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MAGNETISM: Attraction and Repulsion.
By placing two bar magnets (magnetic iron) end to end they either attract or repel one
another.
A bar magnet is referred to as a permanent magnet because it retains its magnetic
properties. Around each magnet there exists a magnetic flux which when brought into
the range of a similar field will cause the two bodies to be either attracted or repelled.
When positioned to attract, the result is that of one long magnet where the path of the
flux is directed through both magnets.
The strength of a magnet (flux, unit Weber) may be expressed by the number of lines
of force leaving the 'N' pole and entering the 'S'. Flux density varies around the
magnet but is concentrated at the poles and is the number of lines of force passing
through an area of 1sq cm. Here the unit of measurement is the 'tesla', symbol (T)
where one tesla is a density of one Weber of magnetic flux per square metre.
The physical size of a magnet is not related to its magnetic strength, this depends
upon the flux density.
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MAGNETISM: Magnetic Field of a Straight Conductor.
For this experiment iron filings are first sprinkled on a card and randomly arranged by
tapping the card. If a piece of straight wire is connected as shown across a DC supply,
a current will flow when closing the switch. The direction of current is for
conventional current, i.e. from positive to negative which magnetises the filings,
giving each a 'N' and 'S' pole, as for a compass needle.
Concentric circles are formed due to the magnetic field around the conductor.
The direction of the lines of force is determined by the current direction, using the
right-hand screw rule. Tightening a screw represents the current flowing down
through the card making the field clockwise. Reverse the current and the field is now
in the opposite direction, as if the screw were inserted from below.
Flux density is greatest close to the wire and reduces towards the outer edges if the
card.
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MAGNETISM: Magnetic Field of a Single Coil.
Remembering the right-hand-screw rule we can now develop this topic further by
examining the effect on the magnetic flux pattern of a coil.
Note, you are viewing the coil through one single loop. With the battery connected
one way round the current flows up through the card forming an anti-clockwise
pattern of filings and then down through the card clockwise. Changeover the current
direction by reversing the battery and the flux pattern is now in the opposite direction.
The central arrow on the diagram indicates the direction of flux. Adding more turns to
the coil makes the field inside the coil winding stronger, as the flux becomes
concentrated in a small area. In a multi-turn coil this flux concentration can be
improved by winding the coil on a bar of magnetic material, i.e. ferrite or soft iron
which provides a path for the flux.
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MAGNETISM: Electro-magnet.
An electro-magnet can be constructed by winding a coil of wire around a piece of
ferrous material, i.e. soft iron.
This is often called a 'solenoid'. When the switch is closed current flows in the
winding causing a magnetic field around the coil, thereby magnetising the soft iron
core. Increasing the current through the winding or adding more turns to the coil can
improve the strength of the electro-magnet. Magnetic saturation of the core will
eventually occur.
The direction of the magnetic flux determines which end of the core is 'North' and
which is 'South'. Reversing the direction of current through the coil will change this.
An example of the use of an electro-magnet in electronics is as a relay switch. Or as
an electrically operated crane, whereby the magnetic field generated is used to lift,
say, a car body and drop it by switching off the energising current. Winding the coils
onto a horseshoe shaped magnet brings the poles closer together thereby
concentrating their magnetic force around a smaller area.
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MAGNETISM: A Closed Magnetic Circuit.
A magnetic circuit is the path of the magnetic field. The diagram shows a permanent
magnet where the poles are connected via three pieces of soft iron. The closed
magnetic flux path is therefore continuous. All the magnetic effect is enclosed within
this path, with little detected outside.
The reluctance of the iron is the resistance of the material to magnetic flux. Iron is a
good magnetic conductor, providing a path of lowest resistance.
The second diagram is one found in transformers. Current through the coil replaces
the permanent magnet and can be determined from the formulae given. The type of
core material determines the strength of the field. Permeability is the ratio of the
magnetic flux density to the magnetic force producing it.
In an electric circuit current is due to the existence of an electromotive force.
Similarly, in a magnetic circuit flux is due to a magnomotive force (m.m.f.) and
proportional to the current and number of turns. The constant of 1.257 (4 × pi)
converts m.m.f. to the equivalent SI unit (ampere-turns).
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MAGNETISM: Magnetic Circuit with an Air Gap.
We now introduce an air gap in the magnetic circuit. Air or non-magnetic materials
have a very high 'reluctance' compared to iron, requiring that its path be kept as short
as possible. In practice this air gap could enclose a moving coil as part of an electric
motor or analogue meter movement.
The presence of the air gap considerably increases the number of ampere-turns
required to produce a given value of flux lines.
Compare the amount a current required to produce the same number of lines with a
core of the same dimensions, with or without the presence of the air gap by referring
back to the previous closed magnetic circuit topic. Note: the length of the path must
always include the width of the air gap.
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MAGNETISM: Force on a Conductor in a Magnetic Field.
A conductor is positioned inside a magnetic field created by two opposing permanent
magnet poles. With current switched off there is no influence on the permanent
magnetic field.
A current flow sets up a series of concentric lines of force around the conductor in a
clockwise direction. The effect is a crowded magnetic field above the conductor as the
field is reinforced and a less dense field below as the flux in the lower part of the
conductor and the 'N' to 'S' field is in opposition.
As the lines of force try to shorten the path above the conductor from 'N' to 'S' this
exerts a mechanical force on the upper part of the conductor pushing it in a downward
direction. If the magnet poles or the current were reversed the resultant direction
would be upward.
The force imposed on the conductor can be calculated as shown.
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