6.3 - Magnetic Force and Field


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6.3 - Magnetic Force and Field

  1. 1. 6.3 – Magnetic Force and Field
  2. 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. 3. Earth’s Magnetic FieldBecause magnets will turn so that UNLIKE poles come together, the polesare really called ‘North seeking poles’ or ‘South seeking poles’Compasses containsmall magnets whichturn towards theEarth’s poles. N S http://phet.colorado.edu/en/simulation/magn ets-and-electromagnets
  4. 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. 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 Field’Until we know more about Magnetism it isn’t possible to define TheMagnetic Field Density (B) in the same way as we do for GravitationalField Strength (g) and Electric Field Strength (E)
  6. 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. 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. 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. 9. Force on a current carrying conductor review
  10. 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. 11. Fleming’s 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. Fleming’s left-hand rule is usedto do this. thuMb = Motion First finger = magnetic Field seCond finger = Current
  12. 12. Increasing the size of the force review
  13. 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. 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. 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 Fleming’s Left Hand Rule the electron experiences a force downwards at rightangles to it’s motion. It’s 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 it’s motion, so itends up moving in a circle.The Magnetic Field provides the Centripetal Force.
  16. 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 Fleming’s Left Hand Ruleto see what the force is on I1 F
  17. 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. 18. If the currents are flowing in opposite directions: I IIf the currents are flowing in opposite directions:
  19. 19. What would happen to a coil? What happens to the shape of the coils?
  20. 20. What would happen to a coil? http://ocw.mit.edu/ans7870/8/8.02T/f04/visualizations/magnetostatics/15 -MagneticForceAttract/MagForceAtt_640.mpg
  21. 21. Aurora BorealisKnut Birkeland (1867–1917) 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. 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. 23. Aurora Borealis – the PhysicsCharged particles from spaceexperience a force on them fromthe earth’s 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 theearth’s 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.