Magnetism. Magnets Source of magnetism. Magnetic field. Magnetic force

Size: px

Start display at page:

Download "Magnetism. Magnets Source of magnetism. Magnetic field. Magnetic force"

Madlyn Harmon
5 years ago
Views:

1 Magnetism Magnets Source of magnetism Magnetic field Magnetic force

2 Magnets and magnetic force Historical First magnets were pieces of iron-bearing rock called loadstone (magnetite, Fe 3 O 4 ) found originally in Asia (magnesia) Iron is one of a few materials (also ickel, Cobalt) that can be permanently magnetised. These are called ferromagnetic materials avigational compass developed in 12 th century

3 Magnets and Magnetic Force All magnets have two poles, orth and South. named with reference to alignment in Earth s magnetic field. S Magnetic pole that points to Earth s geographic north pole is called the north pole of the magnet S S S S

4 Magnets and magnetic force A magnetic field surrounds the magnet. Lines indicate direction and magnitude of magnetic field When two magnets are near to each other or in contact, they exert a force on one another, in a manner similar to electrical charges. This force is the magnetic force. F S S F S F S F Like poles repel, unlike poles attract.

5 Earth s Magnetic Field South magnetic pole Rotation Axis 11 orth Geographic Pole Earth s magnetic field pattern similar to that of a bar magnet. orth Geographic Pole is approximately 2,000km from the south magnetic pole

6 Earth s Magnetic Field South magnetic pole Rotation Axis 11 orth Geographic Pole Earth s Magnetic Field Caused by circulating currents of molten iron etc in the Earth s core Magnetic poles change position with time Complete reversal of direction 850,000 yrs ago Evidence: ancient magnetic fields left in rocks Supports continental drift theory Paleomagnetism: residual magnetism in ancient rocks past movement of tectonic plates

7 Compass needle Essentially small bar magnet free to rotate in horizontal plane If suspended on bearing such that it can rotate also in vertical plane: As needle is moved northward from the equator it will rotate in the vertical plane increasingly towards the surface of the Earth Angle between the horizontal and the direction of the magnetic field is called the angle of dip

8 avigation Humans: Compass needles provide directional information umber of animals also use magnetism for navigation Built in directional sensor Some bacteria evolved the facility to produce minute magnetic crystals of Fe 3 O 4 --used for guidance Fe 3 O 4 crystals of length 50nm Pigeons,Dolphins, bees etc navigate using internal magnetic needles (magneto-tactic bacteria) and the Earth s magnetic field (small)

9 avigation Magneto-tactic bacteria Magnetic crystal chain in bacteria found in orthern Hemisphere have opposite polarity to those found in Southern Hemisphere orthern Hemisphere: Earth s field has downward component Bacteria migrate along the direction of the Earth s field Southern Hemisphere: Earth s field has upward component pond pond Bacteria migrate opposite to magnetic field Why: Deep water: more food, less toxic Oxygen. S S

Source of magnetism Permanent magnets are not the only items with magnetic properties Example: When a current passes through a simple wire, a magnetic field is created around the wire, this is due

10 Source of magnetism Permanent magnets are not the only items with magnetic properties Example: When a current passes through a simple wire, a magnetic field is created around the wire, this is due to the flow of the electrons. I Magnetic field lines Direction of field given by right hand rule motion of electrical charges (current) is the only source of magnetism. What about permanent magnets?

11 Source of magnetism The microscopic origin of the magnetism in magnets; Simple view Atom Electrons are moving around the nucleus: electrons orbiting constitute circular current loop electrons generates a tiny magnetic field

12 Source of magnetism Spinning electrons? Electrons also act as though they are spinning about an axis through their centres. Spinning electron also act like a current loop so creates a tiny magnetic field Both these electron motions in atoms, orbital and spins create magnetic fields.

