1. 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

2. MAGNETS AND FIELDS s magnetic north N geographic north

3. 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

4. Hans Christian Oersted
MAG FIELDS AND CURRENT
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.
Professor
Hans Christian Oersted

5. Oersted noticed that if the current was running in one direction the red part of the needle pointed toward the wire. B I

6. However if the current flowed in the opposite direction then the red part of the needle pointed way from the wire B I

7. From this observation a rule was developed
RH GRIP RULE
From this observation a rule was developed I
Thumb = direction of current.
Fingers = direction of mag field lines
RIGHT HAND GRIP RULE:

8. Complete exercises from Page 180 – 181-
current outwards
current inwards
Complete exercises from Page 180 – 181
Rutter
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.

9. SOLENOIDS 3 things effect size of magnetic field
How do we increase the magnetic field generated by the current?
Answer: Use more wires
This led to the production of the SOLENOID
3 things effect size of magnetic field
More current
More wires
Iron core inserted into coil
This combined the magnetic field produced in each wire together producing a larger field

10. Do Exercise Page 182 Rutter
length l
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. I
Do Exercise Page 182 Rutter

11. MOTOR EFFECT What happens if two magnetic fields interact?
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. Interaction between a magnet and the field due to a currentS N S

13. 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.

14. RIGHT HAND SLAP RULE Try the exercises on page 183 – 184 Rutter
How do I know which way the force is acting?
Palm = Force direction
Try the exercises on page 183 – 184
Rutter
Fingers =
field direction
Thumb = I direction
Right Hand Slap Rule

15. D.C. MOTOR A B D C C A B S N D N S I I brushes Split ring commutator
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

16. How to build a simple motor
Now try pages 185 – 8 Rutter
How big is the force?
This can be calculated using the following equation:
F = BILsin
Note:
F = 0
Where:
F = size of the force (N)
B = magnetic field strength (T)
I = current (A)
 = angle made between two mag fields
F = max

17. FORCE ON A CHARGE F = Bvq F = Bev We have two types of charges:
Positive and
negative
Two equations can be utilised to determine the force acting on each charge
Thus we use different slap rules for each when a charge enters a magnetic field
Positive = RH slap rule
Negative = LH slap rule
F = Bvq
Note:
If the field is large enough the charge will travel in a complete circle.
F= mv2/r can be used
F = Bev
Where:
B = mag field strength (T)
v = velocity (ms-1)
q = charge (C)
e = charge of electron (C)

18. Charged particle moving through a magnetic field
THE FORCE IS ALWAYS PERPENDICULAR TO THE MOTION F F
THE PARTICLE MUST MOVE ALONG AN ARC OF A CIRCLE I
Try Pages 188 – 190 Rutter

19. ELECTROMAGNETIC INDUCTION Demo of electromag 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
So what is different?
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.
Demo of electromag induction

20. The variables that are involved are:
Hey, Mr Lees how much voltage is generated when a magnet moves through a coil?
The variables that are involved are:
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)

21. How a hydroelectric dam uses induction to make electricity?
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?

22. 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

23. IDEAS THAT CAN BE USED FOR THIS QUESTION
The Solenoid
Induction
Inside a generator
Water Height