ELECTROMAGNETISM
NCEA LEVEL 2
PHYSICS
•
•
•
•
•
•
•
•
Magnets and fields
Mag fields and current
Right hand grip rule
Solenoids
Motor Effect
DC Motor
Force on a charge
Induction
geographic
north
MAGNETS AND FIELDS
geographic
north
magnetic
north
N
S
Magnetic field lines ALWAYS run north to south.
The number of magnetic field lines denotes the strength of
the magnetic field (B). The more lines the stronger the
magnet
B = number of field lines in
units called Tesla (T) after
Nikoli Tesla
MAG FIELDS AND CURRENT
Professor
Hans Christian
Oersted
1820
Professor Oersted was
demonstrating an
experiment for students
when he accidentally
discovered that a compass
needle moved when it
was close to a wire
connected to a battery.
Oersted noticed that if the current was running in
one direction the red part of the needle pointed
toward the wire.
B
I
However if the current flowed in the opposite direction then the red part of the
needle pointed way from the wire
B
I
From this observation a rule
was developed
RH GRIP RULE
I
Thumb = direction
of current.
RIGHT HAND GRIP RULE:
Fingers = direction
of mag field lines
-
current outwards
current inwards
Current is coming out of the
page
Current going into the
page
Complete
exercises
from Page
180 – 181
Rutter
Use thumb and fingers of right hand grip rule to work out
direction of field.
SOLENOIDS
How do we increase
the magnetic field
generated by the
current?
Answer: Use more wires
This led to the production of
the SOLENOID
This combined the magnetic
field produced in each wire
together producing a larger
field
3 things effect size of
magnetic field
1. More current
2. More wires
3. Iron core inserted
into coil
length l
I
Do Exercise Page 182
Rutter
The field lines of each
wire interact to increase
the overall strength of
the field. Again use
thumb and fingers of
the right hand grip rule
to calculate which way
the field runs. NORTH
always has field lines
coming out of the
solenoid.
What happens if two
magnetic fields
interact?
MOTOR EFFECT
Answer: Unbalanced forces are produced
Hang on unbalanced
force must produce
motion. Newton told
us this!
F=ma
Answer: Of course we can
Can we get things to
move by using two
magnetic fields?
Interaction between a magnet and the field due to a current
12
N
S
N
S
F
N
S
The interaction of 2 fields. One from the magnet and one
from the current carrying wire interact. Where they are
going in the same direction they add together in strength.
Where they go in opposite directions they cancel out. A
force is then applied in the direction of the greatest force.
In this case upwards.
RIGHT HAND SLAP RULE
How do I know which way
the force is acting?
Palm = Force direction
Fingers =
field direction
Thumb = I direction
Right Hand Slap Rule
D.C. MOTOR
B
C
C
I
I
N
S
A
brushes
B
N
D
S
D
Split ring
commutator
A
The DC
Motor
EVERY TIME THE COIL PASSES THROUGH THE VERTICAL POSITION, THE
COMMUTATOR REVERSES THE CURENT IN THE COIL, IN ORDER TO SUSTAIN THE
ROTATION
How to build a simple motor
How big is
the force?
Now try
pages 185 – 8
Rutter
This can be calculated using the
following equation:
F = BILsin
Where:
F = size of the force (N)
B = magnetic field strength (T)
I = current (A)
 = angle made between two mag fields
Note:
F = max
F=0
FORCE ON A CHARGE
We have two types of charges:
1. Positive and
2. negative
Thus we use different slap rules for each
when a charge enters a magnetic field
1. Positive = RH slap rule
2. Negative = LH slap rule
Note:
If the field is large enough the
charge will travel in a complete
circle.
F= mv2/r can be used
Two equations can be utilised
to determine the force acting
on each charge
F = Bvq
F = Bev
Where:
B = mag field strength (T)
v = velocity (ms-1)
q = charge (C)
e = charge of electron (C)
Charged particle moving through a magnetic field
F
F
I
THE FORCE IS ALWAYS
PERPENDICULAR TO THE
MOTION
THE PARTICLE MUST
MOVE ALONG AN ARC
OF A CIRCLE
Try Pages 188 – 190
Rutter
ELECTROMAGNETIC INDUCTION
If using electricity and
magnetism can generate
movement can it work the
other way?
ANS: Yes, we call it a generator and it
works by electromagnetic induction
ANS: If we change the movement of
charge to kinetic movement then the
charges are still moving and so a
magnetic field is still generated. This
means two sets of fields will interact.
This means a force will be produced
which acts on the only thing that can
move, the charge along the wire.
So what is
different?
Demo of
electromag
induction
The variables that are
involved are:
1.
2.
3.
Hey, Mr Lees how much
voltage is generated
when a magnet moves
through a coil?
Size of the magnets
magnetic field (B)
Speed of movement (v)
Amount of wire (l)
This results in:
V = Bvl
Where:
V = Voltage (V)
B = magnetic field strength (T)
l = length of wire (m)
The major use of this
phenomenon is in hydro-electric
dams in the production of
electricity.
BENMORE DAM NZ
How a
hydroelectric
dam uses
induction to
make
electricity?
OPEN ENDED ESSAY QUESTION
Use the diagram
opposite to help you
describe how New
Zealanders have
embraced the Physics
principles of
“Electromagnetism” to
generate electricity
from water?
Try Pages 190 – 193
Rutter
IDEAS THAT CAN BE USED FOR THIS QUESTION
The Solenoid
Induction
Inside a
generator
Water Height