13 Source of magnetism In most atoms the magnetic effect of the electrons cancel each other out In ferromagnetic materials the magnetic effects of the electrons do not fully cancel each other out, atoms then are like tiny magnets. Ferromagnetic materials consists of small regions (called domains) in which all the magnetic effects of atoms are aligned. domains Disorder: non-magnetic magnetised Externally applied magnetic field can align the domains so that the material becomes magnetised. S

14 Magnetisation (Ferromagnetic) metals (iron most often), can be magnetised under an external magnetic field Applications Electromagnet

15 Dia/Para-magnetism In the presence of an external magnetic field three types of materials exist: Ferromagnetic Under an external field, material produces magnetic field in the same direction as external field. Material retains field after external field is switched off Paramagnetic Under an external field, material produces magnetic field in the same direction as external field. Much weaker than ferromagnetism. Field is not retained. Diamagnetic Under an external field, material produces magnetic field in the opposite direction of external field. Field is not retained. e.g. levitating frog

16 Magnetic force Charged particle moving through a magnetic field with velocity (v) experiences a magnetic force (F) B acting on it q + q + q + v θ v F=qvBsinθ F = 0 v F = maximum =qvb o movement no force. Minimum value of force: (zero) direction of charged particle is parallel to magnetic field lines Maximum value of force: direction of charged particle is perpendicular to magnetic field lines F = qvb Sinθ B: magnetic field strength B F qv sin θ = Units of B = Tesla ( T ) 1 Cms

Magnetic force Direction of force on the charged particle: always perpendicular to direction of both v and B F V + θ B Right hand rule may be used to determine the direction of the force on a moving

17 Magnetic force Direction of force on the charged particle: always perpendicular to direction of both v and B F V + θ B Right hand rule may be used to determine the direction of the force on a moving charge in a magnetic field positive charge moving with velocity v in magnet field B Thumb points in the direction of current. Index finger points in direction of magnetic field Middle finger points in direction of force Important: Direction of current opposite to motion of negative charges (e.g. electrons)

18 Magnetic force Find the direction and magnitude of the magnetic force on a proton travelling at a speed of 7.5x10 6 ms -1 in a direction at 50 to the direction of the magnetic field B of magnitude 2 Tesla. Calculate the initial acceleration of the proton. Mass of proton 1.67x10-27 kg z F Direction of force given by right hand rule v x + 50 B F = qvb sinθ F = ( C)( ms )(2.0 T ) sin y F = Initial acceleration F = ma a 13 F = = = m kg ms 14 2

19 Magnetic force Vector notation: How to indicate the third dimension in 2D sketches. Vector pointing out of the plane (tip of the arrow) Vector pointing into the plane (tail of the arrow) e.g. positron (positively charged) moving through an uniform magnetic field e + B v F

20 Motion of charged particle in a magnetic field Charged particle moves in a plane perpendicular to a uniform magnetic field Positively charged particle Magnetic field at 1 moving with velocity v x x x x x x x x x x v Magnetic force F at x x x right angles to v x x x x F x 2 x x x x x x Magnetic force F x always perpendicular to v x x x can t change magnitude of v x x x x magnitude v constant x x + x 1 v Change direction of v only x x x B x Particle moves under the influence of force of constant magnitude that is always perpendicular to the velocity Path of particle is a circle + Centripetal acceleration v 2 /R 2 v F = qvb = m R R = R radius of circle mv qb

21 Example: Mass Spectrometer Two ionised isotopes, hydrogen and deuterium, with an initial speed of 1.3x10 6 ms -1 pass into a magnetic field of strength 0.15 T as shown. Calculate the distance between them when they strike the image plate. x x x x x x x mv x x x x R = B x x x x x x x qb x x x x Image plate Mass (hydrogen nucleus) proton =1.67x10-27 kg x x x slit Mass (deuterium nucleus) proton + neutron =3.34x10-27 kg R H m v H ( kg)(1.3x10 ms ) = = = 19 qb ( C)(0.15 T ) 0.09m R D mv D ( kg)(1.3x10 ms ) = = = 19 qb ( C)(0.15 T ) 0.18m Distance =2R -2R = 0.18m D H

CHAPTER 20 Magnetism

CHAPTER 20 Magnetism Units Magnets and Magnetic Fields Electric Currents Produce Magnetic Fields Force on an Electric Current in a Magnetic Field; Definition of B Force on Electric Charge Moving in a Magnetic