magnetic fields 08
DESCRIPTION
SACE Physics Section 2 Topic 3TRANSCRIPT
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Magnetic Fields
Section 2 Topic 3
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Magnetic Fields
Magnetic fields are produced by moving electric charges; hence by electric currents.
In a bar magnet; iron atoms have electrons that spin.
Each spinning electron; tiny ‘magnet’.
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Magnetic Fields
As all the electrons spin in the same direction; there is no cancellation, magnetic field is stable.
Field lines can represent magnetic fields; As they did in electric fields.
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Magnetic Fields
The drawing of field lines follows the same rules as with electric fields.A compass moved in a magnetic field; follows the field lines.
Field lines follow the direction; North to South.
Bar Magnet B Field
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Magnetic Fields
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Oersted’s Law
Permanent magnets are not the only source of magnetic fields.Hans Christian Oersted discovered; a magnetic field, around a wire carrying an electric
current.
He showed this by placing compasses around a wire; carrying an electric current.
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Oersted’s Law
The field is concentric circles centred on the wire; strongest near the wire.
This magnetic field is in addition to; electric field produced by the
charges.
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Oersted’s Law
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Oersted’s Law
To determine the direction of the magnetic field around a wire; use Oersted’s right hand rule.
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Oersted’s Law
Grab the wire with your right hand,Thumb in the direction of the conventional current, I; (i.e. +ive to -ive),
Field is in the direction of; curl of your fingers.
Oersted's Law
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Oersted’s Law
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Oersted’s Law
If the wire is shown as vertical; the field is hard to draw, it will be into and out of the page.
Crosses (x) are used to show the field; directed into the page.
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Oersted’s Law
Dots () are used to show the field; directed out of the page.
Think of an arrow; when it is flying towards you, see a point (out of the page).
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Oersted’s Law
When it is flying away from you; you will see the feathers, cross (into the page).
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Oersted’s Law
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Oersted’s Law
This concept can also be used to illustrate current. Current flow through a straight wire
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Oersted’s LawTo increase the strength of the field increasing the current; the wire can be bent into a loop.
Current flow through a circular coil
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Oersted’s Law
To further increase the strength of the field at the centre of the loop; several loops are used instead of the
single wire, to form a flat coil.
Each loop of current carrying wire contributes; to a stronger magnetic field.
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Oersted’s LawCurrent flow through a solenoid
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Magnetic Force Around a Current-Carrying Conductor
When a current carrying wire is placed in a magnetic field; the two magnetic fields interact.
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Magnetic Force Around a Current-Carrying Conductor
The two fields are; the permanent field around the
magnet, the field created around the wire.
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Magnetic Force Around a Current-Carrying Conductor
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Magnetic Force Around a Current-Carrying Conductor
When the supply is switched on; wire is pushed out until it no longer
touches the mercury, breaks the current, falls back.
It then makes contact again; again is forced out.
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Magnetic Force Around a Current-Carrying Conductor
This process repeats and the wire; continues to bounce in and out.
This is due to the two interacting magnetic fields producing; a resultant force, at right angles.
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Magnetic Force Around a Current-Carrying Conductor
This shows that the current; the applied magnetic field, and the force on the wire, mutually perpendicular.
Fleming’s right hand rule shows this.This is also called; right hand palm rule.
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Magnetic Force Around a Current-Carrying Conductor
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Magnetic Force Around a Current-Carrying Conductor
The experiment shown above can also be used to show; the force will increase, when any of the following factors
increase:
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Magnetic Force Around a Current-Carrying Conductor
The current, I, in the wire.The length, l, of the wire.The strength of the external magnetic field (B).
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Magnetic Force Around a Current-Carrying Conductor
Vector B, the direction of which is the same as the magnetic field lines; describes the strength of the
magnetic field.
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Magnetic Force Around a Current-Carrying Conductor
Several names are given to B including; magnetic induction, magnetic field intensity, magnetic flux density.
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Magnetic Force Around a Current-Carrying Conductor
The S.I. units for magnetic induction B are the tesla (T); or weber per square metre (Wbm-2).
The magnetic induction of the Earth’s field; is approx. 10-4 T.
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Magnetic Force Around a Current-Carrying Conductor
Electromagnets have values; in the order of 2 T;
Superconducting magnets have values; around 10 T.
The relationship between the above quantities is:
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Magnetic Force Around a Current-Carrying Conductor
F = BIl F is the force on the wire,
in newtons, I is the current flowing in the wire,
in amperes,
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Magnetic Force Around a Current-Carrying Conductor
B is the magnetic induction of the magnetic field, in tesla,
l is the length of wire in the magnetic field, in metres.
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Magnetic Force Around a Current-Carrying Conductor
This formula only applies when; wire and the magnetic field is
perpendicular.
If the angle is reduced; force will be reduced.
The formula is more correctly shown by:
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Magnetic Force Around a Current-Carrying Conductor
F = BIlsin is the angle; between the wire, and the magnetic field.
Note sin is at a maximum when; = 90o, ie when B and I are perpendicular.
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Magnetic Force Around a Current-Carrying Conductor
The quantity Il is known as; the current element.
