Physics Unit 6 - Magnetism 1

Magnetism General term for properties and interaction of magnets Very closely related to electricity Magnetism is caused by moving electric charges

Producing a Magnet Electromagnets Electrons moving through a wire produce a magnetic field Magnetic materials Electrons move around in an atom produce a magnetic field in a specific direction Non-magnetic materials have lots of atoms producing magnetic fields in all random directions Magnetic materials have atoms where fields align

Magnetic Domains Magnetic domain - Group of atoms with aligned magnetic poles In non-magnetic objects, magnetic domains are randomly aligned A permanent magnet or a magnetized object has aligned magnetic domains domain

Magnetization Magnetic objects can be made into a magnet when their magnetic domains line up Rubbing an iron bar with a magnet or passing it through an electromagnetic field makes most of the domains line up. The field very close to another magnet can magnetize another object (EX: paper clips stuck to a magnet also act as a magnet) If magnetic domain remains aligned in the object, it is called a permanent magnet

Magnetic Poles Similar to charges, there are two types of poles: North and South Like poles repel (same as with charges) Opposite poles attract (same as with charges)

Magnetic Poles There is no such thing as an individual magnetic “charge” Every north pole needs a south pole, and vice versa A broken magnet creates new poles Each magnet has both a N and a S pole

Magnetic Field A magnet is surrounded by a magnetic field This field is what exerts a force on other magnets or other magnetic materials Magnets produce a force without needing to touch Magnetic field lines are used to represent the magnetic field in the area around a magnet

Magnetic Field Lines Direction: Strength: Lines point from N to S pole This is the direction of force on another magnet’s north pole. Another N is forced away from N pole and toward S pole. Strength: Field is strongest where lines are closest together This happens at the poles of the magnets

Earth’s Magnetic Field Earth is surrounded by magnetic field Caused by the rotation of its molten iron-nickel core Earth acts as if a giant bar magnet were thrust through its center Magnetic poles close to but slightly off from geographic north and south poles (where Earth spins on its axis)

Compasses A compass has a narrow needle-shaped magnet that freely rotates on a small pin The red side of the magnet has been called the north pole, since it points north in direction Since opposites attract, this means Earth’s geographic north pole is actually a magnetic south pole!

Magnetic Field from Currents Remember, currents are moving charges. Moving charges produce magnetic fields The magnetic field around a single wire is in co-centric loops around the wire This is not terribly useful, but when the wire is wrapped in solenoid (coil) the magnetic field acts like a bar magnet

Electromagnets Electromagnet – strong, temporary magnet formed by current passing through a wire coil Doubling the number of coils or doubling the current will double the strength of the magnet Two ends are different – N and S poles (like other magnets) Can easily be turned on and off Used in electric motors, speakers, and junkyard magnets

Electric Motor Device to convert electric energy to kinetic energy Electromagnet rotates between poles of a magnet It alternates being repelled and attracted by magnets around it, causing it to keep rotating Ex. Blender, toy car, fan

Inside real electric motor assembled motor field magnet (stationary parts) brushes & wires to battery armature & commutator (rotating parts)

Speaker Device to convert electrical energy into sound energy Wire coil acts as electromagnet, moving back and forth as it repels and attracts a permanent magnet The coil makes a paper cone vibrate, producing sound

Galvanometer Device that measures small electric current by running it through an electromagnet that deflects a needle

Electromagnetic Induction Electromagnetic induction – a current caused by a changing magnetic field Caused by relative motion between a wire coil and a magnet Reverse of electromagnet Electromagnet uses a current to produce a magnet Electromagnetic induction uses a magnet to induce a current

Electromagnetic Induction Generators are electric motors in reverse. Microphones are speakers in reverse Generators convert kinetic energy into electrical energy Microphones convert sound energy into electrical energy By rotating the armature, a current is produced in the coil as it rotates between the fixed magnet Sound vibrates the paper cone, causing the coil to move in and out relative to the fixed magnet and a current is produced in the coil

Electric Motor vs. Electric Generator Electrical energy output Permanent Magnet Electromagnet Permanent Magnet MOTOR Commutator Source of mechanical energy Electromagnet Source of electrical energy Commutator Mechanical energy output

Electric Generator PE of lake water is converted to KE Hydroelectric Dam PE of lake water is converted to KE KE turns the generator shaft which creates electrical energy

More on Induction “The Forever Flashlight™ uses induction and electromagnetic energy to eliminate the need for batteries.” Shaking the flashlight moves a magnet that induces a current in a stationary coil, which is then stored in a capacitor.

DC & AC Direct Current (DC) Current flows in one direction EX: batteries Alternating Current (AC) Current reverses its direction at regular intervals EX: electrical outlets 7th 12-5

Transformers Increases or decreases AC voltage Primary coil AC produces a magnetic field that induces AC in the secondary coil Voltage ratio = ratio of turns in each coil 8th 12-5

Transformers Increases AC voltage Step-Up Tranformer Increases AC voltage More turns in output (secondary) coil Used at power plants Step-Down Tranformer Decreases AC voltage More turns in input (primary) coil Used into households and into devices 8th 12-5

Transformers Examples 8th 12-5

Other Applications of Magnetism Magnetic strips – store and read information from the arrangement of tiny magnetized particles Traffic light sensors – use electromagnetic induction to sense changes in magnetic field in the presence of large metal object (car) 8th 12-5