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Electromagnetism.
We found out earlier that electric charges
produce electric force fields
 So
what is a magnetic field?
Magnetic Field
 So
how are magnetic fields produced?
This experiment
shows that the moving
electrons in the wire
also produce a……
Magnetic Field
 So
electric charges always produce an
electric field.
 If
the electric charges are moving, they
also produce a magnetic field
Wire with
electrons
flowing OUT
Produces
circular
magnetic field
 GRIP
RULE
 Fingers curl in direction of thing going
round a circle
 Thumb points in the direction of thing
going straight
Left Hand Grip Rule for Wires
Electron
flow
Magnetic
field
What direction
do the electrons
flow?
Left Hand Grip Rule for Coils
Magnetic
field
Electron
flow
GRIP RULE
Fingers curl in direction of thing going round a circle
Thumb points in the direction of thing going straight
What direction is the field in the coil?
Which end is North??
Electrons Moving in Fields
200 Volts
 link
to cathode ray tube
Magnetic Force on Electrons
electron
flow
SLAP
Magnetic force
on electron
(out of page)
Magnetic
field
electron
flow
magnetic
field
FORCE
Find the direction of the Magnetic Force
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Find the direction of the Magnetic Force
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Find the direction of the Magnetic Force
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ELECTRON x
COMING OUT x
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Trajectory of a Moving
Electron
Electrons cutting across a
magnetic field bend in a circle

LINK TO CHARGE IN A MAGNETIC FIELD
 The
size of the magnetic force on a
charge depends on:
 The
strength of the magnetic field.
 The speed of the charge across the
field
 The amount of charge
F  Bvq
Find the size and direction of the
Magnetic Force
3.0x106 ms-1
B = 0.040 T
Find the size and direction of the
Magnetic Force
3.0x106 ms-1
B = 0.040 T
Find the size and direction of the
Magnetic Force
B = 0.040 T
3.0x106 ms-1
 link
to F=Bvq
Magnet on a TV
Aurora, Invercargill
Aurora, Antarctica
LHC Cern
circumference = 27 km
depth= 100 m
Proton Tube
Particle Detector
What keeps the protons going in
a circle?
If the protons go clockwise, what
direction is the magnetic field??
Your Particle Accelerator
CRT. Electrons can be deflected
using electric OR magnetic fields
Colour Monitor
 link
to electrons in a TV
 link
to TV
Ex.
Proton mass = 1.67 x 10-27 kg.
Proton charge = 1.6 x 10-19 C
Circumference = 27 km
calculate the magnetic field strength
required when the protons are travelling
at 0.99c
Extension: there is a flaw in your
calculation, what is it????
 Measuring
the mass of an electron with
a “magic eye tube”
No magnetic field
small magnetic field
large magnetic field
 By
measuring the radius of the curve,
we can calculate the magnetic field
strength.
mv
Bvq 
r
2
Bqr
m
v
Force on a Current Carrying Wire
Electron flow
current flow
1: electrons in roller are moving...
2: causing electrons to be pushed…
3: causing roller to be pushed……
This is called the motor effect
electron motion
(this is the moving wire)
SLAP
Magnetic force on
electron
(this is the current)
Magnetic
field
The size of the electromagnetic force
depends on:
1.
2.
3.
Strength of magnetic field
Current in wire
Length of wire
F  BIl
Applications
 Thousands
of devices use this force to
do something useful, e.g.
 The electric motor in……

The electric motor in……
The speakers in your phone or stereo
ELECTRIC
CURRENT
MOTION
ELECTRIC
MOTOR
Magnet + Wire
current flow
 link
to Fendt Electric Motor
Parts of a Motor
World’s Simplest Motor
+
e- flow
Mag Field
Loudspeaker
The Current into a Speaker is
Alternating
S
N
AC from amplifier
N
S
S PUSH
N
Electron flow
AC from amplifier
S
S
N
N
PULL
AC from amplifier
S
N
Electromagnetic Induction
 We
have seen that……
ELECTRIC
CURRENT
MOTION
ELECTRIC
MOTOR
Magnet + Wire
 Do
you think that……
If you move a wire through a magnetic
field you can produce electric current?
MOTION
ELECTRIC
MOTOR
Magnet + Wire
ELECTRIC
CURRENT

Here’s a hint
 Tyger
Tyger, burning bright,
In the forests of the night;
What immortal hand or eye,
 Could frame thy fearful symmetry

Remember this?
Electron motion
Force
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How about moving a wire full of
electrons across a magnetic field?
Move the
wire
Force on
electrons
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Electron
xmotion
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There is a voltage induced
across the wire
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Force
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electrons
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Electron
motion
This is called………
Electromagnetic Induction
It is used to generate almost all the
electricity you use.

It is also used in things like ……
Voltage Induced in Moving Wire
Y
X
1: electrons in roller are moving
2: causing them to be pushed
3:The bottom end (X) becomes negative (Y becomes positive)
4:There is an induced voltage

1.
2.
3.
The size of the induced voltage
depends on:
Strength of magnetic field
Speed or wire across the magnetic field
Length of wire
V  Bvl
 link
to Fendt Generator
Motor Effect and Generator Effect
Electron flow
Force on roller
1: electrons in roller are moving
2: causing them to be pushed
3: electrons in roller are now flowing
4: causing them (and roller) to be pushed
diagram shows conventional current
Microphone
S
N
AC to amplifier
S
N
Sound waves
vibrate coil
This causes alternating current
 We
have seen that……
ELECTRIC
CURRENT
MOTION
ELECTRIC
MOTOR
Magnet + Wire
MOTION
Generator
Magnet + Wire
ELECTRIC
CURRENT
ALTERNATING
CURRENT
SOUND
SPEAKER
Magnet + Wire
ALTERNATING
CURRENT
SOUND
MICROPHONE
Magnet + Wire
 Extension
 James
Clerk Maxwell showed that……
 a changing electric field created a
changing magnetic field.
 and
a changing magnetic field created a
changing electric field.
 he
worked out these fields would
travel through space at a speed of
……… 3.0 x 108 ms-1