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1. 6.3 Magnetic Force and Field
2. Magnetic PolesEvery magnet has two poles (North & South)
and is therefore called a DipoleUnlike Electric Fields it is
impossible to have a Monopole.If you cut a magnet in half you end
up with another dipole. N S N S N S Unlike poles attract, Like
poles repel. N S N S N S S N These magnets will turn so that UNLIKE
poles come together.
3. Earths Magnetic FieldBecause magnets will turn so that
UNLIKE poles come together, the polesare really called North
seeking poles or South seeking polesCompasses containsmall magnets
whichturn towards theEarths poles. N S
http://phet.colorado.edu/en/simulation/magn
ets-and-electromagnets
4. Magnetic Field LinesMagnetic FieldThis is a region of space
where a test magnet experiences a turning force
http://www.walter-fendt.de/ph14e/mfbar.htmField LinesThey point
from the North Pole to the South Pole.
5. Magnetic Flux Density (B)The strength of the magnetic field
seemslinked to the density of the magnetic fieldlines.There is a
stronger field at the poles wherethere are more field
lines.Magnetic Flux Density (B)This is the equivalent of:g for
Gravitational Fields (Nkg-1)E for Electric Fields (NC-1)The unit of
Magnetic Flux Density (B) is the Tesla (T) and like the otherfield
strengths it is a Vector.A good way to think about it is that it is
just a measure of how many FieldLines there are in a certain area.
A magnetic field is often called a B FieldUntil we know more about
Magnetism it isnt possible to define TheMagnetic Field Density (B)
in the same way as we do for GravitationalField Strength (g) and
Electric Field Strength (E)
6. Fields Caused by CurrentsIt turns out that if a smallcompass
is placed near awire carrying a current itexperiences a weak
turningforce.This led scientists to realisethat Magnetism is
actuallycaused by moving charges.The field is strongest closest The
fingers show the direction ofto the wire. the field.The direction
of the field canbe found using the RightHand Corkscrew
Rule.http://www.walter-fendt.de/ph14e/mfwire.htm
7. Field inside a coilWhen a current flows All these circles
add This makes a reallyaround a circular loop together. strong
field in thethe magnetic field centre of the circularforms circles.
loop.
8. Field inside a solenoid A solenoid is a coil of wire,
carrying a current.The field that is createdby a solenoid is just
likethat of a bar magnet butthe field lines gothrough the
centre.
9. Force on a current carrying conductor review
10. Changing the direction of the force reviewThe direction of
the force acting on a wire in anelectromagnetic field can be
reversed by:Reversing the Current Reversing the Magnetic FieldThe
direction of the force is therefore relative to both thedirection
of the magnetic field and the current.
11. Flemings left-hand rule reviewIt is possible to predict the
direction of the force acting on awire its motion if the direction
of the current or themagnetic field are known. Flemings left-hand
rule is usedto do this. thuMb = Motion First finger = magnetic
Field seCond finger = Current
12. Increasing the size of the force review
13. Force in a Magnetic Field equationAs you have just seen the
size of the force depends on:B Magnetic Flux DensityI current in
the wirel length of wireIf the field is not at Right Angles to the
wire then the perpendicular componentof the field is used and the
equation is:
14. Force in a Magnetic Field alternative equation B Magnetic
Flux Density I current in the wire l length of wire 1. 2. 3.
4.
15. Charges in Magnetic Fields . = B Field coming out of page1.
Electrons moving in a wire B Field going away into page = Imagine
an arrow coming towards you or going away from you.In the picture
above, the electron is moving to the right, so Conventional Current
(I) ismoving to the left.From Flemings Left Hand Rule the electron
experiences a force downwards at rightangles to its motion. Its the
sum of all the forces on all the electrons that gives thetotal
force on the wire.2. Electrons moving freely through a magnetic
fieldThe force is always perpendicular to its motion, so itends up
moving in a circle.The Magnetic Field provides the Centripetal
Force.
16. What forces are there between two current carrying
wires?Step 1 What does I2 do to I1 ?Use the Right Hand Corkscrew
rule to see what the field lines do.Step 2 Which way does I1
move?Use Flemings Left Hand Ruleto see what the force is on I1
F
17. Now repeat for the other wire:Force caused by I2 on I1
Force caused by I1 on I2 F F The two wires move together!
18. If the currents are flowing in opposite directions: I IIf
the currents are flowing in opposite directions:
19. What would happen to a coil? What happens to the shape of
the coils?
20. What would happen to a coil?
http://ocw.mit.edu/ans7870/8/8.02T/f04/visualizations/magnetostatics/15
-MagneticForceAttract/MagForceAtt_640.mpg
21. Aurora BorealisKnut Birkeland (18671917) is onthe 200
Norwegian kroner note.He was a Physicist best known forhis studies
on the aurora borealis.Timelapse of the Aurora
http://vimeo.com/16917950
22. Aurora Borealis the PhysicsThe Earth has a magnetic field
caused by currents in its core, which channelscharged particles
from solar flares and from our upper atmosphere towardsthe
poles
23. Aurora Borealis the PhysicsCharged particles from
spaceexperience a force on them fromthe earths magnetic field
whichmakes them spiral around themagnetic field lines and
headtowards the poles.As they meet air molecules theyexcite the
molecules causingthem to give out light.Without the protection of
theearths magnetic field we wouldbe constantly bombarded withhigh
energy particles.This is one of the reasons that Space flight is so
difficult. Astronauts reportwhite flashes in their vision as Cosmic
rays pass straight through their heads.Without shielding missions
to Mars will be impossible.