Rearranging the formula above; to make B the subject of the
equation:
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Magnetic Force Around a Current-Carrying Conductor
This leads to the definition of B
lIF
B
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Magnetic Force Around a Current-Carrying Conductor
The magnitude B of a magnetic field is defined as the force per current element placed at right angles to the field.The direction of magnetic induction is perpendicular to both the force and the current element.
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Magnetic Force Around a Current-Carrying Conductor
The direction is given by the right hand rule described above.Force is measured in; newtons (N);
Electric current, in amperes (A),
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Magnetic Force Around a Current-Carrying Conductor
Length of wire, in metres (m),
Magnetic induction, in tesla (T).
1 T = 1 N A-1 m-1.
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Magnetic Force Around a Current-Carrying Conductor
Try Example 1
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Solution
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Solution
B = 4.5 T upI = 0.3 A eastl = 2.0 m = 90o
F = BIlsinF = 4.5 x 0.3 x 2 x sin90o
F = 2.7 N
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SolutionUse the right hand rule to determine the direction.
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Solution
Palm faces out of the page.Direction is southF = 2.7 N south
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Magnetic Force Around a Current-Carrying Conductor
Try Example 2
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Solution
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Solution
I = 0.1 Al = 0.1 mg = 9.8 ms-1
m = 2 g = 0.002 kg
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Solution
For the wire to be supported against gravity, |FB | = | Fg |
BIl sin = mg
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Solution
Use the right hand rule to determine the direction.B = 2.0 T to the right
sinlI
mg
B
1 x 0.1 x 0.1
9.8 x 002.0B
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Moving Coil Loudspeaker
The principle of a moving coil loudspeaker is that; a coil carrying an electric current, oscillating with amplitude, and frequency, proportional to the sound to be
produced, is suspended in a uniform magnetic
field.
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Moving Coil Loudspeaker
The magnetic force on the oscillating current; drives the coil in and out, through the field.
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Moving Coil Loudspeaker
A cone is attached to the coil; the movement of the cone back and forth, sets up compressions and rarefactions, in the adjacent air, creating a sound wave.
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Moving Coil Loudspeaker
Motion of LoudspeakerLoudspeaker Animation
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Components
Frame (or housing or basket) Stationary Component
Generally an open metal frame; supporting the cone, with a magnetic structure attached, at the centre rear.
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Components
Cross section of a Speaker
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Components
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Components
Magnetic Structure - Stationary ComponentA fixed permanent magnet provides; magnetic field,
in almost all modern designs.
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Components
The diagrams above and below show; a ring magnet providing the magnetic
field, with one polarity on each flat face of
the ring.
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Components
Magnetic poles are induced in soft iron pole pieces; as shown below.
Resulting in north and south magnetic poles; on opposite sides of a small air gap.
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Components
The voice coil moves; in the approximately uniform
magnetic field, in the air gap.
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Components
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Components
Cone (or diaphragm) Moving Component
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Components
A conical surface; made of light but rigid material,
such as paper pulp, or plastic, even Kevlar
that sets the air vibrating.
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Components
Voice Coil Moving Component
A coil of wire wound on a lightweight tube; passes through the gap, in the magnetic structure.
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Components
The voice coil is attached to the cone; wires from the two ends of the coil
terminate in, electrical connections on the frame.
Aluminium wire is often used; because it is much lighter than
copper.
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Components
Inner Suspension or Spider
A flat ring of springy material.The outside edge of the ring is connected to the frame; inside edge is connected to the voice
coil, where it joins the cone.
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Components
The purpose of the inner suspension; to hold the cone centrally in the
frame, while allowing it to move in and out
freely, without rocking.
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Components
Outer Suspension Moving Component
Performs a similar function to the inner suspension; but between the outer edge of the
cone and the frame.
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Components
Centre Cap and Dust Dome Moving Component
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Components
Moulded paper or aluminium; covers the centre of the cone at the
end of the voice coil, to prevent dust from entering the
gap, in the magnet assembly.
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Components
It also adds to the moving components; acts as an extension of the cone, moving air to create sound.
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Action of a Loudspeaker
The two ends of the voice coil are connected; to the output terminals of an
amplifier.
Across the terminals, is a P.D.; oscillates in proportion to the sound
waveform.
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Action of a Loudspeaker
This produces an oscillating current in the voice coil.To examine this, we will look at in two stages.The first being when the P.D. is constant.
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Action of a Loudspeaker
A constant P.D. gives rise to; a constant I from V = IR.
The magnetic force is determined by; F = BIl.
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Action of a Loudspeaker
The direction is determined by the right hand rule.Assume the current is at first, into the page.
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Action of a Loudspeaker
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Action of a Loudspeaker
The voice coil moves in the direction of; the magnetic force.
The inner and outer suspensions; oppose the motion and, exert a restoring force.
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Action of a Loudspeaker
Eventually the forces have equal but opposite magnitude and; the voice coil no longer moves.
The rest position is approximately given by; F = BIl.
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Action of a Loudspeaker
As F I and V IF V.If the P.D. is reversed; hence the current.
The force and displacement; are also reversed.
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Action of a Loudspeaker
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Action of a Loudspeaker
Effect of Changing the CurrentSummary of How it WorksIt appears from above that the P.D. is; proportional to the cone
displacement, therefore is proportional to the sound
waveform.
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Action of a Loudspeaker
There are some factors that prevent this from being true.The skill in producing a good speaker is to reduce these factors.These factors, are outside the scope of this course.