole (direction) Tangent gives the direction at that point Never intersect Crowded near poles (strong) & far separated near middle of the magnet (weak) Parallel & equi-distant uniform magnetic field Behave like stretched elastic rubber strings

15 Magnetic Field of Earth Freely suspended magnet points north-south direction Iron rod burried inside earth along north-south direction becomes a magnet Neutral points are obtained on plotting field lines of a magnet

16 Magnetic Field of Earth Magnetic needle rests making different angles with the horizontal

17 S N Plotting of Magnetic Field lines N 1. Place a magnet on a white paper & draw an outline 2. North pointing geographic north 3. Place compass at north end 4. Put a dot at the other end of the needle 5. Move the compass so that the other end of needle coincides with the dot 6. Repeat steps 4 & 5 until the other end is reached 7. Join the dots with a smooth curve 8. Repeat 4 to 7 on the left of the magnet

18 Plot Magnetic Field lines Uniform magnetic field lines Non-uniform magnetic field lines

19 Plot Magnetic Field lines Magnetic field lines of a bar magnet when its north pole N faces geographic north Magnetic field lines of a bar magnet when its north pole S faces geographic north

20 Neutral points Magnetic field of magnet = magnitude of earth s horizontal magnetic field But in opposite direction Resultant (net) magnetic field at the neutral point is zero Compass placed at the neutral points remains unaffected (rests pointing in any direction)

21 The End