1. MAGNETISM

2. PROPERTIES OF MAGNETS Magnets have two poles north and south pole.
A magnet arracks magnetic materials such as iron, cobalt nickel and steel.

3. Like poles of the magnets repel each other.
Unlike poles of the magnets attract each other.

4. Around the magnet there are magnetic field lines starting from north pole and ends at south pole.

5. PLOTTING MAGNETIC FIELD LINES
You will need
• a bar magnet
• a plotting compass
• plain paper, a pencil
What to do
1 Place the magnet in the centre of the sheet of paper and draw round it.
2 Place the compass near one of the poles of the magnet.
Mark dots 1 and 2 on the paper to indicate the two ends
of the compass needle.

6. 3 Move the compass away from the magnet and position it so that one end of its needle is marked by dot 2. Mark dot 3 at the other end. 4 Continue this process, until you have moved round to the other pole of the magnet. 5 Remove the compass. The sequence of dots shows one of the field lines of the magnet’s field. Draw a smooth line through the dots. 6 Repeat the process, starting at a slightly different position, to obtain another field line.

7. INDUCED MAGNETISM
A permanent can attract or repel another permanent magnet. It can also attract other unmagnetised magnetic materials. For example a permanent magnet can attract steel pins, paper clips or iron nails. What is going on here? Paper clips are made up of a magnetic material. When the south pole of the magnet is brought closed to a clip the clip is attracted. The attraction tell us that the end of the paper clip nearest magnetic pole must be a north pole as shown below. This is known as induced magnetism. When the permanent magnet is removed, the pin will return to its unmagnetised state.

8. MAGNETIC MATERIALS
Materials that attract the magnets and can be magnetised. Eg: iron, steel, cobalt, nickel etc.. NON-MAGNETIC MATERIALS Materials that do not attract the magnets. Eg: brass, copper, zinc, tin, aluminum and as well as non metals. MAGNETISED MATERIALS Materials which acts as magnets and having all magnetic properties.

9. MAGNETIC EFFECT OF A CURRENT
Magnetic field due to current in straight wires
When an electric current is passed through a wire, as shown below, a weak magnetic field is produced around the wire.
If the current through the wire increases the strength of magnetic field also increases.
If the direction of current flowing through the wire changes the direction of magnetic field around the wire also change.

10. So the direction of magnetic field produce by a current is given by the right-hand grip rule as shown below. Imagine gripping the wire with your right hand so that your thumb points in the conventional direction. Your fingers then point in the same direction as the field lines.

11. Magnetic field due to current in solenoid A current produces a stronger magnetic field if the wire it flows through is wound into a coil. The diagram below shows the magnetic field patterns produced by current carrying coil. A long coil is called solenoid.

12. If the current through the solenoid increases, the strength of magnetic field also increases.
If we increases the number of terns in the solenoid, the strength of magnetic field also increases.
If the direction of current flowing through the solenoid changes, the field around the solenoid and the poles of solenoid also changes.
So to work out which way round the poles are, we can use another right-hand grip rule, as shown below. Imagine the gripping the coil with your right hand so that your fingers point in the conventional current direction. Your thumb then points towards the North pole of the coil.

13. METHODS OF MAGNATISATION Magnetising by electrical method
1. A steel bar is placed inside a solenoid through which a direct current passed for a while.
2. A strong magnetic field is produced and the steel bar is magnetised.
3. When viewed at one end of the bar, the current flows in clockwise direction that end becomes a South pole. If current flows anti-clockwise direction, it becomes a North pole. And we can easily find the poles by using right hand grip rule.

14. Magnetising by stroking method
A piece of the material may be stroked with a permanent magnet. By stroking it consistently from one end to another (never going in the reverse direction), it becomes magnetised.

15. METHODS OF DEMAGNATISATION Demagnetising by an Alternating current
1. A magnet to be demagnetized is places inside a solenoid.
2. The solenoid is connected to an a.c. supply.
3. The magnet is withdrawn far away through the solenoid in the east-west direction while the a.c. current still flowing.
4. As the process is repeated, the magnet will be demagnetised.

16. Demagnetising by heating
If we heat a magnet strongly by means of bunsen flame, the magnet will lose its magnetisation.
Demagnetising by hammering
As in the case of heating, hammering also cause a magnet to lose its magnetism. If the magnet is hammered vigorously while lying in an east west direction, its magnetism become weaker and weaker.

17. PROPERTIES OF TEMPORARY MAGNETS AND PERMANENT MAGNETS
Temporary magnets (eg. Iron)
Permanent magnets (eg. Steel)
Soft magnetic material
Hard magnetic material
Easy to magnetise and demagnetise ( lose its magnetism easily)
Harder to magnetise and demagnetise (can retain its magnetism)
Used to make electromagnets
Used to make permanent magnets
Uses: transformers, audio/video tape, electric bells, magnetic relays, reed relays
Uses: D.C motors, A.C generators, galvanometers, loud speakers and magnetic door catches.

18. APPLICATION OF THE MAGNETIC EFFECT OF A CURRENT Magnetic relay
A magnetic relay is a switch operated by an electromagnet. With a relay a small switch with thin wires can be used to turn on the current in a much more powerful circuit – for example, one with large electric motor in it.
When the switch S in the input circuit is closed, a current flows through electromagnet and it will magnetised. This pulls the iron armature towards it, which closes the contact C. As a result current flows through the output circuit and motor turn on.

19. Circuit breaker
A circuit breaker is a automatic switch which cuts off the current in a circuit if this rises above a specific value.
In the type shown below, the current flows through two contacts and also through an electromagnet. If the current gets too high, the pull of electromagnet becomes strong enough to release the iron armature, so the contact open and stop the current. Pressing the reset button closes the contact again.

20. Loud speaker
Most loudspeakers are of the moving-coil type shown below. The cylindrical magnet produces a strong radial (‘spoke-like’) magnetic field at right angles to the wire in the coil. The coil is free to move backwards and forwards and is attached to a stiff paper or plastic cone. If a current is passed through the coil, a backward and forward force acts on it; this follows from Flemings left hand rule.
The loud speaker is connected to an amplifier which gives out alternating current. This flows backwards, forwards, backwards… and so on, causing a force on the coil which is also backwards, forwards, backwards…. As a result, the cone vibrates and gives out sound waves. The nature of sound produced depends on the frequency and amplitude of the alternating current flowing through the coil.

21. Tape recorder
A sound wave sent into a microphone is transformed into an electric current (alternative current), amplified, and allowed to pass through a wire coiled around a doughnut-shaped piece of iron, which functions as the recording head. The iron ring and the wire constitute an electromagnet, in which the lines of the magnetic field are contained completely inside the iron except at the point where a slot is cut in the ring. Here the magnetic field fringes out of the iron and magnetizes the small pieces of iron oxide embedded in the tape. As the tape moves past the slot, it becomes magnetized in a pattern that reproduces both the frequency and the intensity of the sound signal entering the microphone.

22. Magnetic screening or magnetic shielding
Magnetic screening is protecting delicate instruments from stray magnetic fields by placing delicate instrument inside the soft iron box. Since the soft iron is a magnetic material, so stray magnetic field lines can go through the soft iron as shown below. But the field lines cannot reach into delicate instrument it will protect the layer of soft iron around it.