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### Magnetism.

posted Nov 26, 2015, 1:28 AM by Upali Salpadoru   [ updated Sep 24, 2016, 2:00 PM ]

### Fig 1  A powerful electromagnet in action.

Switching a current on and off can pick up a load and drop. No need of tying, hooks or baskets to carry.

Properties of Permanent magnets.
 Fig.2.Iron filing clinging on. . 1.  A magnet attracts iron.The attraction is strongest at the two ends. These two ends are called North pole and south pole.The attraction is greatest at the 2 poles.Which is North and which is south? Fig.3. A magnet to find the direction. -Compass. 2. When a magnet freely rotates in a horizontal plane, one side points the north while the other points the south.  According to this naming, North Pole is the North seeking side.Fig.4. A hanging magnat. Fig.5.. Unlike poles attract. 3. North pole attracts South and vice versa.Anypole can atOld AA torch cells are used as rollers for the magnets to move easily.tract de-magnetised iron. Fig.6. Like poles repel. Old AA torch cells are used as rollers for the magnets to move easily. Fig.7.Breaking a magnet 4. Breaking a magnet will result in two magnets.

Magnetic Lines of Force.

 Fig.8. Magnetic lines of force.Method.1.Sprinkle some iron filings around  a magnet and tap the paper on which it is placed. Method 2. Place a pocket compass at different places around a magnet. Then mark the two ends of the needle and join those points.This method will help you to find the direction of the force lines.Magnetic lines of force  do not cross.What are force lines?Fig.9.There are magnetic lines of force around a magnet resembling a pumpkin.

A line of force is the path a North pole may take.

 But you cannot get a N pole without a South?If there is a will there is a way!

 Home experiment Fig.10 Movement of a N pole. Take a plastic basin filled with water. Float a styrofoam sheet and place a bar magnet on it. Then magnetise a needle and insert it into a very small float such as a cork.Keep the N pole of the needle closer to the water surface while the other end is immersed deep.Place this at different places as in the diagram and see how it moves

Why do Magnets align in a North, South direction?

 Fig.11.Terrestrial magnetism. For an unknown reason the Earth behaves as there is a magnet in it as shown here.According to the magnetic force of lines magnetic pole exist in the Arctic sea.The geaographic North, according to the axis of rotationlies in Alaska.Every year this is changing by a few kilo meters.

Electromagnetism.

 Fig.12.Magnetism from Current A  few turns of a wire are wrapped round a direction finding compass. As a current passes through the wire  the pointer will move to one side depending on the direMction of the current Fig.13 . Magnetic lines arond a current carrying wire. What is the direction? Clock-wise or counter clock-wise?  This can be determined by using a small pocket compass. There is a rule to find this. Fig.14,Cork-screw rule. The Cork screw ruleThere is a simple rule to remember the direction of the concentric magnetic lines of force induced by a current.If you send a screw driver in the direction of the current, the way you have to turn it gives the direction of the force.

Fig.15.  Getting the direction of the force

Several loops of wire are wound using the holes made in a thick piece of paper.  The direction of the current is shown by the black arrows. When the current is going into the paper the magnetic lines are clockwise. Shown in red. When the current is coming up they are anti-clockwise. Shown in blue. It could be seen that the lines of force  clash in between the turns. Then they cancel and those areas become ‘null points’. Shown by the letter ‘n’. The forces join at the axis of the coil. This strong magnetic field is shown in green. If you insert a soft iron into the coil the strength of magnetism will increase as the lines prefer to pass through iron rather than through air.

Poles of the Electromagnet.

The side from which the lines emanate is the North pole and at the South pole the lines go in. There is a very easy way to find this.  It is called the Fleming's Right hand rule.

Fig.16. Fleming's right hand rule.

Grip the coil so that your fingers will point the direction of the current. Then the thumb will point the North pole.

Fig.17. Use the right hand rule to verify  the N and S poles.

Electric Bell

The Electromagnet is shown in grey.  When the current passes it attracts the steel strip which acts as a spring. This is fixed at the base. Then the striker hits the bell. This will cut off the contact of the strip with the screw. As magnet does not get current the steel spring will pull the striker back This can hit can hit another bell. So on and so forth it will go on ringing as long as the current supply is there.

Fig.18. Electric bell.

### Fig.19. A barmagnet is dropped through a coil connected to  an ammeter or a voltmeter.  As the magnet falls the meter will indicate that an electric current has been generated.

Fig.20. Magnetism from Current

A  few turns of a wire are wrapped round a direction finding compass. As a current passes through the wire  the pointer will move to one side depending on the direction of the current

 Q.1.01.  If a pocket compass is placed at C  will N side point  right or left ?A- Right.    B. Left     C. North     D. down.2.X and Y stand for ..A- Positive  and Negative.B. Negative and Positive.C. North pole and South pole.D. South pole and North pole.3. What do the arrows indicate?A. Direction of current.        B. The path a magnet will move.        C. The path South pole of a magnet will move.        D. The path North pole of a magnet will move.4.Which out of these chemicals is most suitable to detect the magnetic lines of force?A   Magnesium powder.   B.  Copper powder  C. Sulphur powder. D. Iron powder.6x4 =24 marks.

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Q.2.0

1.What kind of a magnet would have given this impression with iron filings?

2. What letter indicates the North pole of the magnet? A, B or C?

3.What would happen if a conductor DE is moved to right and left?

4. What would happen if it is moved vertically up and down?

6x4=24 mark

s.

1.What do these  indicate

a. thumb  b. nails c. Latter A.  d. Letter B ?

2. Could these actions be used to increase or decrease the strength of the magnetic field?

a.Increasing the resistance of the wire.

b.Increasing the number of turns in the coil.

c. Inserting a wooden core into the coil.

d.Inserting  a soft iron core into the coil.

3. What may happen if you..

a. Change the direction of current?

b. If you increase the voltage?                                                                 6x3 = 18 marks.

Q.4.0

Jakie wound a cleaned,uninsulated copper wire around an aluminium core. She passed a stepped down current from the domestic supply.

When she was not successful in obtaining a magnetic force, her teacher pointed out several errors.

Find out four errors in this set up.                      5x4 =20 marks.

Q.5.0

The figure shows a partly drawn diagram for an electric bell.

1. Complete the diagram for the bell to function properly.

2. Explain how the bell works.