Transcript y12electro-magnetism-onscreenpresentation2

Electro-magnetism
NCEA AS2.6
Text Chapters:
Magnetism
Permanent or ferromagnetism
Like poles repel,
unlike poles attract
Magnetic field
strength B measured
in Tesla T
Magnetic Fields
Magnetic fields can be represented by
field lines
 Closer together=stronger field
 Point from N to S
Magnetism
Electromagnetism.
 Caused by moving
electric charges.
 The earth has a
magnetic field
around it because
of iron atoms
spinning around
the core.
Electromagnetism

Fields are formed around current
carrying wires
Right Hand Curl Rule
Thumb = points in
direction of current
flow
Fingers curl in
direction of field lines
Magnetic Force
Two parallel current carrying wires
F=kI1I2L/d
Current flowing same direction in both – wires
are attracted
Current flowing opposite directions – wires repel
each other
Solenoids
Fields are
formed in
solenoids or
coils.
Right Hand Curl Rule
Fingers curl in
direction of
current flow
around the coil
Thumb = points
towards the
north pole
Electromagnets
Electromagnets have a range of uses:



Attached to cranes in scrap metal yards
Electric bells
Relay switches
More useful than permanent magnets as they
are stronger and can be switched off
Magnetic force
Force on moving charges - a charged particle
moving in a magnetic field will experience a
force
F=Bqv (sinθ)



B=mag field strength +Q
q= charge
v=velocity
B
F
v
(This formula applies to a small charge moving
at right angles to magnetic field lines – if not
right angles multiply by sinθ)
Right Hand Slap Rule
For a positive charge
moving in a magnetic
field:
 Thumb = direction
of movement
 Fingers = direction
of magnetic field
 Palm = direction of
force acting on
charge
Right hand rule gives direction for positive charge – negative
must be reversed
Force on Moving Charge
This principle can be
used to deflect the
electron beam in a TV
set to make it scan
across the screen
Force on Moving Charge
It can also be used to
determine the mass
of unknown
substances in a
device called a “mass
spectrometer”
Force on Current-carrying Wire
A wire carrying a current will experience a
force when placed into a magnetic field
F=BIL (sinq)
Applies to current flowing in a wire running
at right angles to magnetic field lines. (if
not 90°, then multiply by sinq)
Direction (for conventional current) given
by right hand rule.
Right Hand Slap Rule
For a wire carrying
current in a magnetic
field:

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
Thumb = direction of
current
Fingers = direction of
field lines
Palm = direction of
force on wire
Magnetic Force
This principle can be
used in magnetohydro propulsion
units…
Magnetic Force
And in loud-speakers...
Electric Motors
If magnetic force acts on opposite sides of a
current carrying coil which is mounted on an
axis, a torque is produced which makes the coil
spin
Electric Motors
This idea is the basis
for all devices that run
by an electric motor.
Induction
If a wire is moved
through a magnetic
field then a voltage
can be induced
across the ends the
wire.
If the wire is
connected to a circuit
then current will flow.
The direction of
induced current is
determined by a right
hand rule.
Right Hand Slap Rule
v=direction of wire
movement
B=direction of
magnetic field lines
F= force on a positive
charge (ie direction of
current flow)
Induction
The size of this
induced voltage is
given by:
 V=BvL
 (B=mag field
strength, v=velocity
of movement,
L=length of wire in
field)
 This is known as
Faraday’s Law
Induction
The direction of the induced current is such that
it creates an opposing force on the motion that is
causing it.

This is known as Lenz’s Law
Induced voltage/current can be made larger if:

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The mag field is stronger
The wire is longer
The movement is faster
(Solenoid has an iron core)
Induction
Induction is the principle behind the microphone
Induction
And the dynamo..
Generators
If a coil is spun around inside a magnetic field
then current can be induced that can be used to
operate devices that run on electricity
This is called
generation
Both DC and AC can
be generated
depending on
whether slip rings or
split rings are used
Transformers
These consist of 2
coils wound close to
each other.
Changing the current
in one coil makes the
field around it change.
This changing field
induces current in a
nearby coil.
Transformers
The ratio of the windings
determines how much
voltage/current is induced
The voltage can be
calculated using:
N=number of turns
V=Voltage
P=primary coil
S=secondary coil
Np
Ns

Vp
Vs
Transformers
No transformer is 100% efficient, but
assuming it was:

Power in = Power out
Pin  Pout
V p I p  Vs I s
Transformers
3 Types:
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Step up : Vs > Vp
Step down : Vs < Vp
Isolating (a safety device) : Vs = Vp