Electro-magnetism NCEA AS2.6 Text Chapters:

Magnetism Permanent or ferro-magnetism Earths field shape, causing by currents in core Permanent or ferro-magnetism Like poles repel, unlike poles attract Magnetic field strength B measured in Tesla T

Magnetic Fields Magnetic fields can be represented by field lines Closer together=stronger field Point from N to S

Magnetism Electromagnetism. Caused by moving electric charges. B=kI/d (k=2x10-7NA-2) Demo with “wire” and charm compasses Coils can have an iron core inserted into them and become electromagnets, very strong, able to be turned on and off. Uses, MRI scanners, scrap metal yards, relay switches Demo with electric bell Right hand rules for wires and solenoids Practice questions 10A Problem sheet Electromagnetism. Caused by moving electric charges. The earth has a magnetic field around it because of iron atoms spinning around the core.

Electromagnetism Fields are formed around current carrying wires

Right Hand Curl Rule Thumb = points in direction of current flow Fingers curl in direction of field lines

Magnetic Force Two parallel current carrying wires F=kI1I2L/d Transmission Lines Qu 10D No.3,4 Two parallel current carrying wires F=kI1I2L/d Current flowing same direction in both – wires are attracted Current flowing opposite directions – wires repel each other

Solenoids Fields are formed in solenoids or coils.

Right Hand Curl Rule Fingers curl in direction of current flow around the coil Thumb = points towards the north pole

Electromagnets Electromagnets have a range of uses: Attached to cranes in scrap metal yards Electric bells Relay switches More useful than permanent magnets as they are stronger and can be switched off

Magnetic force TV tubes Detection of radioactive particles Qu 10C Force on moving charges - a charged particle moving in a magnetic field will experience a force F=Bqv (sinθ) B=mag field strength q= charge v=velocity (This formula applies to a small charge moving at right angles to magnetic field lines – if not right angles multiply by sinθ) +Q v F B

Right Hand Slap Rule For a positive charge moving in a magnetic field: Thumb = direction of movement Fingers = direction of magnetic field Palm = direction of force acting on charge Right hand rule gives direction for positive charge – negative must be reversed

Force on Moving Charge This principle can be used to deflect the electron beam in a TV set to make it scan across the screen

Force on Moving Charge It can also be used to determine the mass of unknown substances in a device called a “mass spectrometer”

Force on Current-carrying Wire Demo with wire in field Diagram showing the addition of fields Current balances to measure field strength. Qu 10B A wire carrying a current will experience a force when placed into a magnetic field F=BIL (sinq) Applies to current flowing in a wire running at right angles to magnetic field lines. (if not 90°, then multiply by sinq) Direction (for conventional current) given by right hand rule.

Right Hand Slap Rule For a wire carrying current in a magnetic field: Thumb = direction of current Fingers = direction of field lines Palm = direction of force on wire

Magnetic Force This principle can be used in magneto-hydro propulsion units…

Magnetic Force And in loud-speakers...

Electric Motors Qu 10D No. 1, 2, Handout on motors Oht Demo with ruler and boxes Split rings/commutator If magnetic force acts on opposite sides of a current carrying coil which is mounted on an axis, a torque is produced which makes the coil spin

Electric Motors This idea is the basis for all devices that run by an electric motor.

Induction Demo with magnet, coil and galvanometer If a wire is moved through a magnetic field then a voltage can be induced across the ends the wire. If the wire is connected to a circuit then current will flow. The direction of induced current is determined by a right hand rule.

Right Hand Slap Rule v=direction of wire movement B=direction of magnetic field lines F= force on a positive charge (ie direction of current flow)

Induction The size of this induced voltage is given by: V=BvL (B=mag field strength, v=velocity of movement, L=length of wire in field) This is known as Faraday’s Law

Induction Prevents violation of cons of energy law Qu 11A How voltage is induced in terms of forces on charges Qu 11B The direction of the induced current is such that it creates an opposing force on the motion that is causing it. This is known as Lenz’s Law Induced voltage/current can be made larger if: The mag field is stronger The wire is longer The movement is faster (Solenoid has an iron core)

Induction Induction is the principle behind the microphone

Induction And the dynamo..

Generators Hand out on generators OHT of dynamo If a coil is spun around inside a magnetic field then current can be induced that can be used to operate devices that run on electricity This is called generation Both DC and AC can be generated depending on whether slip rings or split rings are used

Transformers These consist of 2 coils wound close to each other. Changing the current in one coil makes the field around it change. This changing field induces current in a nearby coil.

Transformers The ratio of the windings determines how much voltage/current is induced The voltage can be calculated using: N=number of turns V=Voltage P=primary coil S=secondary coil

Transformers No transformer is 100% efficient, but assuming it was: Power in = Power out

Transformers 3 Types: Step up : Vs > Vp Step down : Vs < Vp Isolating (a safety device) : Vs = Vp