Magnetism - Physics Resources 5 – Unit 2– Theme 6: Magnets and Motors Page 1 Magnetism Magnetism is an effect that we cannot see, hear or touch. It is caused by magnets

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  • Form 5 Unit 2 Theme 6: Magnets and Motors Page 1

    Magnetism Magnetism is an effect that we cannot see, hear or touch.

    It is caused by magnets.

    A magnet is a body which attracts iron, combinations of

    metals (known as alloys) or other materials which are

    composed of iron or iron-like substances. Additionally, the magnet can "magnetize" other objects which in turn act like

    magnets. Other iron objects will be attracted to the

    magnetized object.

    Types of magnets .

    There are many different types of magnets, the most

    common are Bar magnet, Flat magnet, horseshoe magnet

    and ring magnets.

    The Earth itself is a magnet and

    contains a great deal of iron. On the other hand the Earth's moon is largely

    made of aluminum, not iron. The

    moon does not act like a magnet and

    magnetic compasses do not work on

    the moon.

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  • Form 5 Unit 2 Theme 6: Magnets and Motors Page 2

    A magnetic compass has a non-magnetic

    casing (container). The compass needle is

    magnetic and is free to pivot around in a

    circle. The compass needle points to the

    earth's magnetic north. A magnetic compass

    works through air, water, glass, and walls.

    The Magnetic Compass Points Towards Magnetic North

    The two ends of the magnet are different and are referred to

    as the north and south pole (or north-seeking pole and

    south-seeking pole).

    Like poles repel. The south poles of two magnets will not

    attract one another. Neither will the north poles of two

    magnets. The north pole of one magnet and the south pole

    of another magnet will attract one another. Unlike poles

    attract.

    Like Poles Repel -- Unlike Poles Attract

  • Form 5 Unit 2 Theme 6: Magnets and Motors Page 3

    So, what is the difference between a magnet and a

    piece of magnetic material?

    The easiest way to tell them apart is that a magnet can

    repel and attract another magnet. Whereas, a piece of

    magnetic material can only attract a magnet!

    There are only three magnetic elements, iron, nickel and

    cobalt that can form a magnet. These are called

    Ferromagnetic materials.

    In practice you will only use iron,or steel which is an alloy of

    iron. Even some plastics are magnetic because they have

    magnetic substance mixed in them.

    If you break up a magnet, you will see, new north and

    south poles form immediately. A magnetic pole cannot

    be isolated.

    Cutting a magnet in half will not isolate a single north or south. One magnet becomes two, then four, and so on.

    This process will never end; even when the last electron spin dipole is reached, it cannot be cut to reveal a single north or single south pole.

  • Form 5 Unit 2 Theme 6: Magnets and Motors Page 4

    Magnetic field and magnetic lines of force

    The space around a magnet is

    called its "magnetic field".

    The pattern is called the magnetic

    lines of force. The effect the magnet

    has around its surrounding is called

    the magnetic field. The strength of

    a magnetic force,is inversely

    proportional to the square of the

    distance from the magnet.

    The properties of lines of force :

    1. They are said to originate from the north pole and end at

    the south pole.

    2. The lines of forces of a particular magnet do not intersect

    with each other.

    3. A line of force is continuous : starts from the north pole

    and ends at the south pole.

    4. There is no line of force within the magnet.

  • Form 5 Unit 2 Theme 6: Magnets and Motors Page 5

    Magnetic effects can be forced on substances that can then

    become magnetic themselves. The substances have to be

    made of magnetically favourable materials like iron, nickel,

    steel, etc. This is called Magnetic Induction.

    Take an iron nail. It is not magnetic initially. Bring

    its head close to a bar magnet. Let it get attached

    to the north pole of the magnet. Slowly the nail

    itself will start attracting other nails. Thus the first nails head gets induced as a south pole and

    it itself becomes a magnet. Its free end acquires a

    north pole. Thus the iron nails become induced as

    magnets. After the nails are removed, they may

    continue to behave as tiny magnets, but will loose

    their magnetic effect after some time.

