Post on 24-Feb-2016
description
ELECTROMAGNETISMNCEA 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
geographic north
magnetic north
s
N
MAGNETS AND FIELDS
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
N S
Professor Hans Christian
Oersted
1820Professor 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.
MAG FIELDS AND CURRENT
B
I
Oersted noticed that if the current was running in one direction the red part of the needle pointed toward the wire.
I
B
However if the current flowed in the opposite direction then the red part of the needle pointed way from the wire
RIGHT HAND GRIP RULE:
I
From this observation a rule was developed
Thumb = direction of current.Fingers = direction of mag field lines
RH GRIP RULE
current outwards current inwards
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Current is coming out of the page
Current going into the page
Use thumb and fingers of right hand grip rule to work out direction of field.
Complete exercises
from Page 180 – 181
Rutter
3 things effect size of magnetic field
How do we increase the magnetic field generated by the
current?
SOLENOIDS
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
1. More current2. More wires3. Iron core inserted
into coil
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.
length l
I
Do Exercise Page 182 Rutter
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
Can we get things to move by using two
magnetic fields?Answer: Of course we can
12
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Interaction between a magnet and the field due to a current
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N S
F
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.
How do I know which way the force is acting?
RIGHT HAND SLAP RULE
Palm = Force direction
Thumb = I directionFingers = field direction
Right Hand Slap Rule
Try the exercises on page 183 –
184Rutter
D.C. MOTOR
NA
B
D
C
I
S
brushes
EVERY TIME THE COIL PASSES THROUGH THE VERTICAL POSITION, THE COMMUTATOR REVERSES THE CURENT IN THE COIL, IN ORDER TO SUSTAIN THE
ROTATION
C
A
B
SN
D
I
Split ring commutator
The DC Motor
How big is the force?
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 = 0
F = max
Now try pages 185 – 8
Rutter
How to build a simple motor
FORCE ON A CHARGEWe have two types of charges:1. Positive and2. negative
Thus we use different slap rules for each when a charge enters a magnetic field1. Positive = RH slap rule2. 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)
I
FF
Charged particle moving through a magnetic field
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
Hey, Mr Lees how much voltage is generated
when a magnet moves through a coil?
The variables that are involved are:
1. Size of the magnets magnetic field (B)
2. Speed of movement (v)3. 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.
How a hydroelectric
dam uses induction to
make electricity?
BENMORE DAM NZ
Use the diagram opposite to help you describe how New Zealanders have embraced the Physics principles of “Electromagnetism” to generate electricity from water?
OPEN ENDED ESSAY QUESTION
Try Pages 190 – 193 Rutter
IDEAS THAT CAN BE USED FOR THIS QUESTION
The Solenoid
Induction
Water Height
Inside a generator