    Making magnets.

    1. Hammerring steel

    2. Stroking method - This involves inducing magnetism

    in bars of iron by rubbing them in a particular manner over

    other permanent magnets.

    Stroking a piece of

    unmagnetised iron or

    steel with a known

    magnet can make it

    into a magnet.

    This causes the

    molecules to line

    up in the North-

    South

    orientation.

  • Form 5 Unit 2 Theme 6: Magnets and Motors Page 6

    3. Solenoid method This involves introducing a magnet

    into a solenoid and switching the direct current for a short

    time.

    Handling or breaking, heating, etc may destroy magnetism.

    Such effects are called demagnetizing effects. In order to

    avoid demagnetizing effects, a permanent magnet is always

    kept stuck to soft iron materials called keepers. These

    keepers help the poles to stay apart and not get stuck to

    each other or bang against each other.

    Iron Soft magnetic material .

    Iron is very easy to become a magnet but it also loses its

    magnetism easily as well. Very useful for making

    temporary magnets. E.g. electromagnets, transformers,

    reed switches, magnetic switches

    Steel Hard magnetic material .

    Steel is much more difficult to become a magnet but it

    retains its magnetism. Very useful for making permanent

    magnets. E.g. compass, fridge door rubber, oil sump tap, magnetic disks, tapes

  • Form 5 Unit 2 Theme 6: Magnets and Motors Page 7

    Magnetic Field patterns

    Notice that the lines of magnetic force between the poles

    are parallel.

    The region in the centre of the poles shown by the red star

    has no magnetic field, as the two opposing fields cancel

    each other out.

  • Form 5 Unit 2 Theme 6: Magnets and Motors Page 8

    Electro Magnetism

    An electric current produces a magnetic field. The Danish

    physicist H. C. Oersted first discovered this relationship.

    This magnetic field strength could be increased when the

    electrical wire was wound into a coil. Winding the wire

    around a soft-iron core can increase magnetic strength hundreds or thousands of times. Such a device is known as

    an electromagnet.

    Advantages of an electromagnet over a natural magnet include strength and the ability to control the current

    and direction.

    Examples :

    1. Salvage yard cranes.

    2. Lifting magnets are also used to load machine parts,

    steel rails and scrap iron or steel.

    3. Burglar alarms use magnets.

    The magnetic field around

    a straight wire is circular, at

    right angles to the wire.

    You can work out the direction

    of the field using your right

    clenched fist. Point your

    thumb upwards in the same

    direction as the current. The

    direction of the field is the

    same direction in which your

    fingers curl. Reversing the

    direction of the current will reverse the magnetic field

    direction.

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  • Form 5 Unit 2 Theme 6: Magnets and Motors Page 9

    Electromagnetic Coil.

    The magnetic field can be made stronger by coiling the wire

    around a piece of soft iron. This electromagnet is

    sometimes called a solenoid.

    The shape of the magnetic field is the same as a bar

    magnet.

    The soft iron inside the coil makes the magnetic field

    stronger because it becomes a magnet itself when the

    current is flowing.

    Soft iron is used because it loses its magnetism

    as soon as the current stops flowing. Soft iron is said to form

    a temporary magnet. In this way, the electromagnet can

    be switched on and off by turning the electricity on and off.

    Steel forms a permanent magnet. If steel was used inside

    the coil, it would continue as a magnet after the electricity

    was switched off.

    It would not be useful as an electromagnet. Permanent

    magnets are needed for electric motors, generators,

    loudspeakers and microphones.

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  • Form 5 Unit 2 Theme 6: Magnets and Motors Page 10

    The strength of the magnetic field around the coil can

    be increased by

    1. Using a soft iron core (core means middle bit).

    2. Using more turns of wire on the coil.

    3. Using a bigger current.

    Reversing the direction of the current will reverse the

    magnetic field direction.

    An electromagnet is used in the electric bell, relay,

    circuit breaker, loudspeaker

    and microphone.

    Electric Bell

    When the switch is pushed closed

    the circuit is completed

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